Optical spectroscopy, thermal quenching and electron–vibrational interaction of the octahedrally coordinated Eu2+ ion in CaAl2B2O7 and BaAl2B2O7

Optical Materials xxx (2014) xxx–xxx

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Optical Materials journal homepage: www.elsevier.com/locate/optmat

Optical spectroscopy, thermal quenching and electron–vibrational interaction of the octahedrally coordinated Eu2+ ion in CaAl2B2O7 and BaAl2B2O7 S.J. Camardello a,⇑, P.J. Toscano b, M.G. Brik c, A.M. Srivastava a a b c

GE Global Research, One Research Circle, Niskayuna, NY 12309, United States University of Albany, 1400 Washington Avenue, Albany, NY 12222, United States Institute of Physics, University of Tartu, Ravila 14C, Tartu 50411, Estonia

a r t i c l e

i n f o

Article history: Received 10 April 2014 Received in revised form 7 June 2014 Accepted 17 June 2014 Available online xxxx Keywords: Eu2+ Six-fold coordination Thermal quenching Electron–vibrational interaction

a b s t r a c t The spectroscopic properties of the six-fold coordinated Eu2+ in CaAl2B2O7 and BaAl2B2O7 are investigated and reported in this paper. Relevant optical parameters such as the Stokes shift, crystal-field splitting, the centroid shift and the red shift of the Eu2+ 4f65d1 electronic configuration are calculated from the experimental data. Further, the Huang–Rhys factor and the effective phonon energy, which are responsible for the electron–vibrational interaction, are theoretically estimated for the Eu2+ ion in CaAl2B2O7 and BaAl2 B2O7. The low quenching temperature of the luminescence in these materials is tentatively connected with the thermally induced promotion of the Eu2+ 4f65d1 electron to the host lattice conduction band (photoionization). Evidence for the limited solubility of Eu2+ in BaAl2B2O7 is presented. A comparative study of the Eu2+ luminescence in M2+Al2B2O7 (M2+ = Ca2+, Sr2+, Ba2+) family of materials is also presented. Ó 2014 Published by Elsevier B.V.

1. Introduction Reports on the spectroscopic properties of the divalent europium ion (Eu2+) in a six-fold octahedral coordination are available in the literature [1,2]. Fig. 1 exhibits (schematically) the energy level structure of the Eu2+ ion in an octahedral coordination. The ground state of the Eu2+ (4f7) ion is the spherically symmetrical 8 S7/2. The octahedral crystal field splits the Eu2+ 4f65d1 configuration into the lower t2g (triply degenerate, dxy, dzx, and dyz orbitals) and the upper eg levels (doubly degenerate, dx2  y2 and dz2 orbitals), respectively. The total crystal field splitting is denoted by 10Dq. Depending on the strength of the crystal field, the excitation/absorption spectrum is characterized by two broad and well-isolated bands. The low and high-energy bands are associated with the 4f7[8S7/2] ? 4f65d1[t2g] and the 4f7[8S7/2] ? 4f65d1[eg] optical transitions, respectively. Frequently, the low energy excitation band exhibits the characteristic ‘‘staircase’’ spectrum, which retains the character of the seven (Eu3+) 4f6 levels (7F0–6) [3,4]. The staircase spectrum is due to the transition from the Eu2+ 8S7/2 ground state to the seven 7FJ (J = 0–6) multiplets of the excited 4f6 [7FJ(=0–6)]5d1 electronic configuration.

⇑ Corresponding author. E-mail address: [email protected] (S.J. Camardello).

The complicated energy level scheme of the Eu2+ 4f65d1 electronic configuration [3] only readily permits the determination of the total crystal-field splitting and the centroid shift (ec) in crystal structures where the local site symmetry is that of an octahedron. Most of our knowledge on the variation of these parameters with the crystal composition is restricted to lattices with octahedral, cubic and cuboctahedral site symmetries [2]. The goal of our work is to extend the knowledge of the spectroscopic properties of the Eu2+ ion when it occupies a well-defined six-coordinated octahedral site in various solids and to present a cross-cutting comparative study of the experimental results. In this respect we have reported on the spectroscopic properties of the octahedrally coordinated Eu2+ ion in Cs2M2+P2O7 (M2+ = Ca2+, Sr2+) [5] and more recently in SrAl2B2O7 [6]. In this paper we evaluate the spectroscopic properties and the thermal quenching behavior of the Eu2+ luminescence in CaAl2B2O7 and BaAl2B2O7. The Ca2+ ion in CaAl2B2O7 occur in an octahedral coordination with six equal Ca–O bonds (238.1 pm) [7]. The O– Ca–O bond angles deviate from 90° within a few degrees. The Ca2+ ion is bonded to trigonal borate groups and AlO4 tetrahedral groups. The trigonal BO3 groups which are located above and below the AlO4 tetrahedral groups are slightly offset from one another (Fig. 2). The symmetry of the BaAl2B2O7 only varies slightly from that of CaAl2B2O7, but the variation is enough to lower the Ba2+ ion local site symmetry to D3h. The Ba2+ ion also occur in a

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Fig. 1. Schematic representation of the Eu2+ energy level scheme in an octahedral coordination.

coordination of six oxygen ions with six equal Ba–O bond distances (274.6 pm) [8]. As in CaAl2B2O7, the Ba2+ ion is bonded to trigonal borate groups and to AlO4 tetrahedral group except that instead of being offset, the borate and AlO4 groups eclipse one another (Fig. 3). This slight change in the structure lowers the Ba2+symmetry to D3h. For both materials we also present theoretical calculations of the electron–vibration interaction parameters. 2. Experimental 2+ Polycrystalline M2+ = Ca2+, Ba2+; x = 0.005, 0.01, 1xEuxAl2B2O7 (M 0.02) were synthesized by the classic solid state reaction technique. The starting materials, CaCO3, BaCO3, Al2O3, H3BO3 (5% excess), Eu2O3 were weighed and ball milled for 1 h to obtain a homogeneous powder. The samples were then loaded in high purity alumina crucibles and fired at 700 °C/800 °C/900 °C/ for 5 h. Between each firing step, the samples were re-homogenized via ball milling. All samples were synthesized in a slightly reducing atmosphere of 2% H2–98% N2 gas mixture. The XRD pattern confirms single phase formation of the intended compositions. Excitation and emission spectra were carried out on an Edinburgh FL920 spectrometer equipped with an ozone generating

Fig. 3. The six-coordinated (D3h) Ba2+ in BaAl2B2O7 and the eclipsed AlO4 tetrahedrals and trigonal BO3 groups (looking down the Z-axis).

bulb for extended UV coverage and using a 250 nm blaze optical grating on the excitation arm for enhanced UV efficiency. Low temperature measurements were accomplished by using an ARS cryostat that was optically coupled into the spectrometer. The excitation and emission spectra that are presented in this work have been properly corrected. 3. Results and discussions 3.1. The excitation spectrum of CaAl2B2O7:Eu2+ Fig. 4 exhibits the low temperature (T = 10 K) excitation spectrum of CaAl2B2O7:Eu2+. The low and high-energy bands in the excitation spectra are centered at 28,400 cm1 and 44,450 cm1, respectively. These bands are associated with the Eu2+ 4f7[8S7/2] ? 4f65d1[t2g] and the 4f7[8S7/2] ? 4f65d1[eg] transitions, respectively. The low-energy excitation band shows evidence for the staircase spectrum while the high-energy band is essentially devoid of any structure. Fig. 4 also shows the positions of the multiplets in Eu3+ with intervals taken from reference [9] when

7 F5 F6

7 7

F4

7

F3

Intensity

7

F2

7

F1

7

F0

25000

30000

35000

40000

45000

50000

Energy cm-1 Fig. 2. The six-coordinated (octahedal) Ca2+ in CaAl2B2O7 and the offset between the AlO4 tetrahedrals and trigonal BO3 groups (looking down the Z-axis).

Fig. 4. The excitation spectrum of CaAl2B2O7:Eu2+ at 10 K (kem = 23,256 cm1).

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7

F6

7

Intensity

the 7F6 level is fixed at the highest energy peak. In this way the energy position of the Eu2+ 4f7[8S7/2] ? 4f6[7F0]5d1 transition (Eex = 24,668 cm1 or 3.06 eV) in CaAl2B2O7 is determined. In order to calculate the crystal-field splitting of the Eu2+ 4f65d1 electronic configuration (10Dq), the center of gravity of the 4f65d1[t2g] and 4f65d1[eg] electronic configurations were estimated by a procedure previously described [5]. The corresponding energy difference gives 10Dq, which is listed in Table 1 along with the corresponding parameters that have been obtained for the octahedrally coordinated Eu2+ ion in the pyrophosphates, Cs2M2+P2O7 (M2+ = Ca2+, Sr2+) [5] and in SrAl2B2O7 [6].

F5

7 7 7

7 F1 F2 F0

F4

F3

7

3.2. The excitation spectrum of BaAl2B2O7:Eu2+ Fig. 5 displays the low temperature (10 K) excitation spectra of BaAl2B2O7:Eu2+. Note that the individual t2g and eg bands are not distinguished. We attribute this to the D3h site symmetry which splits the t2g and eg bands into (a1 + e) and e, respectively, and to the anticipated smaller crystal-field splitting due to the longer Ba2+–O2 bond distance. The resulting overlap between the various components of the 4f65d1 excited state configuration makes it difficult to clearly identify the t2g and eg bands in BaAl2B2O7. The staircase structure is, however, resolved in the excitation spectrum. This places the Eu2+ 4f7 [8S7/2] ? 4f6 [7F0]5d1 transition at Eex = 27,322 cm1 or 3.39 eV in BaAl2B2O7. 3.3. The centroid shift (ec) and red-shift [D(2+, A)] The centroid shift (ec) can be calculated from the following relations:



ec ¼ Efree  Eex þ 0:37 þ c ec ¼ Efree  c

 10Dq eV r

ð1Þ

   1 3  CGt2g þ 2  CGeg eV 5

ð2Þ

In Eq. (1), ec ¼ Efree is the free ion value of Eu2+ (4.93 eV), and for c octahedral symmetry r = 5/2 [2]. In Eq. (2), CGt2g and CGeg are the center of gravity of the t2g and the eg excitation bands, respectively [5]. The centroid shift can only be readily determined when there is clear identification of both the eg and t2g bands. Thus the centroid shift cannot be estimated in the BaAl2B2O7 composition. The red-shift [D(2+, A)], which is a combination of the centroid shift and the crystal-field splitting of the Eu2+ 4f65d1 electronic configuration, can be calculated from the following relationship [1,2]:

Dð2þ; AÞ ¼ Efree  Eex

ð3Þ

free

where E is the energy difference between the ground state and the Eu2+ 4f6[7F0]5d1 excited level in the free ion (34,036 cm1; 4.22 eV). These optical parameters, along with the Stokes shift of emission (estimated in the next section) which have been determined from the low temperature (T = 10 K) excitation spectra of CaAl2B2Table 1 The center of gravity of the Eu2+ 4f65d1 (t2g) [CGt2g] and the 4f65d1 (eg) [CGeg] electronic configuration and the crystal field splitting [10Dq] in Cs2CaP2O7, Cs2SrP2O7, SrAl2B2O7 and CaAl2B2O7; all values in cm1. The metal–ligand bond-distance (d(M– O) in pm) is also indicated. Host

CGt2g

CGeg

10Dq

d(M–O)

Reference

Cs2CaP2O7 Cs2SrP2O7 SrAl2B2O7 CaAl2B2O7

27,993 28,268 29,308 28,413

46,194 45,655 44,346 44,450

18,200 17,387 15,038 16,037

236 249 255 238

[5] [5] [6] This work

30000

40000

50000

Wavelength (cm -1) Fig. 5. The excitation spectrum of BaAl2B2O7:Eu2+ at 10 K (kem = 26,667 cm1).

Table 2 Peak energy of the Eu2+ 4f7 ? 4f6 [7F0]5d1 excitation transition [Eex], peak energy of the emission band [Eem], full width at half maximum of the emission band [Cem], the red shift [D(A, 2+)], centroid shift [ec], and the Stokes shift [DES = Eex  Eem] in Cs2CaP2O7, Cs2SrP2O7, CaAl2B2O7, SrAl2B2O7 and BaAl2B2O7. All values in cm1. Host

Eex

Eem

Cem

D(A, 2+)

ec

DE S

Reference

Cs2CaP2O7 Cs2SrP2O7 SrAl2B2O7 CaAl2B2O7 BaAl2B2O7

25,029 25,655 25,641 24,668 27,322

16,637 17,635 24,390 23,258 26,684

2191 2147 1074 1158 1435

9000 8388 8395 9294 7366

4517 4175 4468 5646 –

8392 8020 1251 1410 638

[5] [5] [6] This work This work

O7:Eu2+ and BaAl2B2O7:Eu2+, are collected in Table 2 along with the corresponding parameters that have been obtained for the Eu2+ ion in Cs2M2+P2O7 (M2+ = Ca2+, Sr2+) [5] and SrAl2B2O7 [6]. The weak covalence of the Eu2+-O2- bonding in M2+Al2B2O7 (M2+ = Ca2+, Sr2+) is suggested by the small centroid shift of the Eu2+ 4f65d1 electronic configuration that is not too different from the pyrophosphate materials (see Table 1). In the pyrophosphates, the weak covalence of the Eu2+–O2 bonding was attributed to the strong covalent bonding within the phosphate moieties which renders the corresponding Eu2+–O2 bonding more ionic due to the inductive effect [5]. In M2+Al2B2O7 (M2+ = Ca2+, Sr2+), the O(2) anions of the CaO6/SrO6 octahedral groups share their electron density with the small and highly charged B3+ and Al3+ cations. The resulting polarization of the O(2) electron density towards the B3+ and Al3+ cations will render the Eu2+–O2 bonding ionic by the inductive effect [10,11]. This explains the relatively small centroid shift of the Eu2+ 4f65d1 electronic configuration in M2+Al2B2O7 (M2+ = Ca2+, Sr2+) and their near equivalence to that in the pyrophosphate materials. 3.4. The emission spectra and the Stokes shift of CaAl2B2O7:Eu2+ and BaAl2B2O7:Eu2+ The 10 K emission spectra of CaAl2B2O7:Eu2+ is exhibited in Fig. 6. The emission spectrum has a FWHM of about 1158 cm1 and is centered at 23,258 cm1. Since the lowest energy Eu2+ 4f7[8S7/2] ? 4f6[7F0]5d1 transition is located at 24,668 cm1, the Stokes shift of the Eu2+ emission is 24,668 cm1– 23,258 cm1 = 1410 cm1 or 0.17 eV. The 10 K emission spectra of BaAl2B2O7:Eu2+ is exhibited in Fig. 6. The emission spectrum has a FWHM of about 1435 cm1 and is centered at 26,684 cm1.

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CaAl2B2O7

Intensity

BaAl2B2O7

20000

22000

24000

26000

28000

-1

Wavelength (cm ) Fig. 6. Emission Spectra (kex = 30,303 cm1) at 10 K.

of

CaAl2B2O7

(kex = 27,397 cm1)

and

BaAl2B2O7

Since the lowest energy Eu2+ 4f7[8S7/2] ? 4f6[7F0]5d1 transition is located at 27,322 cm1, the Stokes shift of the Eu2+ emission is 27,322 cm1–26,684 cm1 = 638 cm1 or 0.08 eV. An interesting feature in the emission spectrum of BaAl2B2O7:Eu2+ is the presence of a low energy broad band, the intensity of which increases with increasing Eu2+ concentration (Fig. 7). This band which is only observed at low temperatures is completely quenched at room temperature. It is possible that the solubility of Eu2+ in BaAl2B2O7 is limited because of the considerable difference between the ionic radii of Ba2+ (150 pm) and the Eu2+ (131 pm) ions. The Shannon and Prewitt ionic radii are used here [12]. The increasing intensity of this band with increasing Eu2+ concentration (Fig. 7) suggests the preferential occupation of Eu2+ in a secondary phase. The presence of the secondary phase was, however, not identified in the XRD patterns of the samples. By measuring the relative intensities of the two emission bands as a function of temperature, the absence of energy transfer from the high energy site to the low energy site was inferred. This also sug-

Eu=0.005 Eu=0.01 Eu=0.02

gests that a secondary phase is responsible for the low energy emission band. The most frequent value for the Stokes shift of the normal Eu2+ 6 4f 5d1 ? 4f7 emission transition in solids is 1350 cm1 (0.16 eV) [1]. The anomalous emission is typically characterized by a large Stokes shift (approaching 1 eV) and large width of the emission band. It has been proposed that the anomalous emission of the Eu2+ ion is suggested when the Stokes shift of emission exceeds 4000 cm1 (0.50 eV) and/or when the FWHM of the emission band is larger than 3000 cm1 (0.37 eV) [1]. In this regard, the luminescence of CaAl2B2O7:Eu2+ and BaAl2B2O7:Eu2+ can be connected with the normal Eu2+ 4f65d1 ? 4f7 emission transition. With the data assembled in Table 2 we can infer that (1) the Eu2+ emission wavelength shifts to higher energy in the sequence CaAl2B2O7 (23,258 cm1)–SrAl2B2O7 (24,390 cm1)–BaAl2B2O7 (26,684 cm1) and, (2) the Stokes shift of emission increases in the sequence BaAl2B2O7 (638 cm1)–SrAl2B2O7 (1215 cm1)–CaAl2 B2O7 (1410 cm1). The energy of the Eu2+ emission transition can be written as [1,2]:

Eem ¼ 4:22  DðA; 2þÞ  DES

ð4Þ

Starting from BaAl2B2O7 and moving to CaAl2B2O7 the highest energy emission in BaAl2B2O7 can be understood on the basis of Eq. (4) and the data assembled in Table 2: the smallest red shift and Stokes shift is found for the Ba analog. The variation in the Stokes shift is more interesting as it shows the dependence of the relaxation of the local structure in the excited state on the ionic radii on the host lattice cation. Clearly, the reorganization in the excited state is more constrained in crystals with cations of large ionic radii. A similar dependence of the Stokes shift on the host lattice cation ionic radii is found in the MSi2O2N2 (M2+ = Ca2+, Sr2+, Ba2+) family of materials, where the Stokes shift increases in the sequence BaSi2O2N2–SrSi2O2N2–CaSi2O2N2 [13]. The explanation advanced for the observed variation in the Stokes shift of Eu2+ emission in MSi2O2N2 (M2+ = Ca2+, Sr2+, Ba2+) family of materials is linked to the observation that certain 4fn15d1 excited states contract during excitation [13]. In the case of an octahedrally coordinated Ce3+ ion, the change in the bond-length between the 4f1 and 5d1 (t2g) configuration is contractive [14,15]. It is argued in Ref. [13] that the change in the bond-length is maximal when the Eu2+ substitutionally replaces a cation with smaller ionic radii (Ca2+) and that the significant ionic radii difference between Ba2+ and Eu2+ limits the contraction of the Eu2+-ligand bond length in the excited state. Thus in a single configurationalcoordinate model there is a smaller offset of the excited state parabola relative to the ground state parabola when the Eu2+ replaces the larger Ba2+ ion and this results in a smaller Stokes shift of emission.

Intensity

3.5. Luminescence thermal quenching behavior of CaAl2B2O7:Eu2+and BaAl2B2O7:Eu2+ In Table 3 we have collected the host lattice band gap energy, the emission energy and the T50 (the temperature at which the emission intensity has quenched by 50% of its low temperature

Table 3 The host lattice optical absorption edge [EHL; cm1], peak energy of the emission band [Eem; cm1] and T50 (K) of the Eu2+ emission in M2+Al2B2O7 (M2+ = Ca2+, Sr2+, Ba2+). Note that the references pertain to the EHL data.

16000

18000

20000

22000

24000

26000

28000

Energy (cm-1) Fig. 7. The emission spectra of BaAl2B2O7 with varying Eu2+ dopant concentrations at 10 K (kex = 30,303 cm1).

Host

EHL

Eem

T50

Reference

CaAl2B2O7 SrAl2B2O7 BaAl2B2O7

55,555 54,040 <50,000

23,258 24,390 26,684

60 260 300

[21] [22] [23]

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CaAl2B2O7 and BaAl2B2O7, respectively. The trend in the T50 variation, BaAl2B2O7–SrAl2B2O7–CaAl2B2O7, shows that the thermal quenching temperature increases with increasing ionic radii of the alkaline earth ion. This has been observed before in many materials [16]. In absence of further optical data which may help in placing the ground and the excited states of the Eu2+ ion within the band gap of M2+Al2B2O7 (M2+ = Ca2+, Sr2+, Ba2+), we refrain from providing further analysis of the experimental data.

100

Normalized Intensity

80

60

40

4. Analysis of the excitation and emission spectra of Eu2+ ions in CaAl2B2O7 and BaAl2B2O7 with estimation of the electron– vibrational (EVI) parameters

20

0 0

50

100

Temperature (K) Fig. 8. Thermal quenching of 1% Eu2+ doped CaAl2B2O7 from T = 10–130 K.

100

There are three main parameters which effectively describes the electron–vibrational interaction (EVI) in impurity centers: the Stokes shift DES (the energy difference between lowest energy absorption and emission peaks), the Huang–Rhys factor S (which is proportional to DES) and the effective phonon energy  hx. All these parameters arise from the shift of the equilibrium positions of the potential energy surfaces in the ground and excited states, as shown in the framework of a simple single configurational coordinate model [17]. The values of S and  hx can be estimated using the following equations:

DES ¼ ð2S  1Þhx

90

Normalized Intensity

80

CðTÞ ¼

70 60

ð5Þ

 

pffiffiffiffiffiffiffiffiffiffiffiffi hx 1=2 8 ln 2hx S coth 2kT

ð6Þ

In Eq. (5), C(T) is the full width at half maximum (FWHM) of the emission band as a function of the absolute temperature T. The system of non-linear Eqs. (5) and (6) can be solved graphically with the values of C(T) and DES extracted from the experimental emission and excitation spectra to yield the values of S and  hx. To check if the calculated values of S and  hx are reasonable, the emission band shape can be modeled and compared with the experimental spectrum. The intensity of the emission band at energy E can be approximated by the following expression:

50 40 30 20 10

0

100

200

300

400

Temperature (K) Fig. 9. Thermal quenching of 1% Eu2+ doped BaAl2B2O7 from T = 10–350 K.

value) for the Eu2+ luminescence in M2+Al2B2O7 (M2+ = Ca2+, Sr2+, Ba2+) family of material. Clearly, T50 varies as BaAl2B2O7 (300 K)– SrAl2B2O7 (260 K)–CaAl2B2O7 (60 K). Figs. 8 and 9 exhibit the thermal quenching behavior of the Eu2+ (1%) luminescence in CaAl2B2 O7 and BaAl2B2O7, respectively. In the single configuration coordinate model, a large Stokes shift would imply a lower emission quenching temperature because of the increased probability of radiationless crossover of the excited state level into the ground state level. This model appears to be valid for the Eu2+ luminescence in the M2+Al2B2O7 (M2+ = Ca2+, Sr2+, Ba2+) family of material (see Tables 2 and 3). However, the Dorenbos findings has suggested that thermal quenching is not due to level crossing in the configuration coordinate model but due to the ionization of the electron from the lowest energy level of the relaxed Eu2+ 4f65d1 electronic configuration to the host lattice conduction band level [16]. With references to Figs. 8 and 9, thermal quenching due to energy migration over Eu2+ was ruled out because the intensity quenching profile was independent of the Eu2+ dopant levels up to 2 atomic percent. From the thermal quenching behavior it may be concluded that the relaxed Eu2+ 4f65d1 electronic configuration is in close proximity and relatively well separated from the host lattice conduction band states in

I¼

  eS Sp ehx=kT 1 þ S2 ; p! pþ1

p¼

E0  E hx

ð7Þ

where E0 is the ZPL energy (which should be located in the vicinity of the point of intersection of the excitation and emission spectra), and p is the number of the effective phonons involved into the emission transition. All other quantities entering Eq. (7) have been described above. The EVI parameters can be estimated using Eqs. (5)–(7). Despite the simplicity of this model, it has been proven to be very successful in describing the EVI in the 5d states of Eu2+ ion in a number of host lattices [18–20]. The excitation spectra of CaAl2B2O7:Eu2+ and BaAl2B2O7:Eu2+ can be decomposed into Gaussian functions, which is a result of splitting of the Eu2+ 4f65d1 states by the crystal-field. It should be mentioned here that, in principle, the degeneracy of all 4f65d1 states in a crystal-field can be completely lifted so that all the five peaks may be seen in the absorption/excitation spectrum. However, it is not possible to identify five distinct peaks. A possible explanation may be a strong EVI, which considerably broadens the excitation spectra. This is the reason why we did not use five Gaussians to fit the excitation spectra (which, in any event, would result in too large a number of fitting parameters, the values of which can hardly be physically justified) and focused our attention on the lowest excitation peak only. The number of individual Gaussians used for fitting simply equals the number of prominent features in the excitation bands for each compound considered. Multi-phonon processes and anharmonicity of the electron–phonon

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CaAl2B2O7:Eu

The ZPL energy position can be also estimated from the point of intersection of the excitation and emission spectra (for an ideal system, the emission and absorption/excitation spectra should be the mirror images of each other). For real systems, such mirror symmetry is rarely met due to anharmonicity, interaction with different vibrational modes, presence of defects and different emitting centers, etc. Although the emission and absorption spectra in CaAl2B2O7 and BaAl2B2O7 are not the mirror images of each other, the position of the point of their intersection (23,963 cm1 for CalAl2B2O7 and 27,003 cm1 for BaAl2B2O7) is reasonably close to the ZPL energy position of obtained from the emission band shape modeling.

2+

S=2.96; hw=286 cm -1 , ZPL=24000 cm-1

Normalized intensity

1.0

0.5

emission abs calc 0.0 20000

22000

24000

26000

28000

Energy, cm

30000

5. Conclusions

32000

34000

-1

Fig. 10. The excitation and emission spectra of CaAl2B2O7:0.01Eu2+ in comparison with the calculated emission band shape (T = 10 K).

BaAl2B2O7:Eu

2+

S=1.03; hw=601 cm-1 , ZPL=27230 cm-1

Normalized intensity

1.0

0.5

emission abs calc 0.0 22000 24000 26000 28000 30000 32000 34000 36000 38000

Energy, cm-1 Fig. 11. The excitation and emission spectra of BaAl2B2O7:0.01Eu with the calculated emission band shape (T = 10 K).

The optical properties of the Eu2+ ion in CaAl2B2O7 and BaAl2B2 O7 are examined in this paper. For the first time, the crystal field splitting, the centroid shift and the electron phonon interaction parameters have been determined for the octahedrally coordinated Eu2+ ion in CaAl2B2O7. Using the experimental absorption and emission spectra, we have estimated the Huang–Rhys factor, the energy of an effective phonon interacting with the Eu2+ 5d states, and the zero-phonon line position (ZPL). To test the validity of the obtained parameters, the Eu2+ 4f65d1–4f7 emission band was modeled and compared with the corresponding experimental result; good agreement was demonstrated, which serves as a proof of reliability of the estimated and reported EVI parameters. The thermal quenching of the luminescence is related to photoionization of the excited state. We have demonstrated that the optical parameters derived from our experimental data are consistent with the theoretical calculations which have been presented in this paper. Finally, we find evidence for the limited solubility of Eu2+ in BaAl2B2O7. References [1] [2] [3] [4] [5]

2+

in comparison

interaction (which are not included in the present model) may also contribute to the formation of the excitation and emission spectra. Comparison of the excitation and emission spectra of the CaAl2 B2O7 (Fig. 10) and BaAl2B2O7 (Fig. 11) yields all the necessary data that are needed for the solution of a system of Eqs. (5) and (6). The Stokes shift of the CaAl2B2O7 is 1410 cm1 and the FWHM of the emission band 1158 cm1, Huang–Rhys factor S = 2.96 and energy of effective phonons  hx = 286 cm1. The Stokes shift of the BaAl21 B2O7 is 638 cm and the FWHM of the emission band 1435 cm1, Huang–Rhys factor S = 1.03 and energy of effective phonons  hx = 601 cm1. Using the values of S and  hx, it is now possible to model the emission band shape with the help of Eq. (7), allowing the ZPL energy to vary freely until the best agreement between the experimental and calculated band shapes is reached. Good agreement between the experimental and simulated emission band shapes justifies the values of S and  hx obtained from Eqs. (5) and (6).

[6] [7] [8] [9] [10] [11] [12] [13] [14]

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