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Optical spectroscopy analysis of the Eu3+ ions local structure in calcium diborate glasses
Journal of Non-Crystalline Solids 319 (2003) 200–216 www.elsevier.com/locate/jnoncrysol
Optical spectroscopy analysis of the Eu3þ ions local structure in calcium diborate glasses V. Lavın, U.R. Rodrıguez-Mendoza, I.R. Martın, V.D. Rodrıguez
*
Departamento de Fısica Fundamental y Experimental, Electr onica y Sistemas, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain Received 5 February 2002; received in revised form 13 August 2002
Abstract A detailed analysis of the optical properties of Eu3þ ions in calcium diborate glasses has been performed in order to correlate them with the local environment of the lanthanide ions in these glasses. From the excitation and emission spectra under broadband excitation the energy level diagram of the Eu3þ ion and the atomic parameters have been obtained. Moreover, it is found that the Eu3þ ions are predominantly bonded with the bridging and non-bridging oxygens of different borate groups containing the BO3 units. Under site selective excitation in the 7 F0 ! 5 D0 inhomogeneously broadened peak, valuable information about the splitting of the 7 F1 and 7 F2 levels and the lifetime of the 5 D0 level has been obtained as a function of the excitation wavelength. The linewidth of the low-energy components of the 5 D0 ! 7 F1 emission shows an increase with the splitting of this level that can be well explained considering nonradiative de-excitation processes towards lower 7 F1 Stark levels. From the splitting of the 7 F1 and 7 F2 levels, the second and fourth rank crystal-field parameters have been calculated as a function of the excitation wavelength. The observed behaviour suggests the existence of an unique class of site for the Eu3þ ions, with a large range of values for the crystalfield strength parameter NV ðB2q Þ, from 1000 to 3200 cm1 , that includes weak, medium and strong crystal-field environments for the Eu3þ ions in the calcium diborate glass. The J -mixing is shown to play a relevant role in these calculations for medium and strong crystal-field environments, i.e., for values of the crystal-field strength parameter NV ðB2q Þ over 1400 cm1 . Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.70.)a; 78.55.)m; 32.50.þd; 71.70.Ch
1. Introduction Crystals and glasses doped with lanthanide ions have been widely used in optoelectronic devices
*
Corresponding author. Tel.: +34-922 31 83 04; fax: +34-922 31 82 28. E-mail address: [email protected] (V.D. Rodrıguez).
such as lasers and amplifiers [1,2]. Particular attention has been paid to glasses due to their relatively easy fabrication and the possibility of obtaining large bulk samples compared to crystals. For all these applications, the role of the environment of the optically active ion in the glass matrix is of paramount importance since its local symmetry rules the intraconfigurational optical transitions.
0022-3093/03/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(02)01914-2
V. Lavın et al. / Journal of Non-Crystalline Solids 319 (2003) 200–216
Many techniques, such as NMR, X-rays, IR absorption or Raman spectroscopy, may give information on the structure of glasses, although they do not reflect the distribution and real symmetry of the environment of the lanthanide ions. On the other hand, EXAFS and neutron scattering have been used to obtain information of the mean bond distances and the coordination numbers of the lanthanides. However, the optical spectroscopy, and especially the technique of fluorescence line narrowing (FLN), is the only one that allows studying the distribution and the structure of the environments of these ions in glasses. Due to the diversity of local environments occupied by the ions in glasses the optical spectra of the lanthanide ions present inhomogeneous broadening. With monochromatic laser light it is possible to excite selectively the optically active ions in a particular type of environment and obtain an emission spectrum which gives information about the crystal-field (CF) acting on them [3,4]. The FLN technique has been used to obtain valuable information about the energy level structure, CF parameters, decay of luminescence, homogeneous linewidths and energy transfer processes for ions in different environments in a wide variety of glasses [5–21]. In the studies of local structure of lanthanides in glasses the most extended choice is the Eu3þ ion [22], due to its relatively simple diagram of levels and the high dependence of its fluorescence on the environment. Moreover, the wave functions of the 7 FJ levels have an almost pure Russell–Saunders character (>95%) [23] and the ground 7 F0 level and the lowest emitting 5 D0 level are non-degenerated. Borate glasses make up one of the families that more clearly show the relationship between glass structure and physical properties. From the scientific point of view, an interesting characteristic of the borate glasses is the appearance of changes in its structural properties when alkaline or alkaline-earth cations are introduced. This effect, known as the Ôboric oxide anomalyÕ, is explained by a continuous change of the boron coordination from three to four oxygens [24,25]. However, according to the Krogh–Moe theory [24,25], the structure of the borate glasses is not a random
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distribution of BO3 triangles and BO4 tetrahedra, but the gathering of these units to form welldefined, stable borate groups (such as diborate, triborate, tetraborate. . .) that constitute the random three-dimensional network. On the other hand, scarce attention has been paid to analyse this effect on the local environments of the lanthanide ions. In this work a study of the optical properties of the Eu3þ ion in a calcium diborate glass has been carried out. The vibronic spectrum, in combination with the IR and Raman spectra, allows analysing the chemical bonds of the Eu3þ ions with the oxygen ions of the different borate groups. The analysis of the 5 D0 ! 7 F1 and 7 F2 emission spectra, obtained after exciting selectively the 5 D0 level for different Eu3þ concentrations, allows the characterization of the CF interaction and to calculate the CF parameters as a function of the excitation wavelength. The effects of the J -mixing are discussed and the conclusions have been applied to understand the line profile of the 7 F0 $ 5 D0 transitions.
2. Experimental The composition of the samples used in this work was (in mol%): (66:66 x=2) CaO, (33:33 x=2) B2 O3 and x Eu2 O3 , with x equals to 0.1 and 2.5. The details of the preparation has been given in a previous paper [26]. Broadband spectra were obtained by exciting the sample with light coming from a 250 W incandescent lamp passed through a 0.25 m monochromator. For FLN spectra a tuneable dye laser operating with Rhodamine 6G, pumped by a Q-switched 532 nm frequency-doubled Nd:YAG laser, was used. The laser spectral linewidth was 0.15 cm1 and the pulse width 5 ns. For time-resolved FLN measurements a phosphorimeter connected to the Qswitching control of the Nd:YAG laser was used. Fluorescence was detected through a 0.25 m double grating monochromator with a cooled photomultiplier using a photon counting technique. Spectra were corrected from instrument response. Fluorescence decays were recorded using a digital oscilloscope controlled by a computer. All
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measurements were made at 13 K with a closecycled helium cryostat.
CF analysis, then the even CF Hamiltonian takes the form [22] ð2Þ
ð2Þ
ð2Þ
ð4Þ
even ¼ B20 C0 þ B22 ðC2 þ C2 Þ þ B40 C0 HCF ð4Þ
ð4Þ
ð6Þ
The optical properties of lanthanides are due to the 4fN electrons. The free lanthanide ion Hamiltonian can be written as a sum of different interactions [22,27–29] X Hfree ion ¼ Ho þ F k fk þ f4f Aso k¼2;4;6
þ aLðL þ 1Þ þ bGðG2 Þ þ cGðR7 Þ X X þ T i ti þ M h mk i¼2;3;4;6;7;8
þ
X
f
P pf ;
ð4Þ
ð4Þ
þ B42 ðC2 þ C2 Þ þ B44 ðC4 þ C4 Þ
3. Theoretical background
h¼0;2;4
ð1Þ
f ¼2;4;6
where the main ones are the inter-electronic repulsion and the spin–orbit interactions, second and third terms respectively, which give rise to the 2Sþ1 LJ levels. The rest only make corrections to the energy of these levels without removing their degeneracy. When a lanthanide ion is immersed in a solid matrix, there is an electrostatic interaction of the 4f electrons with the charge of host ligands, known as the CF interaction, whose magnitude is small compared to the spin–orbit interaction. Thus, apart from the slight shift of the 2Sþ1 LJ multiplets, the main modifications introduced by the CF interaction are the appearance of the Stark levels, arising from the removal of the degeneracy of the 2Sþ1 LJ levels [22,27–29], and the electric dipole transitions, forbidden in the free ion [30]. While the former is due to the even components of the CF Hamiltonian the latter is due to the odd CF Hamiltonian, although both are intimately related to the symmetry of the local CF acting on the optically active ions. The CF Hamiltonian can be written as a sum of one-electron tensor operators in the Wybourne formalism [22,27,28]. As it will be discussed later, an orthorhombic point group symmetry (D2h , D2 or C2V ) has been generally assumed for the local structure of the Eu3þ ions in works devoted to the
ð6Þ
ð6Þ
þ B60 C0 þ B62 ðC2 þ C2 Þ ð6Þ
ð4Þ
ð6Þ
ð6Þ
þ B64 ðC4 þ C4 Þ þ B66 ðC6 þ C6 Þ;
ð2Þ
where Bk;q ¼ Bk;q are real. If the mixing of wave functions of a given J Stark state with those of the J 2, J 4 or J 6 closer states (J -mixing effect) is neglected, the energy positions of the 7 F1 Stark levels, after the diagonalization of the even CF Hamiltonian (2) using symmetrised functions for the 2Sþ1 LJ Stark levels in C2V symmetry, are given by [31] 1 EðA2 Þ ¼ E0 ð7 F1 Þ þ B20 ; 5
pffiffiffi 6 1 B22 ; EðB1 Þ ¼ E0 ð F1 Þ B20 þ 10 10 pffiffiffi 1 6 7 B22 EðB2 Þ ¼ E0 ð F1 Þ B20 10 10 7
ð3Þ
and for the 7 F2 Stark levels by [31] rffiffiffiffiffi 11 1 1 10 B20 B40 þ B44 ; 105 63 9 7 pffiffiffiffiffi 11 11 4 2 10 B20 þ pffiffiffi B22 þ B40 EðB1 Þ ¼ E0 ð7 F2 Þ þ B42 ; 210 63 63 35 6 pffiffiffiffiffi 11 11 4 2 10 B20 pffiffiffi B22 þ B40 þ EðB2 Þ ¼ E0 ð7 F2 Þ þ B42 ; 210 63 63 35 6 rffiffiffiffiffi 7 1 10 EðA1 ; A1 Þ ¼ E0 ð7 F2 Þ B40 B44 63 9 7 2 " rffiffiffiffiffi #2 1 22 5 1 10 4 B20 þ B40 B44 4 105 63 9 7
EðA2 Þ ¼ E0 ð7 F2 Þ
11 1 B22 þ2 105 21
rffiffiffi !2 31=2 5 B42 5 ; 3
ð4Þ 7
where E0 ð FJ Þ is the centre of gravity or barycentre of the 7 FJ multiplets of the free Eu3þ ion and each Stark level is labelled with an irreducible representation of the C2V point group. Finally, some authors [32,33] have tried to simplify the description of the CF defining a
V. Lavın et al. / Journal of Non-Crystalline Solids 319 (2003) 200–216
scalar, rotational invariant parameter called the CF strength. Using that given by Auzel and Malta [33], the CF strength for orthorhombic symmetries takes the form ffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi X 4p 2 2 k NV ðBq Þ ¼ jBk0 j þ 2jBkq j : 2k þ 1 k;q>0
ð5Þ
As can be observed from Eq. (3), if the J -mixing effect is neglected the maximum splitting of the 7 F1 level only depends on the B2q CF parameters [31] and, making use of the theoretical expression of Malta et al. [34], both magnitudes can be correlated as follows sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:3 DEMAX ð F1 Þ ¼ NV ðB2q Þ; pð2 þ a2 Þ 7
ð6Þ
where a is given by Eb Ec a¼ DEMAX =2
203
ð7Þ
and Eb is the barycentre of energy of the 7 F1 level, calculated as the mean energy of the corresponding three Stark levels, whereas Ec is the energy of the central Stark level. 4. Results 4.1. Broadband excitation Excitation and emission spectra of the Eu3þ ions in a calcium diborate glass doped with 2.5 mol% of Eu2 O3 , obtained under broadband excitation at 13 K, are given in Fig. 1. They consist in a set of inhomogeneously broadened bands, each
Fig. 1. Broadband excitation (– – –) and emission (––) spectra of a calcium diborate glass doped with 2.5 mol% of Eu2 O3 at 13 K. Transitions start from the 7 F0 ground level in the excitation spectrum, detecting the 5 D0 ! 7 F2 transition at 615 nm, and from the 5 D0 level in the emission one, exciting the 7 F0 ! 5 L6 transition at 395 nm. Partial energy level diagram of the free Eu3þ ion is also shown.
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one associated to a transition between different 2Sþ1 LJ multiplets within the 4f6 ground configuration. They start from the ground level 7 F0 for the excitation spectrum and from the low emitting level 5 D0 for the emission one. The intraconfigurational 4f6 –4f6 transitions of the Eu3þ ions were identified comparing the peak energies with the free Eu3þ ion energy level diagram (Fig. 1) [22,23,35]. The excitation spectrum is similar to the absorption one and the bands at 319.6 (31289 cm1 ) and 376.8 nm (26539 cm1 ) have been assigned to transitions to the 5 H4 and 5 G4 levels, respectively, although there is a great overlapping with other levels close in energy with the same L and S but different J quantum number [23]. The emission spectrum has been obtained exciting at 395 nm (25316 cm1 ) the 7 F0 ! 5 L6 transition and shows different broad bands corresponding to the 5 D0 ! 7 FJ (J ¼ 0–6) transitions. The absence of emission starting from the 5 DJ (J ¼ 1–3) levels is due to the high-energy phonons found in borate glasses, i.e. when the Eu3þ ions are excited to any level above the 5 D0 one there is a fast non-radiative multiphonon relaxation to this level. From the 5 D0 level the Eu3þ ions decay radiatively, since the large energy difference to the 7 F6 level prevents the possibility of multiphonon relaxation. Thus, its quantum emission efficiency is nearby to the unity. The 5 D0 ! 7 F1 transition is magnetic dipole in nature, it is allowed by all the selection rules and is independent on the composition of the host matrix [36,37] if the J -mixing is negligible [38]. The existence of three overlapped peaks for this transition is the first indication of the low symmetry (orthorhombic, triclinic or monoclinic) of the Eu3þ environments in this glass [22]. The 5 D0 ! 7 FJ (J ¼ 2; 4; 6) transitions are electric dipole in nature and are forced by the odd CF Hamiltonian. The other emission transitions observed in Fig. 1, 5 D0 ! 7 FJ (J ¼ 0; 3; 5), are strictly forbidden in the frame of the intermediate scheme of the Judd– Ofelt theory [30]. As will be discussed later, their low intensities are explained by the J -mixing, which induces an effective borrowing of intensity from the other electric dipole transitions [12,39]. In order to obtain more information about the local structure of the Eu3þ ions and the nature of
their bonds with the oxygens, the vibronic or phonon side band (PSB) spectrum, involving simultaneous vibrational and electronic transitions, has been measured. For the Eu3þ ions, those PSBs coupled to the 7 F0 ! 5 D2 pure electronic, or zero phonon, band (PEB) have been measured detecting the 5 D0 ! 7 F2 transition at 615 nm in a calcium diborate glass with 2.5 mol% of Eu2 O3 at 13 K (Fig. 2). Due to the rather low probability of multiphonon excitation compared to the excitation of a single phonon, the difference in energy between the PSB and the PEB corresponds to the energy of one phonon [40]. The maximum of the PSB spectrum in Fig. 2 corresponds to a vibrational mode coupled to the Eu3þ ions with energy of 1400 cm1 . It is worthy to note that the excitation spectrum of the 5 D0 level does not show any appreciable PSB associated to low-energy phonons. This information is complementary to that of the IR and Raman spectroscopic techniques, which only analyse the intrinsic vibrations of the matrix. However, bands observed by all these techniques show a general correspondence, although the relative intensities can vary drastically according to
Fig. 2. IR absorption and Raman scattering spectra (HH and HV) at RT and PSB(5 D2 ) spectrum at 13 K in a calcium diborate glass doped with 2.5 mol% of Eu2 O3 . For the latter, the mean energy of the 7 F0 ! 5 D2 electronic band has been taken as the zero of energy. Predominant structural units are indicated for each energy range. HH means parallel polarisations in excitation and detection while HV means perpendicular polarisations.
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the different selection rules of each one. Due to the disordered nature of the vitreous materials and the great numbers of possible borate groups, the interpretation of the vibronic spectra is not an easy task. However, it is generally accepted that those bands in the range from 1100 to 1500 cm1 corresponds to borate groups containing BO3 units, while for lower energies are associated to groups containing BO4 units [24,25,41]. The IR and the HH and HV Raman spectra for the glass under study are presented in Fig. 2. Bands associated to the BO3 triangles (1225–1250 cm1 ), to the BO4 tetrahedra (880–1050 cm1 ) and to the B–O–B bonds (690–770 cm1 ) are clearly observed [41].
205
of the existence of a large variety of environments with different local structures and CFs for the Eu3þ ions in calcium diborate glasses. The FLN emission spectra of the 5 D0 ! 7 FJ (J ¼ 1 and 2) transitions exciting selectively within the 7 F0 ! 5 D0 band is shown in Fig. 4 for a calcium diborate glass with 0.1 mol% of Eu2 O3 at 13 K. The 5 D0 ! 7 FJ (J ¼ 3–6) emissions have not been included because they are poorly resolved, show scarce dependence on the excitation wavelength or their intensities are low. Differences in the Eu3þ surroundings are clearly reflected in the FLN spectra, indicating the great sensitivity of the splitting of the 7 F1 and 7 F2 multiplets to variations of the CF from one
4.2. Selective excitation The inhomogeneous line profile associated to the 7 F0 ! 5 D0 transition, obtained detecting at 615 nm the 5 D0 ! 7 F2 transition in a calcium diborate glass doped with Eu2 O3 at 13 K, is shown in Fig. 3. The large full-width at half-maximum (FWHM ¼ 105 cm1 ) is due to overlapping of homogeneous 5 D0 ! 7 F0 spectra with slight different energies. This large FWHM is an indication
7
5
Fig. 3. Inhomogeneous excitation profile of the F0 ! D0 transition in a calcium diborate glass doped with 2.5 mol% of Eu2 O3 at 13 K. Lifetime of the 5 D0 level in calcium diborate glasses doped with 0.1 () or 2.5 (j) mol% of Eu2 O3 at 13 K as a function of the excitation wavelength. The uncertainty of the lifetimes determination is of about 1%.
Fig. 4. FLN emission spectra to the 7 FJ (J ¼ 1; 2) levels exciting selectively the 5 D0 level in a calcium diborate glass doped with 0.1 mol% of Eu2 O3 at 13 K. Spectra are normalised to the maximum of the high-energy peak of the 5 D0 ! 7 F1 transition. Excitation wavelength is indicated for each spectrum in nm and with vertical arrows. The upper spectrum corresponds to the broadband emission from the 5 D0 level.
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environment to another in the glass. Two main features can be observed in these spectra. On the one hand, the CF acting on the Eu3þ ions forces the full degeneracy of the free Eu3þ multiplets. This means that three peaks can be observed for the 5 D0 ! 7 F1 transition, associated to the three 7 F1 Stark levels, and partially resolved spectra suggesting five peaks for the 5 D0 ! 7 F2 transition are obtained, indicating the existence of five 7 F2 Stark levels. This result confirms the low symmetry of the Eu3þ environments previously observed in the broadband emission. On the other hand, there are progressive changes in the intensity and the energy position of the different emission components, especially for the highest energy component of the 5 D0 ! 7 F1 transition that shows a large shift (almost twice the shift of the excitation). Additionally, it is interesting to observe the dependence of the linewidth of the three components of the 5 D0 ! 7 F1 transition on the excitation wavelength. There is an important increase of the linewidth for the two low-energy components of this transition whereas the linewidth of the highenergy component does not change appreciably. This behaviour, shown in Fig. 5, will be analysed afterwards. On the other hand, when the concentration of Eu2 O3 is increased up to 2.5 mol%, more than three peaks can be observed in the 5 D0 ! 7 F1 emission spectra for short excitation wavelengths to the 5 D0 level. Moreover, the time-resolved FLN spectra, as shown in Fig. 6 for excitation at 576.55 nm, change with the delay after the laser pulse. When this delay is increased, the time-resolved spectrum approaches to the inhomogeneous emission spectrum. Finally, the lifetime of the 5 D0 level for the different environments has been also measured exciting selectively the samples doped with 0.1 and 2.5 mol% of Eu2 O3 . The results, shown in Fig. 3, are in the range from 1.6 to 2.1 ms whereas a mean value of 1.86 ms was obtained under broadband excitation. In order to assure the site selectivity in these measurements, the emission corresponding to the high-energy component of the 5 D0 ! 7 F1 transition was monitored. The values for the lifetime obtained exciting at the low-energy side of the 7 F0 ! 5 D0 peak are similar for both Eu3þ con-
Fig. 5. Linewidths of the three emission peaks of the 5 D0 ! 7 F1 transition, low (), medium (j) and high (d) energy, as a function of the difference in energy between each Stark level to next lower one. Curves correspond to the fit to a cubic dependency on the energy difference, see Eq. (8). The uncertainty of the linewidths determination is of about 15 cm1 .
Fig. 6. Time-resolved FLN emission spectra to the 7 FJ (J ¼ 0; 1) at 300 ls (––) and 5 ms (– – –) after the laser flash exciting at 576.55 nm the 5 D0 level in a calcium diborate glass doped with 2.5 mol% of Eu2 O3 at 13 K. Spectra are normalised to the maximum of the high-energy peak of the 5 D0 ! 7 F1 transition. Broadband emission (- - -) from the 5 D0 level is also included for comparison.
centrations, whereas they are shorter for the most doped glass when excitation moves towards the high-energy side of this peak. The changes in the 5 D0 ! 7 F1 emission spectra and the 5 D0 level lifetime with the Eu2 O3 concen-
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tration can be explained considering phononassisted energy migration processes from Eu3þ ions in strong CF environments to ions in weak CF ones, analysed by the authors in the same borate glass [26]. These energy transfer processes increase with the doping concentration and give rise to a decrease of the lifetime. Thus, the migration of energy between Eu3þ ions can be considered negligible for the 0.1 mol% doped glass and the identification of the three 7 F1 Stark levels is straightforward.
5. Discussion 5.1. Broad band excitation From the spectra presented in Fig. 1 it has been possible to obtain the energy level diagram of the Eu3þ ion in the calcium diborate glass up to 31 300 cm1 and to calculate the atomic parameters of the free ion Hamiltonian, using Eq. (1), in the 4f6 electronic configuration in the intermediate coupling scheme. There are several experimental levels but they are not enough to allow all the atomic parameters to vary freely in the fitting procedure. Assuming the hydrogenic approximation [27] only the Slater parameter F2 and the spin–orbit parameter f4f are calculated and the rest of parameters are fixed to the values obtained by Jayasankar et al. [42] for the well known LaF3 :Eu3þ crystal. The values obtained for F 2 and f4f were 86 734 and 1329 cm1 , respectively. The experimental and calculated 2Sþ1 LJ energy levels for the Eu3þ ion in the calcium diborate glasses are presented in Table 1. The simulation is quite reasonable and there are only large differences in a few multiplets. The magnitudes of the atomic parameters are similar to those found in crystals and aqueous matrices [38]. This feature is a consequence of the relatively low influence that the local environment exerts on the energy positions of the 2Sþ1 LJ multiplets and it confirms that the inter-electronic and the spin–orbit interactions are the main responsible for the energy level diagram of the Eu3þ ion. Concerning the local structure of the Eu3þ ions in the glass, a first consideration can be made
207
Table 1 Experimental and calculated 2Sþ1 LJ multiplets (in cm1 ) of the Eu3þ ion in a calcium diborate glass 2Sþ1 7
LJ levels
F0 7 F1 7 F2 7 F3 7 F4 7 F5 7 F6 5 D0 5 D1 5 D2 5 D3 5 L6 5 L7 , 5 G2 5 GJ (J ¼ 3; 4; 5; 6) 5 D4 5 H4
Experimental levels
Calculated levels
0 409 1041 1987 2912 3878 5031 17 301 19 008 21 450 24 396 25 316 26 247 26 539
40 416 1076 1925 2893 3931 5004 17 253 18 966 21 419 24 310 25 353 26 392 26 527
27 586 31 289
27 583
Calculations have been carried out only varying the Slater and spin–orbit parameters, while the other atomic parameters were fixed. F 4 ¼ 0:668 F 2 , F 6 ¼ 0:4943 F 2 , a ¼ 16:82, b ¼ 630, c ¼ 1750, T 2 ¼ 370, T 3 ¼ 40, T 4 ¼ 40, T 6 ¼ 330, T 7 ¼ 380, T 8 ¼ 370, M 0 ¼ 2:38, M 2 ¼ 0:56 M 0 , M 4 ¼ 0:38 M 0 , P 2 ¼ 245, P 4 ¼ 0:75 P 2 , P 6 ¼ 0:5 P 2 . (All values in cm1 .) [42]. Levels with asterisk were not used in the fit. The uncertainty of the experimental levels determination is of about 15 cm1 . The rms of the calculated levels is 60 cm1 .
taking into account the electric 5 D0 ! 7 F2 to the magnetic 5 D0 ! 7 F1 intensities ratio (EMIR). This ratio is considered a measurement of how close the environment of the Eu3þ ion is to a centrosymmetric symmetry [43], since the electric dipole transitions are strictly forbidden when the odd CF Hamiltonian is null. From the emission spectrum an EMIR of 4.5 has been obtained for the calcium diborate glass. It is in the range of values found in other oxide glasses [18,37,43] and is much higher than those found in fluoride ones [8,21]. Another consequence of the high probability of the electric dipole transitions is that the lifetime of the 5 D0 level is relatively low, i.e., 1.86 ms in the calcium diborate glass. According to the Judd approach for the vibronically induced forced electric dipole transition [44], the intensity of the PSB shows an R6 dependence on the lanthanide to ligand distance, indicating that the PSBs are dominated by the ligands forming the immediate environment of the
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Eu3þ ions. The PSB spectrum in Fig. 2 shows a range of energy for the phonons up 1600 cm1 , with the maximum about 1400 cm1 , corresponding to different Eu3þ -ligand vibrational modes. This spectrum can include different contributions of the vibrational modes of the BO3 triangles: the boron-bridging oxygen bonds inside the various borate rings (1350–1400 cm1 ) and those from the boron-non-bridging oxygens bonds (1420–1550 cm1 ) associated to the great borate groups [41]. The IR-active band centred at 1200 cm1 , associated to the chains found in the metaborate group [25], shows a lower contribution to the PSB. In the range from 850 to 1100 cm1 the contribution of the vibrational modes of the BO4 units present in diborate and tetraborate groups is also weak. The overall PSB spectrum is similar, although somewhat broader, to that found by Tanabe et al. [45] in a sodium diborate glass. As it can be observed, not all the borate groups present in the glass take part in the formation of the local structure around the Eu3þ ions. Although Konijnendijk [25] suggests that the structure of these glasses is formed principally from diborate groups, it is clearly observed that there is not a predominant bond of the Eu3þ ion with the bridging oxygen of these groups. On the contrary, the presence of bridging and non-bridging oxygen of borate groups with BO3 units with great tendency to co-ordinate with the Eu3þ ions give rise to a much lower relative electron–phonon coupling factor to the BO4 units of the diborate groups than expected. Tanabe et al. [45] have invoked the difference in the affinity of the different borate groups with the Eu3þ ions and their selectivity in the glass to explain it. However, this feature could be associated to the high rigidity of the BO4 units of the diborate groups, which have more sharing points with other polyhedra than those borate groups with BO3 triangles [41,46,47]. 5.2. Selective excitation 5.2.1. Broadening mechanism of the 5 D0 ! 7 F1 line profile Three 7 F1 Stark levels are obtained from the FLN emission spectra of the 0.1 mol% of Eu2 O3 doped glass. However, as it has been pointed out,
for this sample the low-energy components of the 5 D0 ! 7 F1 transition show an important broadening of their linewidth with the excitation wavelength as the splitting of the 7 F1 multiplet increases. This broadening could be explained by the existence of non-radiative de-excitation processes with the emission of low-energy acoustic phonons due to the interactions of the 4f electrons with the matrix. These processes diminish the lifetimes of the Stark levels and, consequently, increase their linewidths [29]. In order to analyse this behaviour, the linewidth of each of these three peaks is plotted in Fig. 5 as a function of the energy separation between each 7 F1 Stark level and its immediately lower one. Assuming the Debye approximation for the phonon spectra in the glass, the linewidth of each Stark level would increase with the third power of the separation in energy between two consecutive Stark levels, in accordance with the expression for the transition probability [29]
3 3 DE W ðaJ ; bJ 0 Þ ¼ jhaJ jHcc–din jbJ 0 ij2 2pqv5 h h ½nðDEÞ þ 1;
ð8Þ
where q is the density of the glass, v is the mean velocity of sound, DE is the difference in energy between two consecutive levels, nðDEÞ is the density of phonons of energy DE and Hcc–din is the dynamic CF Hamiltonian [3]. The fit of the linewidths to Eq. (8) is shown in Fig. 5 as a function of the difference in energy between two consecutive levels. The fit includes an energy independent contribution to the linewidth that recovers different contributions such as the multiphonon, the inhomogeneous and/or the instrumental broadenings. However, the non-radiative rate from the lowest-energy 7 F1 Stark level is relatively weak and does not contribute appreciably to the linewidth of this peak. This result may be associated to the matrix element of the dynamic CF Hamiltonian in Eq. (8). Thus, the good fit observed for the change in linewidth of these peaks could be explained adequately considering the interaction of the Eu3þ ions with a distribution of phonons described by the Debye approximation. This approximation has been also invoked to
V. Lavın et al. / Journal of Non-Crystalline Solids 319 (2003) 200–216
explain the spectral widths observed in crystalline host lattices [48]. 5.2.2. Crystal-field analysis From the splitting observed in the emission transitions to the 7 F1 and 7 F2 multiplets it is possible to carry out a description of the CF acting on the Eu3þ ions in the calcium diborate glasses. This analysis is made assuming a local symmetry different to the trivial one C1 , for which at least 13 CF parameters are necessary to define the CF Hamiltonian, while only eight Stark levels could be available. The most extended choice is to assume a C2V symmetry for the local environments of the Eu3þ ions in the glass [6,8,10–13,15,20,21]. This is because it is a subgroup of almost all the higherorder point groups, it allows optical activity from
Fig. 7. Gaussian de-convolution of the 5 D0 ! 7 F2 emission profiles exciting selectively the 5 D0 level at 576, 577.5 and 579 nm in a calcium diborate glass doped with 0.1 mol% of Eu2 O3 at 13 K. Sums of the five gaussian functions are included.
209
the 5 D0 level to practically all the 7 FJ Stark levels and it is the highest order non-centrosymmetric point group that breaks completely the degeneracy of the 7 FJ multiplets [6]. To determine the components of the 5 D0 ! 7 F2 emission spectra in Fig. 4, a de-convolution of the spectra in five Gaussian line-shape functions has been carried out. Progressive changes in the energy positions and relative intensities have been assumed in the de-convolution process, since there are neither crosses between components nor relevant changes in their intensities from each spectrum to the following one, as already suggested from the FLN spectra. The obtained Gaussian functions are shown in Fig. 7 for the spectra obtained exciting at 576, 577.5 and 579 nm. The
Fig. 8. 7 F1 and 7 F2 Stark levels of the Eu3þ ions respect to the 7 F0 ground level in a calcium diborate glass as a function of the excitation wavelength. Solid lines indicate calculated Stark levels. The uncertainty of the 7 F1 and 7 F2 Stark levels determination is of about 10 and 20 cm1 , respectively. The rms of the calculated Stark levels is 5 cm1 .
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Table 2 Experimental and calculated 7 F1 and 7 F2 Stark levels (in cm1 ) of the Eu3þ ions in a calcium diborate glass as a function of the excitation wavelength to the 5 D0 level at 13 K Excitation wavelength (nm) 575 575.5 576 576.5 577 577.5 578 578.5 579 579.5
Obs. Calc. Obs Calc. Obs. Calc. Obs. Calc. Obs Calc. Obs Calc. Obs. Calc. Obs. Calc. Obs. Calc. Obs. Calc.
7
7
F1
F2
B1
A2
B2
A1
B2
A2
A1
B1
137 141 143 144 151 152 160 163 176 178 189 191 209 211 223 227 237 241 257 260
389 383 388 388 382 389 379 386 375 382 376 384 371 377 367 366 364 363 360 355
702 701 684 687 657 659 639 640 616 617 593 598 569 577 540 551 511 523 488 499
883 886 890 891 900 897 906 903 916 913 919 917 919 918 918 921 920 924 919 927
1017 1021 1011 1012 1000 997 986 982 976 972 986 980 980 978 976 976 974 976 967 971
1200 1200 1174 1174 1141 1140 1112 1112 1089 1089 1073 1072 1059 1057 1044 1039 1027 1022 1014 1007
1312 1311 1279 1279 1232 1232 1210 1211 1187 1186 1166 1164 1146 1144 1137 1129 1111 1107 1097 1090
1432 1432 1407 1407 1380 1380 1357 1356 1321 1321 1297 1297 1266 1263 1247 1246 1223 1222 1208 1209
The 7 F0 Stark level has been taken as the zero of energy. The assignments of the irreducible representations correspond to the C2V symmetry. The uncertainty of the 7 F1 and 7 F2 Stark levels determination is of about 10 and 20 cm1 , respectively.
energy positions of the experimental 7 F0 , 7 F1 and 7 F2 Stark levels are shown in Fig. 8 and Table 2 as a function of the excitation wavelength. The analysis of the CF begins considering the splitting of the 7 F1 multiplet. In a first approximation, if the J -mixing is neglected only the second rank CF parameters, B20 and B22 , of the even CF Hamiltonian, Eq. (2), will affect significantly to the breakdown of the degeneracy of the 7 F1 multiplet into three Stark levels [22]. The irreducible representations of the 7 F1 and 7 F2 Stark levels are given in Eqs. (3) and (4) [31]. As can be observed from Eq. (3), the sign and magnitude of the B20 parameter determine the relative position of the A2 Stark level with respect to the barycentre of the other two Stark levels. On the other hand, the sign of the B22 parameter determines the relative positions of the B1 and B2 Stark levels and, hence, its magnitude will be proportional to the difference in energy between these two levels. It is interesting to note that if B20 is low enough, or zero, there will be three equally spaced Stark levels.
Due to the almost symmetrical splitting observed for the three 7 F1 Stark levels with the increase of the CF, the B22 CF parameter presents a large variation in magnitude, while the B20 axial parameter is rather low and will not affect significantly the splitting [22,34,49]. The irreducible representation assigned to the central Stark level will be A2 , while the others can be ambiguously assigned to the B1 and B2 representations. However, care must be taken in the assignations since for orthorhombic symmetries six different combinations of B2q CF parameters can be obtained from the different assignations possible for the three 7 F1 Stark levels that give the same splitting [28,50]. The B2q CF parameter calculated from Eq. (3) are shown in Table 3 (in parentheses). The second rank CF strength parameter NV ðB2q Þ is shown in Fig. 9 as a function of the experimental maximum splitting of the 7 F1 manifold, DEMAX (7 F1 ), obtained from Table 2. Its values are between the largest ones found in glasses, only overcome by silicate [6] and borosilicate
V. Lavın et al. / Journal of Non-Crystalline Solids 319 (2003) 200–216 Table 3 Second and fourth rank CF parameters (in cm1 ) considering the J -mixing as a function of the excitation wavelength to the 5 D0 level Excitation wavelength (nm)
B20
B22
B40
B42
575
)49 ()102) )6 ()83) 35 ()86) 40 ()70) 35 ()67) 34 ()54) 0 ()60) )15 ()47) )16 ()34) )26 ()40)
1398 (1155)
1308
)600
48
1329 (1105) 1205 (1048) 1125 (979) 1010 (902) 917 (825) 803 (736) 716 (647) 614 (560) 522 (471)
1287 1254 1160 1076 1075 1028 1004 966 967
)551 )519 )524 )480 )380 )336 )358 )357 )360
17 )12 )105 )145 )150 )158 )225 )239 )300
575.5 576 576.5 577 577.5 578 578.5 579 579.5
B44
Values in parentheses have been calculated neglecting the J -mixing. The uncertainty of the CF parameters determination is of about 10 and 20 cm1 for the second and fourth rank
Fig. 9. CF strength parameter NV ðB2q Þ, with (j) and without () taking into account the J -mixing, as a function of the maximum splitting observed for the 7 F1 level of the Eu3þ ions in a calcium diborate glass. The solid line indicates the fit to the theoretical expression of Malta et al. [34]. The uncertainty of the CF strength parameter determination is of about 20 cm1 .
[18] glasses. Another consequence of the almost symmetrical splitting of the 7 F1 Stark levels observed in this glass is that the factor a in Eqs. (6) and (7) is rather low. Thus, the proportionality factor between the CF strength and the maximum splitting of the 7 F1 level is practically a constant of value 0.218 for all the environments, giving rise to
211
the linear dependence shown in Fig. 9. This result is rather similar to that obtained for fluoride glasses [38] and closer to 0.213 obtained for oxide crystals [34]. In order to give more confidence to the assignations of the irreducible representations of the 7 F1 Stark levels in C2V symmetry one may observe that the CF strength for an ideal symmetrical splitting (a ¼ 0), Eq. (6), rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pð2 þ a2 Þ DEMAX ð7 F1 Þ NV ðB2q Þ ¼ 0:3 1 DEMAX ð7 F1 Þ ð9Þ ¼ 0:218 and the maximum splitting of the 7 F1 multiplet obtained with the B22 CF parameter from Eq. (3), assuming no J -mixing, pffiffiffi 2 6 DEMAX ð7 F1 Þ ¼ B22 ffi 0:218NV ðB2q Þ ð10Þ 10 gives rise to the same relation. Although the agreement between experimental and calculated Stark levels is quite good, the CF interaction in the calcium diborate glasses is so strong that it is necessary to analyse the influence of the J -mixing. This is particularly important for those Eu3þ ions in environments under strong CF, for which the 7 F1 Stark wave functions are mixed with the 7 F2 and 7 F3 ones through the second rank CF Hamiltonian. In order to take into account this interaction in the splitting of the 7 F1 manifold, the complete Hamiltonian (free-ion plus the second rank CF Hamiltonian) has been diagonalised taking as a base linear combinations of jJMi wave functions of the 7 FJ multiplets in the C2V symmetry [31]. The fitting program [51] minimises the sum of square residuals between the experimental and calculated Stark levels. The B2q CF parameters calculated neglecting J -mixing have been taken as initial values. From the calculated parameters the CF strength NV ðB2q Þ is obtained and represented in Fig. 9 as a function of the maximum splitting of the 7 F1 multiplet. As can be clearly observed, when J -mixing is assumed the energy contributions to the 7 F1 Stark levels give rise to larger values for the CF strength to explain the same experimental
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splitting. For Eu3þ ions excited at 575 nm the CF strength parameter for these environments increases in about 20%, compared to that obtained neglecting the J -mixing effects. Thus, from the comparison of both sets of values for the CF strength a measurement of the influence of the J mixing has been obtained. Taking the J -mixing into account G€ orllerWalrand and Binnemans [22] have re-defined the Ôweak CFÕ (J -mixing negligible) and Ôstrong CFÕ (J mixing unnegligible) for the lanthanide systems. According to this definition in the calcium diborate glasses it can be obtained a distribution of environments under weak (with maximum 7 F1 splitting DEMAX ð7 F1 Þ < 300 cm1 ), medium (300 450 cm1 ) CF. Although these ranges are roughly taken, these results suggest that the B2q CF parameters calculation must include the J -mixing when the lanthanide ion is immersed in a strong CF environment. Moreover, the theoretical expression obtained by Malta et al. [34] must be used with caution. The importance of the J -mixing in the calcium borate glasses suggests the interest of extending the analysis to the 7 F2 multiplet. One important difficulty to overcome is the assignation of the irreducible representations of the 7 F2 Stark levels. Once the assignation of the three 7 F1 Stark levels is fixed there are 60 possible combinations for the C2V symmetry, besides those obtained after the change of the B1 and B2 assignations. Thus, it is essential to give some criteria to reduce the possibilities. Brecher et al. [6,8] gave some of them that have been used since then: the assignation of the Stark levels will be the same for all the spectra, the relative splitting of the B1 and B2 Stark levels must be similar for both multiplets and, finally, only those assignations that give a good fit for all the spectra are taken into account. After assuming these criteria the resulting combination is straightforward (see Table 2). Using the diagonalisation program developed by Reid [51] the fitting process of the 7 F1 and 7 F2 Stark levels was carried out taking as initial parameters the atomic parameters given in Table 2 and the second and fourth rank CF parameters calculated from Eqs. (3) and (4). The calculated
energy levels are presented in Fig. 8 (solid lines) and Table 2, showing a rather good agreement with the experimental ones. The calculated second and fourth rank even CF parameters are shown in Table 3 and Fig. 10. They show large differences from one environment to another. It must be noted that the signs of the B20 , B40 and B44 parameters are determined from the chosen assignation of irreducible representations, while those of the B22 and B42 are arbitrary, depending on the interchange of the B1 and B2 irreducible representations. Results indicate that a great percentage of the splitting of the 7 F2 multiplet is due to the B22 parameter, while the contribution of the B4q parameters is less important. This is not surprising since the splittings are similar for the 5 D0 ! 7 F1 and 7 F2 transitions (see Figs. 4 and 8). Moreover, the B2q parameters do not differ from those obtained from the analysis of the 7 F1 Stark splitting assuming J -mixing. Thus, it seems that although the B4q are large in magnitude, they cancel each other in such a way that their contribution to the splitting is not significant. This conclusion seems to be right for all the Stark levels except for the two higher energy 7 F2 ones. With the atomic parameters and the second and fourth rank CF parameters it is possible to simulate the energy level diagram of the 7 FJ Stark levels
Fig. 10. CF parameters versus the excitation wavelength within the 7 F0 ! 5 D0 peak.
V. Lavın et al. / Journal of Non-Crystalline Solids 319 (2003) 200–216
213
of the Eu3þ ions in all the environments possible in the calcium diborate glasses. Although the sixth rank CF parameters cannot be calculated with the experimental levels available, the confidence of the simulation is reasonable since it can be assumed that the influence of CF parameters B6q is only appreciably in the strong CF environments. One of the most interesting consequences of the influence of the J -mixing for strong CF environments in calcium diborate glasses is the increase of the electric dipole contribution to the 5 D0 ! 7 F1 magnetic dipole transition. For excitation at 575 nm the wave functions of the three 7 F1 Stark levels are j7 F1 ðB1 Þi ffi 98%j7 F1M i þ 2%j7 F3M i j7 F1 ðA2 Þi ffi 90%j7 F1M i þ 8%j7 F2M i þ 2%j7 F3M i
Fig. 11. CF strength NV ðB2q Þ as a function of the excitation wavelength to the 5 D0 level for calcium diborate ðjÞ, sodium silicate ðÞ [6], fluorozirconate ðdÞ [21] and lithium fluoroborate ð Þ glasses [50].
j7 F1 ðB2 Þi ffi 64%j7 F1M i þ 28%j7 F2M i þ 8%j7 F3M i: ð11Þ Thus, the non-dependency of the 5 D0 !7 F1 magnetic dipole transition with the host matrix must be assumed with caution for those Eu3þ environments under strong CF. All these results seem to indicate that there is an unique class of site for the Eu3þ ions present in the calcium diborate glass. This site distribution ranges from weak to strong CF and could be interpreted as continuous variation or distortion of a typical eightfold co-ordinated oxide geometry such as the square antiprism, the triangled dodecahedron or the biccaped trigonal prism [6,8,17]. Finally, in order to compare with results for other glasses, the CF strength parameter, calculated with the second rank CF parameters obtained from the 7 F1 Stark levels considering J mixing, is presented in Fig. 11 versus the excitation wavelength to the 5 D0 level for the diborate and other Eu3þ doped oxide, fluoride and oxyfluoride glasses [6,21,50]. Differences in the environment distribution width (excitation wavelength range) and in the CF strength parameter with the excitation wavelength indicate the influence of the glass composition in the local structure of lanthanides in glasses. Although the influence of the modifier ions is relevant in the optical properties
and, therefore, in the local structure of the Eu3þ ions, some conclusions can be stated for each family of oxide, fluoride or oxyfluoride glasses. For the diborate and silicate oxide glasses a large and close to linear dependency, with the same slope, is found as CF strength parameter increases with the excitation energy. The fluoride glass shows smaller CF strengths and a sharper distribution of environments for the Eu3þ ions. Whereas in the fluoroborate glass an intermediate behaviour for the local structure is observed, since the CF strength values remain lower than those found in oxide glasses but larger than in fluoride ones. Moreover, for a given value of CF strength the excitation wavelength is different for each glass family. From Fig. 11 it can be observed that the pure oxide glasses are red-shifted compared to the pure fluoride glass. According to the nephelauxetic effect, this feature would indicate a higher covalency of the Eu3þ –O2 bonds of the borate groups present in the glass compared to the others. As a conclusion, the lanthanide ion in oxide glasses acts as a glass-modifier, competing with the network formers to satisfy its bonding requirements of charge neutrality. This feature gives rise to a larger variation of the CF and a broader distribution of environments, built with oxygen ions, for the lanthanide ion in these glasses.
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5.2.3. 7 F0 $ 5 D0 transitions The variation observed for the CF strength could also explain the intensity and asymmetry observed in the excitation and emission line profiles of the 5 D0 $ 7 F0 transitions (see Figs. 1 and 3). The higher the CF strength the larger the influence of the J -mixing, i.e., for excitation at 575 nm the wave function of the ground level is j7 F0 i ffi 88%j7 F00 i þ 6%j7 F22 i þ 6%j7 F2–2 i:
ð12Þ
Moreover, there is also an increase in the magnitude of the odd CF parameters. Thus, low concentration of Eu3þ ions in strong CF environments produces an appreciable intensity of the 5 D0 $ 7 F0 electric dipole transitions. The same results were obtained by Nishimura and Kushida [12] and Tanaka et al. [39] for calcium phosphate glasses, and they described the effect as an effective borrowing of intensity from the hypersensitive transition. The large probability of the 5 D0 ! 7 F2 hypersensitive transition supports this hypothesis. However, these changes do not affect appreciably the lifetime of the 5 D0 levels of the Eu3þ ions in different environments, since the contribution of the 5 D0 ! 7 F0 transition to the total decay rate is rather low. The proximity of the 7 F1 and 7 F2 multiplets compared to the energy separation between the 5 D0 and the 5 D2 multiplets have suggested to Nishimura and Kushida [12] that the inhomogeneous broadening is mainly due to the downward energy shift of the 7 F0 level induced by the J -mixing. If only the second rank CF Hamiltonian contribution is considered, the first-order perturbation theory gives an approximate, but realistic, idea of the energy shift obtained and how the J -mixing acts 4 ðjB20 j2 þ 2jB22 j2 Þ 75DE20 1 N 2 ðB2q Þ; ¼ E0 ð7 F0 Þ 15pDE20 V
EðA1 Þ ¼ E0 ð7 F0 Þ
ð13Þ
where E0 (7 F0 ) is the energy of the 7 F0 level of the free Eu3þ ion and DEJJ 0 is the energy difference between the 7 FJ and the 7 FJ 0 multiplets. The energy shift of the 7 F0 level with the CF strength parameters calculated from the last term
of Eq. (13) is in good agreement with the energy variation corresponding to the excitation wavelength. So, when the excitation wavelength goes from 579.5 to 575 nm, the corresponding energy variation is 135 cm1 whereas the energy shift calculated from Eq. (13) is about 150 cm1 . It is clearly observed how the shift increases for environments with higher CF strength. This energy shift may have also contributions from the B4q CF parameters, but they can be ignored if one realise that, applying the first-order perturbation theory, the energy difference between the 7 F4 Stark levels and the ground one is too large (2900 cm1 ). Thus it is principally induced by the B22 parameter.
6. Summary and conclusions The Eu3þ ions cannot be introduced in the simple B2 O3 glass in the absence of modifier cations, since the B2 O3 network is strongly bonded by bridging oxygens. However, if a modifier cation such as Ca2þ is present the network is broken and the non-bridging oxygens appear. In the vicinity of the breakdown the Eu3þ ions could be incorporated as a modifier replacing the Ca2þ cations, since both have a similar ionic radius. Within the Krogh–Moe theory the Eu3þ modifier cations demand and obtain from the network a particular sphere of coordination to reach the charge neutrality, which depends on the structural borate groups present in the matrix and their flexibility. The Eu3þ ions predominantly bond with the bridging and non-bridging oxygens of the BO3 units. These oxygens will form the first coordination sphere, adding a local charge that will balance the þ3 charge of the Eu3þ ions, while the boron and calcium ions will incorporate in the successive spheres, balancing the excess of charge of the oxygen ions. The flexibility shown by the BO3 triangles compared to the BO4 tetrahedra allows to the Eu3þ ion to modify the local environment in order to stabilize it and to achieve the charge neutrality with coordination 8, typical for lanthanides in oxides due to the ionic radius of the oxygen and the Eu3þ ions and to the free orbitals of the latter.
V. Lavın et al. / Journal of Non-Crystalline Solids 319 (2003) 200–216
These features generate a great variety of environments for the Eu3þ ions with different distances and bond angles with the ligand oxygens, giving rise to different CFs and, as a consequence, to small variations in the energy level scheme and the optical properties of the Eu3þ ions. The results obtained in this work allow us to suppose that, although the Eu3þ ions are immersed in an amorphous matrix, some degree of local order persists in the vicinity of these ions. Moreover, it seems that there is a unique class of site for all the distribution of environments present in the calcium diborate glass. In this sense, the eight coordinated environments could be interpreted as continuous distortion of a typical geometry such as the square antiprism, the triangled dodecahedron or the biccaped trigonal prism. From the experimental 2Sþ1 LJ energy levels, the atomic parameters have been calculated and the energy level diagram simulated from these parameter is in good agreement with the experimental one. Moreover, from the splitting of the 7 F1 and 7 F2 multiplets, the second and fourth CF parameters can be calculated as functions of the excitation wavelength. The Stark energy levels obtained from these CF parameters are also in good agreement with the experimental data. A large range of CF strength parameter NV ðB2q Þ values corresponding to a broad site distribution for the Eu3þ ions ranging from weak to strong CF environments is found. An important mixing of the 7 F0 and 7 F1 Stark levels with the 7 F2 ones is found for those Eu3þ ions in strong CF environments. From the analysis of the 7 F1 splitting, a quantitative measure of the J -mixing influence in the 7 F1 maximum splitting for the Eu3þ ions in the different environments have been obtained. As an example, exciting at 575 nm and taking into account the J -mixing in the CF analysis, the obtained CF strength parameter NV ðB2q Þ is 20% larger than without J -mixing. Moreover, for the Eu3þ ions excited at 575 nm an important J -mixing is found, the 7 F0 level is mixed with 12% of the 7 F2 level and the 7 F1 Stark levels are mixed with up to 28% of the 7 F2 level. The J -mixing effect is also reflected in the transition probabilities from the 5 D0 to the 7 FJ levels and in the line profile of the 7 F0 $ 5 D0
215
transitions. Moreover, an important energy shift of the 7 F0 level is observed, which depends on the excitation wavelength and can be explained considering J -mixing. Finally, an increasing of the 5 D0 ! 7 F1 emission linewidths with the splitting of the 7 F1 Stark components is observed, which can be well explained assuming non-radiative de-excitation processes due to interaction of the Eu3þ ions with a Debye phonon distribution. Acknowledgements The authors are indebted to Dr P. Nu~ nez (Dpto. de Quımica Inorganica, Univ. La Laguna) for providing the samples studied in this work and the IR spectrum and to Dr R. Alcala (Dpto. de Fısica de la Materia Condensada, Univ. Zaragoza) for providing the Raman spectra. This research was partially supported by ÔGobierno Aut onomo de CanariasÕ (PI 2001/048) and ÔMinisterio de Ciencia y TecnologıaÕ (MAT2001-3363). References [1] A.A. Kaminskii, Ann. Phys. Fr. 16 (1991) 639. [2] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. [3] G.F. Imbusch, Phys. Scr. T 19 (1987) 354. [4] M.J. Weber, in: Laser Spectroscopy of Solids, Springer, Berlin, 1986, p. 189. [5] N. Motegui, S. Shionoya, J. Lumin. 8 (1973) 1. [6] C. Brecher, L.A. Riseberg, Phys. Rev. B 13 (1976) 81. [7] M.J. Weber, J. Hegarty, D.H. Blackburn, in: Borate Glasses. Structure, Properties and Applications, Plenum, New York, 1978, p. 215. [8] C. Brecher, L.A. Riseberg, Phys. Rev. B 21 (1980) 2607. [9] J.L. Adam, V. Poncßon, J. Lucas, G. Boulon, J. Non-Cryst. Solids 91 (1987) 191. [10] T.F. Belliveau, D.J. Simkin, J. Non-Cryst. Solids 110 (1989) 127. [11] J. Dexpert-Ghys, B. Piriou, N. Jacquet-Francillon, C. Sombret, J. Non-Cryst. Solids 125 (1990) 117. [12] G. Nishimura, T. Kushida, J. Phys. Soc. Jpn. 60 (1991) 683; J. Phys. Soc. Jpn. 60 (1991) 695. [13] R. Balda, J. Fernandez, H. Eilers, W.M. Yen, J. Lumin. 59 (1994) 81. [14] V. Lavın, V.D. Rodrıguez, I.R. Martın, U.R. RodrıguezMendoza, I. Szab o, Z.E. Razstovits, J. Appl. Spectrosc. 62 (1995) 185.
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Optical spectroscopy analysis of the Eu3þ ions local structure in calcium diborate glasses V. Lavın, U.R. Rodrıguez-Mendoza, I.R. Martın, V.D. Rodrıguez
*
Departamento de Fısica Fundamental y Experimental, Electr onica y Sistemas, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain Received 5 February 2002; received in revised form 13 August 2002
Abstract A detailed analysis of the optical properties of Eu3þ ions in calcium diborate glasses has been performed in order to correlate them with the local environment of the lanthanide ions in these glasses. From the excitation and emission spectra under broadband excitation the energy level diagram of the Eu3þ ion and the atomic parameters have been obtained. Moreover, it is found that the Eu3þ ions are predominantly bonded with the bridging and non-bridging oxygens of different borate groups containing the BO3 units. Under site selective excitation in the 7 F0 ! 5 D0 inhomogeneously broadened peak, valuable information about the splitting of the 7 F1 and 7 F2 levels and the lifetime of the 5 D0 level has been obtained as a function of the excitation wavelength. The linewidth of the low-energy components of the 5 D0 ! 7 F1 emission shows an increase with the splitting of this level that can be well explained considering nonradiative de-excitation processes towards lower 7 F1 Stark levels. From the splitting of the 7 F1 and 7 F2 levels, the second and fourth rank crystal-field parameters have been calculated as a function of the excitation wavelength. The observed behaviour suggests the existence of an unique class of site for the Eu3þ ions, with a large range of values for the crystalfield strength parameter NV ðB2q Þ, from 1000 to 3200 cm1 , that includes weak, medium and strong crystal-field environments for the Eu3þ ions in the calcium diborate glass. The J -mixing is shown to play a relevant role in these calculations for medium and strong crystal-field environments, i.e., for values of the crystal-field strength parameter NV ðB2q Þ over 1400 cm1 . Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.70.)a; 78.55.)m; 32.50.þd; 71.70.Ch
1. Introduction Crystals and glasses doped with lanthanide ions have been widely used in optoelectronic devices
*
Corresponding author. Tel.: +34-922 31 83 04; fax: +34-922 31 82 28. E-mail address: [email protected] (V.D. Rodrıguez).
such as lasers and amplifiers [1,2]. Particular attention has been paid to glasses due to their relatively easy fabrication and the possibility of obtaining large bulk samples compared to crystals. For all these applications, the role of the environment of the optically active ion in the glass matrix is of paramount importance since its local symmetry rules the intraconfigurational optical transitions.
0022-3093/03/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(02)01914-2
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Many techniques, such as NMR, X-rays, IR absorption or Raman spectroscopy, may give information on the structure of glasses, although they do not reflect the distribution and real symmetry of the environment of the lanthanide ions. On the other hand, EXAFS and neutron scattering have been used to obtain information of the mean bond distances and the coordination numbers of the lanthanides. However, the optical spectroscopy, and especially the technique of fluorescence line narrowing (FLN), is the only one that allows studying the distribution and the structure of the environments of these ions in glasses. Due to the diversity of local environments occupied by the ions in glasses the optical spectra of the lanthanide ions present inhomogeneous broadening. With monochromatic laser light it is possible to excite selectively the optically active ions in a particular type of environment and obtain an emission spectrum which gives information about the crystal-field (CF) acting on them [3,4]. The FLN technique has been used to obtain valuable information about the energy level structure, CF parameters, decay of luminescence, homogeneous linewidths and energy transfer processes for ions in different environments in a wide variety of glasses [5–21]. In the studies of local structure of lanthanides in glasses the most extended choice is the Eu3þ ion [22], due to its relatively simple diagram of levels and the high dependence of its fluorescence on the environment. Moreover, the wave functions of the 7 FJ levels have an almost pure Russell–Saunders character (>95%) [23] and the ground 7 F0 level and the lowest emitting 5 D0 level are non-degenerated. Borate glasses make up one of the families that more clearly show the relationship between glass structure and physical properties. From the scientific point of view, an interesting characteristic of the borate glasses is the appearance of changes in its structural properties when alkaline or alkaline-earth cations are introduced. This effect, known as the Ôboric oxide anomalyÕ, is explained by a continuous change of the boron coordination from three to four oxygens [24,25]. However, according to the Krogh–Moe theory [24,25], the structure of the borate glasses is not a random
201
distribution of BO3 triangles and BO4 tetrahedra, but the gathering of these units to form welldefined, stable borate groups (such as diborate, triborate, tetraborate. . .) that constitute the random three-dimensional network. On the other hand, scarce attention has been paid to analyse this effect on the local environments of the lanthanide ions. In this work a study of the optical properties of the Eu3þ ion in a calcium diborate glass has been carried out. The vibronic spectrum, in combination with the IR and Raman spectra, allows analysing the chemical bonds of the Eu3þ ions with the oxygen ions of the different borate groups. The analysis of the 5 D0 ! 7 F1 and 7 F2 emission spectra, obtained after exciting selectively the 5 D0 level for different Eu3þ concentrations, allows the characterization of the CF interaction and to calculate the CF parameters as a function of the excitation wavelength. The effects of the J -mixing are discussed and the conclusions have been applied to understand the line profile of the 7 F0 $ 5 D0 transitions.
2. Experimental The composition of the samples used in this work was (in mol%): (66:66 x=2) CaO, (33:33 x=2) B2 O3 and x Eu2 O3 , with x equals to 0.1 and 2.5. The details of the preparation has been given in a previous paper [26]. Broadband spectra were obtained by exciting the sample with light coming from a 250 W incandescent lamp passed through a 0.25 m monochromator. For FLN spectra a tuneable dye laser operating with Rhodamine 6G, pumped by a Q-switched 532 nm frequency-doubled Nd:YAG laser, was used. The laser spectral linewidth was 0.15 cm1 and the pulse width 5 ns. For time-resolved FLN measurements a phosphorimeter connected to the Qswitching control of the Nd:YAG laser was used. Fluorescence was detected through a 0.25 m double grating monochromator with a cooled photomultiplier using a photon counting technique. Spectra were corrected from instrument response. Fluorescence decays were recorded using a digital oscilloscope controlled by a computer. All
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measurements were made at 13 K with a closecycled helium cryostat.
CF analysis, then the even CF Hamiltonian takes the form [22] ð2Þ
ð2Þ
ð2Þ
ð4Þ
even ¼ B20 C0 þ B22 ðC2 þ C2 Þ þ B40 C0 HCF ð4Þ
ð4Þ
ð6Þ
The optical properties of lanthanides are due to the 4fN electrons. The free lanthanide ion Hamiltonian can be written as a sum of different interactions [22,27–29] X Hfree ion ¼ Ho þ F k fk þ f4f Aso k¼2;4;6
þ aLðL þ 1Þ þ bGðG2 Þ þ cGðR7 Þ X X þ T i ti þ M h mk i¼2;3;4;6;7;8
þ
X
f
P pf ;
ð4Þ
ð4Þ
þ B42 ðC2 þ C2 Þ þ B44 ðC4 þ C4 Þ
3. Theoretical background
h¼0;2;4
ð1Þ
f ¼2;4;6
where the main ones are the inter-electronic repulsion and the spin–orbit interactions, second and third terms respectively, which give rise to the 2Sþ1 LJ levels. The rest only make corrections to the energy of these levels without removing their degeneracy. When a lanthanide ion is immersed in a solid matrix, there is an electrostatic interaction of the 4f electrons with the charge of host ligands, known as the CF interaction, whose magnitude is small compared to the spin–orbit interaction. Thus, apart from the slight shift of the 2Sþ1 LJ multiplets, the main modifications introduced by the CF interaction are the appearance of the Stark levels, arising from the removal of the degeneracy of the 2Sþ1 LJ levels [22,27–29], and the electric dipole transitions, forbidden in the free ion [30]. While the former is due to the even components of the CF Hamiltonian the latter is due to the odd CF Hamiltonian, although both are intimately related to the symmetry of the local CF acting on the optically active ions. The CF Hamiltonian can be written as a sum of one-electron tensor operators in the Wybourne formalism [22,27,28]. As it will be discussed later, an orthorhombic point group symmetry (D2h , D2 or C2V ) has been generally assumed for the local structure of the Eu3þ ions in works devoted to the
ð6Þ
ð6Þ
þ B60 C0 þ B62 ðC2 þ C2 Þ ð6Þ
ð4Þ
ð6Þ
ð6Þ
þ B64 ðC4 þ C4 Þ þ B66 ðC6 þ C6 Þ;
ð2Þ
where Bk;q ¼ Bk;q are real. If the mixing of wave functions of a given J Stark state with those of the J 2, J 4 or J 6 closer states (J -mixing effect) is neglected, the energy positions of the 7 F1 Stark levels, after the diagonalization of the even CF Hamiltonian (2) using symmetrised functions for the 2Sþ1 LJ Stark levels in C2V symmetry, are given by [31] 1 EðA2 Þ ¼ E0 ð7 F1 Þ þ B20 ; 5
pffiffiffi 6 1 B22 ; EðB1 Þ ¼ E0 ð F1 Þ B20 þ 10 10 pffiffiffi 1 6 7 B22 EðB2 Þ ¼ E0 ð F1 Þ B20 10 10 7
ð3Þ
and for the 7 F2 Stark levels by [31] rffiffiffiffiffi 11 1 1 10 B20 B40 þ B44 ; 105 63 9 7 pffiffiffiffiffi 11 11 4 2 10 B20 þ pffiffiffi B22 þ B40 EðB1 Þ ¼ E0 ð7 F2 Þ þ B42 ; 210 63 63 35 6 pffiffiffiffiffi 11 11 4 2 10 B20 pffiffiffi B22 þ B40 þ EðB2 Þ ¼ E0 ð7 F2 Þ þ B42 ; 210 63 63 35 6 rffiffiffiffiffi 7 1 10 EðA1 ; A1 Þ ¼ E0 ð7 F2 Þ B40 B44 63 9 7 2 " rffiffiffiffiffi #2 1 22 5 1 10 4 B20 þ B40 B44 4 105 63 9 7
EðA2 Þ ¼ E0 ð7 F2 Þ
11 1 B22 þ2 105 21
rffiffiffi !2 31=2 5 B42 5 ; 3
ð4Þ 7
where E0 ð FJ Þ is the centre of gravity or barycentre of the 7 FJ multiplets of the free Eu3þ ion and each Stark level is labelled with an irreducible representation of the C2V point group. Finally, some authors [32,33] have tried to simplify the description of the CF defining a
V. Lavın et al. / Journal of Non-Crystalline Solids 319 (2003) 200–216
scalar, rotational invariant parameter called the CF strength. Using that given by Auzel and Malta [33], the CF strength for orthorhombic symmetries takes the form ffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi X 4p 2 2 k NV ðBq Þ ¼ jBk0 j þ 2jBkq j : 2k þ 1 k;q>0
ð5Þ
As can be observed from Eq. (3), if the J -mixing effect is neglected the maximum splitting of the 7 F1 level only depends on the B2q CF parameters [31] and, making use of the theoretical expression of Malta et al. [34], both magnitudes can be correlated as follows sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:3 DEMAX ð F1 Þ ¼ NV ðB2q Þ; pð2 þ a2 Þ 7
ð6Þ
where a is given by Eb Ec a¼ DEMAX =2
203
ð7Þ
and Eb is the barycentre of energy of the 7 F1 level, calculated as the mean energy of the corresponding three Stark levels, whereas Ec is the energy of the central Stark level. 4. Results 4.1. Broadband excitation Excitation and emission spectra of the Eu3þ ions in a calcium diborate glass doped with 2.5 mol% of Eu2 O3 , obtained under broadband excitation at 13 K, are given in Fig. 1. They consist in a set of inhomogeneously broadened bands, each
Fig. 1. Broadband excitation (– – –) and emission (––) spectra of a calcium diborate glass doped with 2.5 mol% of Eu2 O3 at 13 K. Transitions start from the 7 F0 ground level in the excitation spectrum, detecting the 5 D0 ! 7 F2 transition at 615 nm, and from the 5 D0 level in the emission one, exciting the 7 F0 ! 5 L6 transition at 395 nm. Partial energy level diagram of the free Eu3þ ion is also shown.
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one associated to a transition between different 2Sþ1 LJ multiplets within the 4f6 ground configuration. They start from the ground level 7 F0 for the excitation spectrum and from the low emitting level 5 D0 for the emission one. The intraconfigurational 4f6 –4f6 transitions of the Eu3þ ions were identified comparing the peak energies with the free Eu3þ ion energy level diagram (Fig. 1) [22,23,35]. The excitation spectrum is similar to the absorption one and the bands at 319.6 (31289 cm1 ) and 376.8 nm (26539 cm1 ) have been assigned to transitions to the 5 H4 and 5 G4 levels, respectively, although there is a great overlapping with other levels close in energy with the same L and S but different J quantum number [23]. The emission spectrum has been obtained exciting at 395 nm (25316 cm1 ) the 7 F0 ! 5 L6 transition and shows different broad bands corresponding to the 5 D0 ! 7 FJ (J ¼ 0–6) transitions. The absence of emission starting from the 5 DJ (J ¼ 1–3) levels is due to the high-energy phonons found in borate glasses, i.e. when the Eu3þ ions are excited to any level above the 5 D0 one there is a fast non-radiative multiphonon relaxation to this level. From the 5 D0 level the Eu3þ ions decay radiatively, since the large energy difference to the 7 F6 level prevents the possibility of multiphonon relaxation. Thus, its quantum emission efficiency is nearby to the unity. The 5 D0 ! 7 F1 transition is magnetic dipole in nature, it is allowed by all the selection rules and is independent on the composition of the host matrix [36,37] if the J -mixing is negligible [38]. The existence of three overlapped peaks for this transition is the first indication of the low symmetry (orthorhombic, triclinic or monoclinic) of the Eu3þ environments in this glass [22]. The 5 D0 ! 7 FJ (J ¼ 2; 4; 6) transitions are electric dipole in nature and are forced by the odd CF Hamiltonian. The other emission transitions observed in Fig. 1, 5 D0 ! 7 FJ (J ¼ 0; 3; 5), are strictly forbidden in the frame of the intermediate scheme of the Judd– Ofelt theory [30]. As will be discussed later, their low intensities are explained by the J -mixing, which induces an effective borrowing of intensity from the other electric dipole transitions [12,39]. In order to obtain more information about the local structure of the Eu3þ ions and the nature of
their bonds with the oxygens, the vibronic or phonon side band (PSB) spectrum, involving simultaneous vibrational and electronic transitions, has been measured. For the Eu3þ ions, those PSBs coupled to the 7 F0 ! 5 D2 pure electronic, or zero phonon, band (PEB) have been measured detecting the 5 D0 ! 7 F2 transition at 615 nm in a calcium diborate glass with 2.5 mol% of Eu2 O3 at 13 K (Fig. 2). Due to the rather low probability of multiphonon excitation compared to the excitation of a single phonon, the difference in energy between the PSB and the PEB corresponds to the energy of one phonon [40]. The maximum of the PSB spectrum in Fig. 2 corresponds to a vibrational mode coupled to the Eu3þ ions with energy of 1400 cm1 . It is worthy to note that the excitation spectrum of the 5 D0 level does not show any appreciable PSB associated to low-energy phonons. This information is complementary to that of the IR and Raman spectroscopic techniques, which only analyse the intrinsic vibrations of the matrix. However, bands observed by all these techniques show a general correspondence, although the relative intensities can vary drastically according to
Fig. 2. IR absorption and Raman scattering spectra (HH and HV) at RT and PSB(5 D2 ) spectrum at 13 K in a calcium diborate glass doped with 2.5 mol% of Eu2 O3 . For the latter, the mean energy of the 7 F0 ! 5 D2 electronic band has been taken as the zero of energy. Predominant structural units are indicated for each energy range. HH means parallel polarisations in excitation and detection while HV means perpendicular polarisations.
V. Lavın et al. / Journal of Non-Crystalline Solids 319 (2003) 200–216
the different selection rules of each one. Due to the disordered nature of the vitreous materials and the great numbers of possible borate groups, the interpretation of the vibronic spectra is not an easy task. However, it is generally accepted that those bands in the range from 1100 to 1500 cm1 corresponds to borate groups containing BO3 units, while for lower energies are associated to groups containing BO4 units [24,25,41]. The IR and the HH and HV Raman spectra for the glass under study are presented in Fig. 2. Bands associated to the BO3 triangles (1225–1250 cm1 ), to the BO4 tetrahedra (880–1050 cm1 ) and to the B–O–B bonds (690–770 cm1 ) are clearly observed [41].
205
of the existence of a large variety of environments with different local structures and CFs for the Eu3þ ions in calcium diborate glasses. The FLN emission spectra of the 5 D0 ! 7 FJ (J ¼ 1 and 2) transitions exciting selectively within the 7 F0 ! 5 D0 band is shown in Fig. 4 for a calcium diborate glass with 0.1 mol% of Eu2 O3 at 13 K. The 5 D0 ! 7 FJ (J ¼ 3–6) emissions have not been included because they are poorly resolved, show scarce dependence on the excitation wavelength or their intensities are low. Differences in the Eu3þ surroundings are clearly reflected in the FLN spectra, indicating the great sensitivity of the splitting of the 7 F1 and 7 F2 multiplets to variations of the CF from one
4.2. Selective excitation The inhomogeneous line profile associated to the 7 F0 ! 5 D0 transition, obtained detecting at 615 nm the 5 D0 ! 7 F2 transition in a calcium diborate glass doped with Eu2 O3 at 13 K, is shown in Fig. 3. The large full-width at half-maximum (FWHM ¼ 105 cm1 ) is due to overlapping of homogeneous 5 D0 ! 7 F0 spectra with slight different energies. This large FWHM is an indication
7
5
Fig. 3. Inhomogeneous excitation profile of the F0 ! D0 transition in a calcium diborate glass doped with 2.5 mol% of Eu2 O3 at 13 K. Lifetime of the 5 D0 level in calcium diborate glasses doped with 0.1 () or 2.5 (j) mol% of Eu2 O3 at 13 K as a function of the excitation wavelength. The uncertainty of the lifetimes determination is of about 1%.
Fig. 4. FLN emission spectra to the 7 FJ (J ¼ 1; 2) levels exciting selectively the 5 D0 level in a calcium diborate glass doped with 0.1 mol% of Eu2 O3 at 13 K. Spectra are normalised to the maximum of the high-energy peak of the 5 D0 ! 7 F1 transition. Excitation wavelength is indicated for each spectrum in nm and with vertical arrows. The upper spectrum corresponds to the broadband emission from the 5 D0 level.
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environment to another in the glass. Two main features can be observed in these spectra. On the one hand, the CF acting on the Eu3þ ions forces the full degeneracy of the free Eu3þ multiplets. This means that three peaks can be observed for the 5 D0 ! 7 F1 transition, associated to the three 7 F1 Stark levels, and partially resolved spectra suggesting five peaks for the 5 D0 ! 7 F2 transition are obtained, indicating the existence of five 7 F2 Stark levels. This result confirms the low symmetry of the Eu3þ environments previously observed in the broadband emission. On the other hand, there are progressive changes in the intensity and the energy position of the different emission components, especially for the highest energy component of the 5 D0 ! 7 F1 transition that shows a large shift (almost twice the shift of the excitation). Additionally, it is interesting to observe the dependence of the linewidth of the three components of the 5 D0 ! 7 F1 transition on the excitation wavelength. There is an important increase of the linewidth for the two low-energy components of this transition whereas the linewidth of the highenergy component does not change appreciably. This behaviour, shown in Fig. 5, will be analysed afterwards. On the other hand, when the concentration of Eu2 O3 is increased up to 2.5 mol%, more than three peaks can be observed in the 5 D0 ! 7 F1 emission spectra for short excitation wavelengths to the 5 D0 level. Moreover, the time-resolved FLN spectra, as shown in Fig. 6 for excitation at 576.55 nm, change with the delay after the laser pulse. When this delay is increased, the time-resolved spectrum approaches to the inhomogeneous emission spectrum. Finally, the lifetime of the 5 D0 level for the different environments has been also measured exciting selectively the samples doped with 0.1 and 2.5 mol% of Eu2 O3 . The results, shown in Fig. 3, are in the range from 1.6 to 2.1 ms whereas a mean value of 1.86 ms was obtained under broadband excitation. In order to assure the site selectivity in these measurements, the emission corresponding to the high-energy component of the 5 D0 ! 7 F1 transition was monitored. The values for the lifetime obtained exciting at the low-energy side of the 7 F0 ! 5 D0 peak are similar for both Eu3þ con-
Fig. 5. Linewidths of the three emission peaks of the 5 D0 ! 7 F1 transition, low (), medium (j) and high (d) energy, as a function of the difference in energy between each Stark level to next lower one. Curves correspond to the fit to a cubic dependency on the energy difference, see Eq. (8). The uncertainty of the linewidths determination is of about 15 cm1 .
Fig. 6. Time-resolved FLN emission spectra to the 7 FJ (J ¼ 0; 1) at 300 ls (––) and 5 ms (– – –) after the laser flash exciting at 576.55 nm the 5 D0 level in a calcium diborate glass doped with 2.5 mol% of Eu2 O3 at 13 K. Spectra are normalised to the maximum of the high-energy peak of the 5 D0 ! 7 F1 transition. Broadband emission (- - -) from the 5 D0 level is also included for comparison.
centrations, whereas they are shorter for the most doped glass when excitation moves towards the high-energy side of this peak. The changes in the 5 D0 ! 7 F1 emission spectra and the 5 D0 level lifetime with the Eu2 O3 concen-
V. Lavın et al. / Journal of Non-Crystalline Solids 319 (2003) 200–216
tration can be explained considering phononassisted energy migration processes from Eu3þ ions in strong CF environments to ions in weak CF ones, analysed by the authors in the same borate glass [26]. These energy transfer processes increase with the doping concentration and give rise to a decrease of the lifetime. Thus, the migration of energy between Eu3þ ions can be considered negligible for the 0.1 mol% doped glass and the identification of the three 7 F1 Stark levels is straightforward.
5. Discussion 5.1. Broad band excitation From the spectra presented in Fig. 1 it has been possible to obtain the energy level diagram of the Eu3þ ion in the calcium diborate glass up to 31 300 cm1 and to calculate the atomic parameters of the free ion Hamiltonian, using Eq. (1), in the 4f6 electronic configuration in the intermediate coupling scheme. There are several experimental levels but they are not enough to allow all the atomic parameters to vary freely in the fitting procedure. Assuming the hydrogenic approximation [27] only the Slater parameter F2 and the spin–orbit parameter f4f are calculated and the rest of parameters are fixed to the values obtained by Jayasankar et al. [42] for the well known LaF3 :Eu3þ crystal. The values obtained for F 2 and f4f were 86 734 and 1329 cm1 , respectively. The experimental and calculated 2Sþ1 LJ energy levels for the Eu3þ ion in the calcium diborate glasses are presented in Table 1. The simulation is quite reasonable and there are only large differences in a few multiplets. The magnitudes of the atomic parameters are similar to those found in crystals and aqueous matrices [38]. This feature is a consequence of the relatively low influence that the local environment exerts on the energy positions of the 2Sþ1 LJ multiplets and it confirms that the inter-electronic and the spin–orbit interactions are the main responsible for the energy level diagram of the Eu3þ ion. Concerning the local structure of the Eu3þ ions in the glass, a first consideration can be made
207
Table 1 Experimental and calculated 2Sþ1 LJ multiplets (in cm1 ) of the Eu3þ ion in a calcium diborate glass 2Sþ1 7
LJ levels
F0 7 F1 7 F2 7 F3 7 F4 7 F5 7 F6 5 D0 5 D1 5 D2 5 D3 5 L6 5 L7 , 5 G2 5 GJ (J ¼ 3; 4; 5; 6) 5 D4 5 H4
Experimental levels
Calculated levels
0 409 1041 1987 2912 3878 5031 17 301 19 008 21 450 24 396 25 316 26 247 26 539
40 416 1076 1925 2893 3931 5004 17 253 18 966 21 419 24 310 25 353 26 392 26 527
27 586 31 289
27 583
Calculations have been carried out only varying the Slater and spin–orbit parameters, while the other atomic parameters were fixed. F 4 ¼ 0:668 F 2 , F 6 ¼ 0:4943 F 2 , a ¼ 16:82, b ¼ 630, c ¼ 1750, T 2 ¼ 370, T 3 ¼ 40, T 4 ¼ 40, T 6 ¼ 330, T 7 ¼ 380, T 8 ¼ 370, M 0 ¼ 2:38, M 2 ¼ 0:56 M 0 , M 4 ¼ 0:38 M 0 , P 2 ¼ 245, P 4 ¼ 0:75 P 2 , P 6 ¼ 0:5 P 2 . (All values in cm1 .) [42]. Levels with asterisk were not used in the fit. The uncertainty of the experimental levels determination is of about 15 cm1 . The rms of the calculated levels is 60 cm1 .
taking into account the electric 5 D0 ! 7 F2 to the magnetic 5 D0 ! 7 F1 intensities ratio (EMIR). This ratio is considered a measurement of how close the environment of the Eu3þ ion is to a centrosymmetric symmetry [43], since the electric dipole transitions are strictly forbidden when the odd CF Hamiltonian is null. From the emission spectrum an EMIR of 4.5 has been obtained for the calcium diborate glass. It is in the range of values found in other oxide glasses [18,37,43] and is much higher than those found in fluoride ones [8,21]. Another consequence of the high probability of the electric dipole transitions is that the lifetime of the 5 D0 level is relatively low, i.e., 1.86 ms in the calcium diborate glass. According to the Judd approach for the vibronically induced forced electric dipole transition [44], the intensity of the PSB shows an R6 dependence on the lanthanide to ligand distance, indicating that the PSBs are dominated by the ligands forming the immediate environment of the
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Eu3þ ions. The PSB spectrum in Fig. 2 shows a range of energy for the phonons up 1600 cm1 , with the maximum about 1400 cm1 , corresponding to different Eu3þ -ligand vibrational modes. This spectrum can include different contributions of the vibrational modes of the BO3 triangles: the boron-bridging oxygen bonds inside the various borate rings (1350–1400 cm1 ) and those from the boron-non-bridging oxygens bonds (1420–1550 cm1 ) associated to the great borate groups [41]. The IR-active band centred at 1200 cm1 , associated to the chains found in the metaborate group [25], shows a lower contribution to the PSB. In the range from 850 to 1100 cm1 the contribution of the vibrational modes of the BO4 units present in diborate and tetraborate groups is also weak. The overall PSB spectrum is similar, although somewhat broader, to that found by Tanabe et al. [45] in a sodium diborate glass. As it can be observed, not all the borate groups present in the glass take part in the formation of the local structure around the Eu3þ ions. Although Konijnendijk [25] suggests that the structure of these glasses is formed principally from diborate groups, it is clearly observed that there is not a predominant bond of the Eu3þ ion with the bridging oxygen of these groups. On the contrary, the presence of bridging and non-bridging oxygen of borate groups with BO3 units with great tendency to co-ordinate with the Eu3þ ions give rise to a much lower relative electron–phonon coupling factor to the BO4 units of the diborate groups than expected. Tanabe et al. [45] have invoked the difference in the affinity of the different borate groups with the Eu3þ ions and their selectivity in the glass to explain it. However, this feature could be associated to the high rigidity of the BO4 units of the diborate groups, which have more sharing points with other polyhedra than those borate groups with BO3 triangles [41,46,47]. 5.2. Selective excitation 5.2.1. Broadening mechanism of the 5 D0 ! 7 F1 line profile Three 7 F1 Stark levels are obtained from the FLN emission spectra of the 0.1 mol% of Eu2 O3 doped glass. However, as it has been pointed out,
for this sample the low-energy components of the 5 D0 ! 7 F1 transition show an important broadening of their linewidth with the excitation wavelength as the splitting of the 7 F1 multiplet increases. This broadening could be explained by the existence of non-radiative de-excitation processes with the emission of low-energy acoustic phonons due to the interactions of the 4f electrons with the matrix. These processes diminish the lifetimes of the Stark levels and, consequently, increase their linewidths [29]. In order to analyse this behaviour, the linewidth of each of these three peaks is plotted in Fig. 5 as a function of the energy separation between each 7 F1 Stark level and its immediately lower one. Assuming the Debye approximation for the phonon spectra in the glass, the linewidth of each Stark level would increase with the third power of the separation in energy between two consecutive Stark levels, in accordance with the expression for the transition probability [29]
3 3 DE W ðaJ ; bJ 0 Þ ¼ jhaJ jHcc–din jbJ 0 ij2 2pqv5 h h ½nðDEÞ þ 1;
ð8Þ
where q is the density of the glass, v is the mean velocity of sound, DE is the difference in energy between two consecutive levels, nðDEÞ is the density of phonons of energy DE and Hcc–din is the dynamic CF Hamiltonian [3]. The fit of the linewidths to Eq. (8) is shown in Fig. 5 as a function of the difference in energy between two consecutive levels. The fit includes an energy independent contribution to the linewidth that recovers different contributions such as the multiphonon, the inhomogeneous and/or the instrumental broadenings. However, the non-radiative rate from the lowest-energy 7 F1 Stark level is relatively weak and does not contribute appreciably to the linewidth of this peak. This result may be associated to the matrix element of the dynamic CF Hamiltonian in Eq. (8). Thus, the good fit observed for the change in linewidth of these peaks could be explained adequately considering the interaction of the Eu3þ ions with a distribution of phonons described by the Debye approximation. This approximation has been also invoked to
V. Lavın et al. / Journal of Non-Crystalline Solids 319 (2003) 200–216
explain the spectral widths observed in crystalline host lattices [48]. 5.2.2. Crystal-field analysis From the splitting observed in the emission transitions to the 7 F1 and 7 F2 multiplets it is possible to carry out a description of the CF acting on the Eu3þ ions in the calcium diborate glasses. This analysis is made assuming a local symmetry different to the trivial one C1 , for which at least 13 CF parameters are necessary to define the CF Hamiltonian, while only eight Stark levels could be available. The most extended choice is to assume a C2V symmetry for the local environments of the Eu3þ ions in the glass [6,8,10–13,15,20,21]. This is because it is a subgroup of almost all the higherorder point groups, it allows optical activity from
Fig. 7. Gaussian de-convolution of the 5 D0 ! 7 F2 emission profiles exciting selectively the 5 D0 level at 576, 577.5 and 579 nm in a calcium diborate glass doped with 0.1 mol% of Eu2 O3 at 13 K. Sums of the five gaussian functions are included.
209
the 5 D0 level to practically all the 7 FJ Stark levels and it is the highest order non-centrosymmetric point group that breaks completely the degeneracy of the 7 FJ multiplets [6]. To determine the components of the 5 D0 ! 7 F2 emission spectra in Fig. 4, a de-convolution of the spectra in five Gaussian line-shape functions has been carried out. Progressive changes in the energy positions and relative intensities have been assumed in the de-convolution process, since there are neither crosses between components nor relevant changes in their intensities from each spectrum to the following one, as already suggested from the FLN spectra. The obtained Gaussian functions are shown in Fig. 7 for the spectra obtained exciting at 576, 577.5 and 579 nm. The
Fig. 8. 7 F1 and 7 F2 Stark levels of the Eu3þ ions respect to the 7 F0 ground level in a calcium diborate glass as a function of the excitation wavelength. Solid lines indicate calculated Stark levels. The uncertainty of the 7 F1 and 7 F2 Stark levels determination is of about 10 and 20 cm1 , respectively. The rms of the calculated Stark levels is 5 cm1 .
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Table 2 Experimental and calculated 7 F1 and 7 F2 Stark levels (in cm1 ) of the Eu3þ ions in a calcium diborate glass as a function of the excitation wavelength to the 5 D0 level at 13 K Excitation wavelength (nm) 575 575.5 576 576.5 577 577.5 578 578.5 579 579.5
Obs. Calc. Obs Calc. Obs. Calc. Obs. Calc. Obs Calc. Obs Calc. Obs. Calc. Obs. Calc. Obs. Calc. Obs. Calc.
7
7
F1
F2
B1
A2
B2
A1
B2
A2
A1
B1
137 141 143 144 151 152 160 163 176 178 189 191 209 211 223 227 237 241 257 260
389 383 388 388 382 389 379 386 375 382 376 384 371 377 367 366 364 363 360 355
702 701 684 687 657 659 639 640 616 617 593 598 569 577 540 551 511 523 488 499
883 886 890 891 900 897 906 903 916 913 919 917 919 918 918 921 920 924 919 927
1017 1021 1011 1012 1000 997 986 982 976 972 986 980 980 978 976 976 974 976 967 971
1200 1200 1174 1174 1141 1140 1112 1112 1089 1089 1073 1072 1059 1057 1044 1039 1027 1022 1014 1007
1312 1311 1279 1279 1232 1232 1210 1211 1187 1186 1166 1164 1146 1144 1137 1129 1111 1107 1097 1090
1432 1432 1407 1407 1380 1380 1357 1356 1321 1321 1297 1297 1266 1263 1247 1246 1223 1222 1208 1209
The 7 F0 Stark level has been taken as the zero of energy. The assignments of the irreducible representations correspond to the C2V symmetry. The uncertainty of the 7 F1 and 7 F2 Stark levels determination is of about 10 and 20 cm1 , respectively.
energy positions of the experimental 7 F0 , 7 F1 and 7 F2 Stark levels are shown in Fig. 8 and Table 2 as a function of the excitation wavelength. The analysis of the CF begins considering the splitting of the 7 F1 multiplet. In a first approximation, if the J -mixing is neglected only the second rank CF parameters, B20 and B22 , of the even CF Hamiltonian, Eq. (2), will affect significantly to the breakdown of the degeneracy of the 7 F1 multiplet into three Stark levels [22]. The irreducible representations of the 7 F1 and 7 F2 Stark levels are given in Eqs. (3) and (4) [31]. As can be observed from Eq. (3), the sign and magnitude of the B20 parameter determine the relative position of the A2 Stark level with respect to the barycentre of the other two Stark levels. On the other hand, the sign of the B22 parameter determines the relative positions of the B1 and B2 Stark levels and, hence, its magnitude will be proportional to the difference in energy between these two levels. It is interesting to note that if B20 is low enough, or zero, there will be three equally spaced Stark levels.
Due to the almost symmetrical splitting observed for the three 7 F1 Stark levels with the increase of the CF, the B22 CF parameter presents a large variation in magnitude, while the B20 axial parameter is rather low and will not affect significantly the splitting [22,34,49]. The irreducible representation assigned to the central Stark level will be A2 , while the others can be ambiguously assigned to the B1 and B2 representations. However, care must be taken in the assignations since for orthorhombic symmetries six different combinations of B2q CF parameters can be obtained from the different assignations possible for the three 7 F1 Stark levels that give the same splitting [28,50]. The B2q CF parameter calculated from Eq. (3) are shown in Table 3 (in parentheses). The second rank CF strength parameter NV ðB2q Þ is shown in Fig. 9 as a function of the experimental maximum splitting of the 7 F1 manifold, DEMAX (7 F1 ), obtained from Table 2. Its values are between the largest ones found in glasses, only overcome by silicate [6] and borosilicate
V. Lavın et al. / Journal of Non-Crystalline Solids 319 (2003) 200–216 Table 3 Second and fourth rank CF parameters (in cm1 ) considering the J -mixing as a function of the excitation wavelength to the 5 D0 level Excitation wavelength (nm)
B20
B22
B40
B42
575
)49 ()102) )6 ()83) 35 ()86) 40 ()70) 35 ()67) 34 ()54) 0 ()60) )15 ()47) )16 ()34) )26 ()40)
1398 (1155)
1308
)600
48
1329 (1105) 1205 (1048) 1125 (979) 1010 (902) 917 (825) 803 (736) 716 (647) 614 (560) 522 (471)
1287 1254 1160 1076 1075 1028 1004 966 967
)551 )519 )524 )480 )380 )336 )358 )357 )360
17 )12 )105 )145 )150 )158 )225 )239 )300
575.5 576 576.5 577 577.5 578 578.5 579 579.5
B44
Values in parentheses have been calculated neglecting the J -mixing. The uncertainty of the CF parameters determination is of about 10 and 20 cm1 for the second and fourth rank
Fig. 9. CF strength parameter NV ðB2q Þ, with (j) and without () taking into account the J -mixing, as a function of the maximum splitting observed for the 7 F1 level of the Eu3þ ions in a calcium diborate glass. The solid line indicates the fit to the theoretical expression of Malta et al. [34]. The uncertainty of the CF strength parameter determination is of about 20 cm1 .
[18] glasses. Another consequence of the almost symmetrical splitting of the 7 F1 Stark levels observed in this glass is that the factor a in Eqs. (6) and (7) is rather low. Thus, the proportionality factor between the CF strength and the maximum splitting of the 7 F1 level is practically a constant of value 0.218 for all the environments, giving rise to
211
the linear dependence shown in Fig. 9. This result is rather similar to that obtained for fluoride glasses [38] and closer to 0.213 obtained for oxide crystals [34]. In order to give more confidence to the assignations of the irreducible representations of the 7 F1 Stark levels in C2V symmetry one may observe that the CF strength for an ideal symmetrical splitting (a ¼ 0), Eq. (6), rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pð2 þ a2 Þ DEMAX ð7 F1 Þ NV ðB2q Þ ¼ 0:3 1 DEMAX ð7 F1 Þ ð9Þ ¼ 0:218 and the maximum splitting of the 7 F1 multiplet obtained with the B22 CF parameter from Eq. (3), assuming no J -mixing, pffiffiffi 2 6 DEMAX ð7 F1 Þ ¼ B22 ffi 0:218NV ðB2q Þ ð10Þ 10 gives rise to the same relation. Although the agreement between experimental and calculated Stark levels is quite good, the CF interaction in the calcium diborate glasses is so strong that it is necessary to analyse the influence of the J -mixing. This is particularly important for those Eu3þ ions in environments under strong CF, for which the 7 F1 Stark wave functions are mixed with the 7 F2 and 7 F3 ones through the second rank CF Hamiltonian. In order to take into account this interaction in the splitting of the 7 F1 manifold, the complete Hamiltonian (free-ion plus the second rank CF Hamiltonian) has been diagonalised taking as a base linear combinations of jJMi wave functions of the 7 FJ multiplets in the C2V symmetry [31]. The fitting program [51] minimises the sum of square residuals between the experimental and calculated Stark levels. The B2q CF parameters calculated neglecting J -mixing have been taken as initial values. From the calculated parameters the CF strength NV ðB2q Þ is obtained and represented in Fig. 9 as a function of the maximum splitting of the 7 F1 multiplet. As can be clearly observed, when J -mixing is assumed the energy contributions to the 7 F1 Stark levels give rise to larger values for the CF strength to explain the same experimental
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splitting. For Eu3þ ions excited at 575 nm the CF strength parameter for these environments increases in about 20%, compared to that obtained neglecting the J -mixing effects. Thus, from the comparison of both sets of values for the CF strength a measurement of the influence of the J mixing has been obtained. Taking the J -mixing into account G€ orllerWalrand and Binnemans [22] have re-defined the Ôweak CFÕ (J -mixing negligible) and Ôstrong CFÕ (J mixing unnegligible) for the lanthanide systems. According to this definition in the calcium diborate glasses it can be obtained a distribution of environments under weak (with maximum 7 F1 splitting DEMAX ð7 F1 Þ < 300 cm1 ), medium (300
energy levels are presented in Fig. 8 (solid lines) and Table 2, showing a rather good agreement with the experimental ones. The calculated second and fourth rank even CF parameters are shown in Table 3 and Fig. 10. They show large differences from one environment to another. It must be noted that the signs of the B20 , B40 and B44 parameters are determined from the chosen assignation of irreducible representations, while those of the B22 and B42 are arbitrary, depending on the interchange of the B1 and B2 irreducible representations. Results indicate that a great percentage of the splitting of the 7 F2 multiplet is due to the B22 parameter, while the contribution of the B4q parameters is less important. This is not surprising since the splittings are similar for the 5 D0 ! 7 F1 and 7 F2 transitions (see Figs. 4 and 8). Moreover, the B2q parameters do not differ from those obtained from the analysis of the 7 F1 Stark splitting assuming J -mixing. Thus, it seems that although the B4q are large in magnitude, they cancel each other in such a way that their contribution to the splitting is not significant. This conclusion seems to be right for all the Stark levels except for the two higher energy 7 F2 ones. With the atomic parameters and the second and fourth rank CF parameters it is possible to simulate the energy level diagram of the 7 FJ Stark levels
Fig. 10. CF parameters versus the excitation wavelength within the 7 F0 ! 5 D0 peak.
V. Lavın et al. / Journal of Non-Crystalline Solids 319 (2003) 200–216
213
of the Eu3þ ions in all the environments possible in the calcium diborate glasses. Although the sixth rank CF parameters cannot be calculated with the experimental levels available, the confidence of the simulation is reasonable since it can be assumed that the influence of CF parameters B6q is only appreciably in the strong CF environments. One of the most interesting consequences of the influence of the J -mixing for strong CF environments in calcium diborate glasses is the increase of the electric dipole contribution to the 5 D0 ! 7 F1 magnetic dipole transition. For excitation at 575 nm the wave functions of the three 7 F1 Stark levels are j7 F1 ðB1 Þi ffi 98%j7 F1M i þ 2%j7 F3M i j7 F1 ðA2 Þi ffi 90%j7 F1M i þ 8%j7 F2M i þ 2%j7 F3M i
Fig. 11. CF strength NV ðB2q Þ as a function of the excitation wavelength to the 5 D0 level for calcium diborate ðjÞ, sodium silicate ðÞ [6], fluorozirconate ðdÞ [21] and lithium fluoroborate ð Þ glasses [50].
j7 F1 ðB2 Þi ffi 64%j7 F1M i þ 28%j7 F2M i þ 8%j7 F3M i: ð11Þ Thus, the non-dependency of the 5 D0 !7 F1 magnetic dipole transition with the host matrix must be assumed with caution for those Eu3þ environments under strong CF. All these results seem to indicate that there is an unique class of site for the Eu3þ ions present in the calcium diborate glass. This site distribution ranges from weak to strong CF and could be interpreted as continuous variation or distortion of a typical eightfold co-ordinated oxide geometry such as the square antiprism, the triangled dodecahedron or the biccaped trigonal prism [6,8,17]. Finally, in order to compare with results for other glasses, the CF strength parameter, calculated with the second rank CF parameters obtained from the 7 F1 Stark levels considering J mixing, is presented in Fig. 11 versus the excitation wavelength to the 5 D0 level for the diborate and other Eu3þ doped oxide, fluoride and oxyfluoride glasses [6,21,50]. Differences in the environment distribution width (excitation wavelength range) and in the CF strength parameter with the excitation wavelength indicate the influence of the glass composition in the local structure of lanthanides in glasses. Although the influence of the modifier ions is relevant in the optical properties
and, therefore, in the local structure of the Eu3þ ions, some conclusions can be stated for each family of oxide, fluoride or oxyfluoride glasses. For the diborate and silicate oxide glasses a large and close to linear dependency, with the same slope, is found as CF strength parameter increases with the excitation energy. The fluoride glass shows smaller CF strengths and a sharper distribution of environments for the Eu3þ ions. Whereas in the fluoroborate glass an intermediate behaviour for the local structure is observed, since the CF strength values remain lower than those found in oxide glasses but larger than in fluoride ones. Moreover, for a given value of CF strength the excitation wavelength is different for each glass family. From Fig. 11 it can be observed that the pure oxide glasses are red-shifted compared to the pure fluoride glass. According to the nephelauxetic effect, this feature would indicate a higher covalency of the Eu3þ –O2 bonds of the borate groups present in the glass compared to the others. As a conclusion, the lanthanide ion in oxide glasses acts as a glass-modifier, competing with the network formers to satisfy its bonding requirements of charge neutrality. This feature gives rise to a larger variation of the CF and a broader distribution of environments, built with oxygen ions, for the lanthanide ion in these glasses.
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5.2.3. 7 F0 $ 5 D0 transitions The variation observed for the CF strength could also explain the intensity and asymmetry observed in the excitation and emission line profiles of the 5 D0 $ 7 F0 transitions (see Figs. 1 and 3). The higher the CF strength the larger the influence of the J -mixing, i.e., for excitation at 575 nm the wave function of the ground level is j7 F0 i ffi 88%j7 F00 i þ 6%j7 F22 i þ 6%j7 F2–2 i:
ð12Þ
Moreover, there is also an increase in the magnitude of the odd CF parameters. Thus, low concentration of Eu3þ ions in strong CF environments produces an appreciable intensity of the 5 D0 $ 7 F0 electric dipole transitions. The same results were obtained by Nishimura and Kushida [12] and Tanaka et al. [39] for calcium phosphate glasses, and they described the effect as an effective borrowing of intensity from the hypersensitive transition. The large probability of the 5 D0 ! 7 F2 hypersensitive transition supports this hypothesis. However, these changes do not affect appreciably the lifetime of the 5 D0 levels of the Eu3þ ions in different environments, since the contribution of the 5 D0 ! 7 F0 transition to the total decay rate is rather low. The proximity of the 7 F1 and 7 F2 multiplets compared to the energy separation between the 5 D0 and the 5 D2 multiplets have suggested to Nishimura and Kushida [12] that the inhomogeneous broadening is mainly due to the downward energy shift of the 7 F0 level induced by the J -mixing. If only the second rank CF Hamiltonian contribution is considered, the first-order perturbation theory gives an approximate, but realistic, idea of the energy shift obtained and how the J -mixing acts 4 ðjB20 j2 þ 2jB22 j2 Þ 75DE20 1 N 2 ðB2q Þ; ¼ E0 ð7 F0 Þ 15pDE20 V
EðA1 Þ ¼ E0 ð7 F0 Þ
ð13Þ
where E0 (7 F0 ) is the energy of the 7 F0 level of the free Eu3þ ion and DEJJ 0 is the energy difference between the 7 FJ and the 7 FJ 0 multiplets. The energy shift of the 7 F0 level with the CF strength parameters calculated from the last term
of Eq. (13) is in good agreement with the energy variation corresponding to the excitation wavelength. So, when the excitation wavelength goes from 579.5 to 575 nm, the corresponding energy variation is 135 cm1 whereas the energy shift calculated from Eq. (13) is about 150 cm1 . It is clearly observed how the shift increases for environments with higher CF strength. This energy shift may have also contributions from the B4q CF parameters, but they can be ignored if one realise that, applying the first-order perturbation theory, the energy difference between the 7 F4 Stark levels and the ground one is too large (2900 cm1 ). Thus it is principally induced by the B22 parameter.
6. Summary and conclusions The Eu3þ ions cannot be introduced in the simple B2 O3 glass in the absence of modifier cations, since the B2 O3 network is strongly bonded by bridging oxygens. However, if a modifier cation such as Ca2þ is present the network is broken and the non-bridging oxygens appear. In the vicinity of the breakdown the Eu3þ ions could be incorporated as a modifier replacing the Ca2þ cations, since both have a similar ionic radius. Within the Krogh–Moe theory the Eu3þ modifier cations demand and obtain from the network a particular sphere of coordination to reach the charge neutrality, which depends on the structural borate groups present in the matrix and their flexibility. The Eu3þ ions predominantly bond with the bridging and non-bridging oxygens of the BO3 units. These oxygens will form the first coordination sphere, adding a local charge that will balance the þ3 charge of the Eu3þ ions, while the boron and calcium ions will incorporate in the successive spheres, balancing the excess of charge of the oxygen ions. The flexibility shown by the BO3 triangles compared to the BO4 tetrahedra allows to the Eu3þ ion to modify the local environment in order to stabilize it and to achieve the charge neutrality with coordination 8, typical for lanthanides in oxides due to the ionic radius of the oxygen and the Eu3þ ions and to the free orbitals of the latter.
V. Lavın et al. / Journal of Non-Crystalline Solids 319 (2003) 200–216
These features generate a great variety of environments for the Eu3þ ions with different distances and bond angles with the ligand oxygens, giving rise to different CFs and, as a consequence, to small variations in the energy level scheme and the optical properties of the Eu3þ ions. The results obtained in this work allow us to suppose that, although the Eu3þ ions are immersed in an amorphous matrix, some degree of local order persists in the vicinity of these ions. Moreover, it seems that there is a unique class of site for all the distribution of environments present in the calcium diborate glass. In this sense, the eight coordinated environments could be interpreted as continuous distortion of a typical geometry such as the square antiprism, the triangled dodecahedron or the biccaped trigonal prism. From the experimental 2Sþ1 LJ energy levels, the atomic parameters have been calculated and the energy level diagram simulated from these parameter is in good agreement with the experimental one. Moreover, from the splitting of the 7 F1 and 7 F2 multiplets, the second and fourth CF parameters can be calculated as functions of the excitation wavelength. The Stark energy levels obtained from these CF parameters are also in good agreement with the experimental data. A large range of CF strength parameter NV ðB2q Þ values corresponding to a broad site distribution for the Eu3þ ions ranging from weak to strong CF environments is found. An important mixing of the 7 F0 and 7 F1 Stark levels with the 7 F2 ones is found for those Eu3þ ions in strong CF environments. From the analysis of the 7 F1 splitting, a quantitative measure of the J -mixing influence in the 7 F1 maximum splitting for the Eu3þ ions in the different environments have been obtained. As an example, exciting at 575 nm and taking into account the J -mixing in the CF analysis, the obtained CF strength parameter NV ðB2q Þ is 20% larger than without J -mixing. Moreover, for the Eu3þ ions excited at 575 nm an important J -mixing is found, the 7 F0 level is mixed with 12% of the 7 F2 level and the 7 F1 Stark levels are mixed with up to 28% of the 7 F2 level. The J -mixing effect is also reflected in the transition probabilities from the 5 D0 to the 7 FJ levels and in the line profile of the 7 F0 $ 5 D0
215
transitions. Moreover, an important energy shift of the 7 F0 level is observed, which depends on the excitation wavelength and can be explained considering J -mixing. Finally, an increasing of the 5 D0 ! 7 F1 emission linewidths with the splitting of the 7 F1 Stark components is observed, which can be well explained assuming non-radiative de-excitation processes due to interaction of the Eu3þ ions with a Debye phonon distribution. Acknowledgements The authors are indebted to Dr P. Nu~ nez (Dpto. de Quımica Inorganica, Univ. La Laguna) for providing the samples studied in this work and the IR spectrum and to Dr R. Alcala (Dpto. de Fısica de la Materia Condensada, Univ. Zaragoza) for providing the Raman spectra. This research was partially supported by ÔGobierno Aut onomo de CanariasÕ (PI 2001/048) and ÔMinisterio de Ciencia y TecnologıaÕ (MAT2001-3363). References [1] A.A. Kaminskii, Ann. Phys. Fr. 16 (1991) 639. [2] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. [3] G.F. Imbusch, Phys. Scr. T 19 (1987) 354. [4] M.J. Weber, in: Laser Spectroscopy of Solids, Springer, Berlin, 1986, p. 189. [5] N. Motegui, S. Shionoya, J. Lumin. 8 (1973) 1. [6] C. Brecher, L.A. Riseberg, Phys. Rev. B 13 (1976) 81. [7] M.J. Weber, J. Hegarty, D.H. Blackburn, in: Borate Glasses. Structure, Properties and Applications, Plenum, New York, 1978, p. 215. [8] C. Brecher, L.A. Riseberg, Phys. Rev. B 21 (1980) 2607. [9] J.L. Adam, V. Poncßon, J. Lucas, G. Boulon, J. Non-Cryst. Solids 91 (1987) 191. [10] T.F. Belliveau, D.J. Simkin, J. Non-Cryst. Solids 110 (1989) 127. [11] J. Dexpert-Ghys, B. Piriou, N. Jacquet-Francillon, C. Sombret, J. Non-Cryst. Solids 125 (1990) 117. [12] G. Nishimura, T. Kushida, J. Phys. Soc. Jpn. 60 (1991) 683; J. Phys. Soc. Jpn. 60 (1991) 695. [13] R. Balda, J. Fernandez, H. Eilers, W.M. Yen, J. Lumin. 59 (1994) 81. [14] V. Lavın, V.D. Rodrıguez, I.R. Martın, U.R. RodrıguezMendoza, I. Szab o, Z.E. Razstovits, J. Appl. Spectrosc. 62 (1995) 185.
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