Unusual luminescence of octahedrally coordinated divalent europium ion in Cs2M2+P2O7 (M2+=Ca, Sr)

ARTICLE IN PRESS Journal of Luminescence 129 (2009) 919–925

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Unusual luminescence of octahedrally coordinated divalent europium ion in Cs2M2+P2O7 (M2+ ¼ Ca, Sr) A.M. Srivastava a,, H.A. Comanzo a, S. Camardello a, S.B. Chaney b, M. Aycibin b, U. Happek b a b

GE Global Research, 1 Research Circle, Niskayuna, NY 12309, United States Department of Physics and Astronomy, University of Georgia, Athens, GA 30602-2451, United States

a r t i c l e in fo

abstract

Article history: Received 23 November 2008 Received in revised form 4 March 2009 Accepted 23 March 2009 Available online 2 April 2009

The excitation and emission spectra of octahedrally coordinated europium ion (Eu2+) ions in Cs2M2+P2O7 (M2+ ¼ Ca, Sr) are reported and discussed. The remarkable features of the Eu2+ luminescence in these phosphate materials include (a) very large Stokes shift of emission (1 eV), (b) high luminescence quenching temperature, and (c) unusually low energy of the emitted photons for Eu2+ luminescence in phosphate-based materials. The broad emission bands of Eu2+ in Cs2CaP2O7 and Cs2SrP2O7 peak at 607 and 563 nm, respectively. The Stokes shift, crystal field splitting, centroid shift and the red shift of the Eu2+ 4f65d1 electronic configuration have been estimated from the relevant optical data. The radiative lifetime of the Eu2+ emission in Cs2M2+P2O7 is 1.2 ms. The nature of the Eu2+ emission in Cs2M2+P2O7 is discussed and arguments are presented to associate the luminescence with an extreme case of normal 4f65d1-4f7[8S7/2] emission. & 2009 Elsevier B.V. All rights reserved.

PACS: 78.55.Hx Keywords: Cs2M2+P2O7 (M2+ ¼ Ca,Sr) Eu2+ luminescence Stokes shift Thermal quenching Normal and anomalous emission

1. Introduction The optical properties of the divalent europium ion (Eu2+) in a six-fold octahedral coordination have been extensively reported in the literature. The host lattices include alkali halides, alkaline earth sulfides and selenides, fluoride perovskites such as M+CaF3 (M+ ¼ Cs, Rb, K) and alkaline earth oxides (M2+O; M2+ ¼ Ca, Sr), to name a few. Much of the available literature data on the optical properties of the Eu2+ ion in a large variety of host lattices has been complied and systematically analyzed by Dorenbos [1]. Fig. 1 shows (schematically) the energy level scheme of the Eu2+ ion in an octahedral coordination. The ground state of the Eu2+ ion is the spherically symmetrical 8S7/2. The octahedral crystal field splits the Eu2+ 4f65d1 electronic configuration into the lower t2g (triply degenerate, xy, zx, and yz orbitals) and the upper eg levels (doubly degenerate, x2–y2 and z2 orbitals), respectively. The total crystal field splitting is denoted by 10Dq. Depending on the strength of the crystal field, the absorption or the excitation spectrum of octahedrally coordinated Eu2+ ion 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,

 Corresponding author. Tel.: +1 518 387 7535; fax: +1 518 387 5299.

E-mail address: [email protected] (A.M. Srivastava). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.03.018

the low energy excitation band exhibits the characteristic ‘‘staircase’’ spectrum, which retain the character of the seven (Eu3+) 4f6 levels (7F06) [2,3]. The staircase spectrum is due to the transition from the Eu2+ 8S7/2 ground state to the seven 7FJ (J ¼ 0–6) multiplets of the excited 4f6[7FJ( ¼ 06)]5d1 electronic configuration. 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 in crystal structures where the local site symmetry is that of an octahedron. Our present knowledge on the variation of these parameters with the crystal composition is restricted to lattices with octahedral, cubic and cubooctahedral site symmetry [4]. In this paper, we present data on a structure where the Eu2+ ion occupies a well-defined six-coordinated octahedral site in solids, yet the emission characteristics are rather unusual. In Cs2M2+P2O7 (M2+ ¼ Ca, Sr) the M2+ site, which the Eu2+ ion substitutionally replaces, is octahedrally coordinated to six oxygen atoms [5]. In Cs2SrP2O7, the SrO6 octahedron is only slightly distorted, R(Sr–O) ¼ 238 pm (2  ), 247 pm (2  ), and 263 pm (2  ), respectively with the average Sr–O bond distance being 249 pm. The O–Sr–O bond angles deviate from 901 within few degrees. Since the ionic radii of the Sr2+ (127 pm) and Eu2+ (131 pm) ions in six coordination are very similar [6], the local symmetry of the Sr2+ site will not deviate much from that in the pure material upon Eu2+ doping.

ARTICLE IN PRESS 920

A.M. Srivastava et al. / Journal of Luminescence 129 (2009) 919–925

x2 - y2

1.2

z2

7F 4 7F 7F 5 3 7F 7F 6 2

eg 5d level 1.0

10 Dq

7

xz

t2g 5d level

Normalized Intensity

yz

E

xy

F1

7

F0

0.8

0.6

0.4

0.2 8S 7/2

Ground State

0.0

15000 20000 25000 30000 35000 40000 45000 50000

Fig. 1. Schematic representation of the Eu2+ energy level scheme in an octahedral coordination (see also text).

Wavenumber (cm-1) Fig. 2. The excitation (solid line; lem ¼ 600 nm) and the emission (dotted line; lex ¼ 360 nm) of Cs2CaP2O7:Eu2+ at T ¼ 80 K.

2. Experimental

1.2

7 F4 F5

7

7F 3

1.0 Normalized Intensity

Polycrystalline samples of Cs2(Ca0.99Eu0.01)P2O7 and Cs2(Sr0.99Eu0.01)P2O7 were synthesized by the classic solid-state reaction technique. The starting materials used were analytical grade Cs2CO3 (10 mole% excess) CaCO3, SrCO3 and (NH4)2HPO4 (10 mole% excess) and Eu2O3. The samples were heated at 300 1C for 1 h in air and then heated twice at 800 1C for 5 h in a slightly reducing atmosphere with intermediate grindings. In both materials, the XRD pattern confirmed the formation of phase pure material. Since the final products were found to be slightly hygroscopic, all optical measurements were carried out in sealed quartz tubes. The techniques employed in the measurement of the excitation and emission spectra and of the temperature dependence of the lifetime have been described previously [7].

7

F6

7F 2

0.8 7

7F 1

F0

0.6

0.4

0.2 3. Results and discussion

0.0 3.1. The excitation spectra and crystal field splitting of the Eu2+ 4f65d1 electronic configuration Figs. 2 and 3 exhibit the excitation and emission spectra of Cs2CaP2O7:Eu2+ and Cs2SrP2O7:Eu2+ at T ¼ 80 K, respectively. The low and high-energy bands in the excitation spectra are associated with the 4f7[8S7/2]-4f65d1[t2g] and the 4f7[8S7/2]4f65d1[eg] transitions, respectively. The low-energy excitation band, which extends from about 300 to 450 nm, shows evidence for the staircase spectrum while the high-energy band extending from 200 to 300 nm is essentially devoid of any structure. In Figs. 2 and 3, the lines drawn on the excitation spectra, indicate the positions of the multiplets in Eu3+ with intervals taken from Chang and Gruber [8] when the 7F6 level is fixed at the highest energy peak. Note that it is possible to satisfactorily resolve the energy position of the 4f7[8S7/2]-4f6[7F0]5d1 transition (Eex) in the respective excitation spectra of the two materials. The full width at half peak intensity (Gex) of the excitation bands is nearly the same in the two materials. In both materials, Gex of the lower energy 4f65d1 (t2g) band is near 6000 cm1 (see Table 1) and this is only slightly larger than the expected 5080 cm1 multiplet splitting of the Eu3+ 7FJ term. In order to calculate 10Dq, the center of gravity (CG) of the 4f65d1[t2g] and 4f65d1[eg] electronic configurations were esti-

15000 20000 25000 30000 35000 40000 45000 50000 Wavenumber (cm-1)

Fig. 3. The excitation (solid line; lem ¼ 560 nm) and the emission (dotted line; lex ¼ 360 nm) of Cs2SrP2O7:Eu2+ at T ¼ 80 K.

Table 1 The center of gravity of the 4f65d1(t2g) [CGt2g ] and 4f65d1(eg) [CGeg ] electronic configurations, crystal field splitting (10 Dq) and the full width at half peak intensity of the t2g [Gt2g ] and eg [Geg ] excitation bands at T ¼ 80 K; all values in cm1. Host

CGt2g

CGeg

10Dq

Gt2g

Ge g

Cs2CaP2O7 Cs2SrP2O7

27,993 28,268

46,194 45,655

18,200 17,387

6285 6218

6270 5659

R mated by calculating the center frequency via I(u)udu/I(u)du. The corresponding energy difference gives the total crystal field splitting, 10Dq, which is listed in Table 1 along with other relevant parameters of the Eu2+ excitation spectra in Cs2M2+P2O7. The 10Dq is larger in Cs2CaP2O7 than in Cs2SrP2O7, as is expected due to the smaller mean /Eu2+–O2S bond distance in the former material.

ARTICLE IN PRESS A.M. Srivastava et al. / Journal of Luminescence 129 (2009) 919–925

For comparison, the magnitude of the octahedral crystal field splitting as a function of the Eu2+–Ligand bond distance in halides, sulfides and selenide materials has been plotted in Fig. 6 of Ref. [4]. With an average Sr–O bond distance of 249 pm, the crystal field splitting of the Eu2+ 4f65d1 electronic configuration in Cs2SrP2O7 (10Dq ¼ 17,387 cm1), fits very well with the data assembled in Fig. 6 of Ref. [4]. To the best of our knowledge, this is the first example, where the 10Dq splitting of the octahedrally coordinated Eu2+ ion in an oxide material has been calculated. It is noteworthy to mention the influence on the center of gravity of the 4f65d1 [t2g] and 4f65d1 [eg] electronic configurations when the mean /Eu2+–O2S bond length decreases from Cs2SrP2O7 to Cs2CaP2O7 (see Table 1). The large internal pressure on the Eu2+ 4f65d1 electronic configuration in Cs2CaP2O7 (due to /Eu2+–O2S bond-length shortening) causes the 4f7[8S7/2]4f65d1[t2g] optical transition to undergo a red shift, while the 4f7[8S7/2]-4f65d1[eg] optical transition undergoes a blue shift. Quantum mechanical calculations have predicted that the pressure induced red shift of the Ce3+ 4f1-d[t2g]1 excitation in the elpasolite Cs2NaYCl6, can be connected with bond-length shortening [9]. Although the influence of hydrostatic pressure and chemical pressure on the total crystal field splitting may not be the same, it is likely that the red shift of the Eu2+ 4f7[8S7/2]4f65d1[t2g] transition in Cs2CaP2O7 relative to that of Cs2SrP2O7 can be connected with /Eu2+–O2S bond-length shortening. Further, the calculations indicate increasing 4f1-d[eg]1 transition energy with increasing pressure [9]. Hence, the blue shift of the 4f7[8S7/2]-4f65d1[eg] transition in going from Cs2SrP2O7 to Cs2CaP2O7 can be connected with decreasing /Eu2+–O2S bond length, which accompanies this excitation. 3.2. The emission spectra The emission spectra of Eu2+ activated Cs2CaP2O7 and Cs2SrP2O7 is a broad symmetrical band centered at 601 nm (16,637 cm1) and 567 nm (17,635 cm1) with full width at half peak intensity (Gem) of 2191 and 2147 cm1, respectively (Figs. 1 and 2; Table 2). The room-temperature quantum efficiency of the emission is high when measured relative to standard commercial lighting phosphors. The remarkable feature of the Eu2+ luminescence in Cs2M2+P2O7 is the spectral location of the emission at very low energy (long wavelength). It is highly unusual for a phosphate host lattice to support orange (Cs2CaP2O7) and yellow (Cs2SrP2O7) color emission of the divalent europium ion. In fact, referring to the presently available set of data on the Eu2+ ion luminescence that is complied in Ref. [1], it can be readily seen that the energy of the emitted photons in Cs2M2+P2O7 is the lowest ever observed in a phosphate-based material. The main reason for this, which will be discussed in detail in Section 3.6.3, is in the unusually high Stoke shift of the Eu2+ emission in Cs2M2+P2O7. The estimation of the Stokes shift of emission is presented in Section 3.3.

Table 2 Peak energy of the 4f7-4f6 [7F0]5d1 excitation transition [Eex], peak energy of emission band [Eem], full width at half peak intensity of the emission band [Gem], the red shift [D(A,2+1)], centroid shift [ec],energy of the zero phonon line [EZPL] and the Stokes shift [DS ¼ EexEem]; all values in cm1 and at T ¼ 80 K. Host

Eex

Eem

Gem

D(A,2+)

ec

EZPL

DS

Cs2CaP2O7 Cs2SrP2O7

25,029 25,655

16,637 17,635

2191 2147

9000 8388

4517 4175

20,833 21,645

8392 8020

921

3.3. The stokes shift of emission (DS) The Stokes shift of the Eu2+ emission (DS) is defined as the difference between the energy of the 4f7[8S7/2]-4f6 [7F0]5d1 transition (Eex) and the emission energy. The values of Eex and DS are listed in Table 2. Alternatively, DS may also be determined by establishing the energy of the zero phonon line (EZPL). It is the common practice to determine EZPL as the intersection of the normalized emission and excitation curves [10,11]. However, this is only valid for cases in which the lowest energy excitation peak is clearly indicated in the excitation data. For our measured spectra, this is not true. Therefore, EZPL is determined by observing the onset of emission as an energetic upper bound and the onset of excitation as an energetic lower bound. The average of these two values is EZPL. The calculated averages are listed in Table 2. The Stokes shift is twice the energy difference between the EZPL and the peak energy of the emission band [10]. The estimated Stokes shift values are consistent with that obtained from the energy position of the Eu2+ 4f7[8S7/2]-4f6[7F0]5d1 transition in the two materials. The most frequent value for the Stokes shift of the normal Eu2+ emission in solids is 1350 cm1 (0.16 eV) [1]. Assuming that the Eu2+ luminescence in Cs2M2+P2O7 corresponds to the normal 4f65d1-4f7[8S7/2], the very large Stokes shift emission (on the order of an electron volt) is quite remarkable. It is indicative of substantial geometric distortions around the Eu2+ ion in the excited state, e.g. due to considerable movement of the Eu2+ ion from its central position, possibly combined with rotations and distortions of the phosphate group. As examples, such distortions have been found for the Ce3+ ion in halides [12] and in LaPO4 [13]. A considerable shortening of the average /Eu2+–O2S bond distance in the excited state will increase the crystal field splitting of the Eu2+ 4f65d1 electronic configuration causing a shift of the emitting level to lower energy and thus leading to a large Stokes shift of emission. We wish to point out that a very large Stokes shift for the 5d-4f emission transition of octahedrally coordinated rare earth ions is rather uncommon, but has been observed in a few cases. For example, the Stokes shift for the Ce3+ 5d-4f emission in the elpasolites, Rb2NaScF6 and Cs2NaYF6 is 6373 cm1 (0.79 eV) and 8474 cm1 (1.05 eV), respectively [14]. The Stokes shift in these materials shows a considerable deviation from the most frequent value of 2200 cm1 (0.27 eV) [15], which is observed for the Ce3+ emission in solids. 3.4. The centroid shift and the red shift The centroid shift (ec) is the shift of the average of the Eu2+ 4f 5d1 electronic configuration relative to the free ion value. For Eu2+ in octahedral coordination, ec can be estimated from the following formula [4]:   10Dq ex c ¼ Efree  E þ 0:37 þ eV (1) c r 6

Ecfree is the barycenter energy of the high spin 4f6 [7F]5d1 level in free Eu2+ ion (4.93 eV) and for octahedral symmetry, r ¼ 5/2 [4]. The 0.37 in Eq. (1) accounts for the energy difference between Eex and the barycenter energy of the first Eu2+ (4f7[8S7/2]4f65d1[t2g]) excitation band in Cs2M2+P2O7 which is 0.77 eV wide (see Table 1). Based on this calculation, the centroid shift in Cs2CaP2O7 exceeds that in Cs2SrP2O7, as shown in Table 2. The centroid shift can also be calculated from the following relation:

c ¼ Efree  f15½3  CGt2g þ 2  CGeg g c

(2)

ARTICLE IN PRESS A.M. Srivastava et al. / Journal of Luminescence 129 (2009) 919–925

1400

1400

1200

1200

1000

1000 Lifetime (ns)

Lifetime (ns)

922

800 600

800 600

400

400

200

200

0

0

300

400

500 600 Temperature (K)

700

300

400

500 600 Temperature (K)

700

Fig. 4. Temperature dependence of the lifetime of the Eu2+ emission in Cs2CaP2O7:Eu2+. The excitation wavelength was 325 nm. The drawn line is the fit to Eq. (2).

Fig. 5. Temperature dependence of the lifetime of the Eu2+ emission in Cs2SrP2O7:Eu2+. The excitation wavelength was 325 nm. The drawn line is the fit to Eq. (2).

Eq. (2) yields the centroid shift of 0.56 eV (4520 cm1) for both materials. This suggests that the difference in ec between the two materials (see Table 2) and as calculated from Eq. (1) is not significant. Nevertheless, ec in these materials is small, which reflects on the weak covalency of the Eu2+–O2 bonding in Cs2M2+P2O7. This is expected since strong covalent bonding within the phosphate units will render the corresponding Eu2+–O2 bonding more ionic due to the inductive effect. Hence, the centroid shift is generally expected to be small in phosphates. 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 [1,4]:

Table 3 Values of the parameters used in fitting the temperature dependence of the lifetime data and the activation energy for thermal quenching (DE).

Dð2þ; AÞ ¼ Efree  Eex

(3)

free

is the energy difference between the ground state and where E the Eu2+ 4f6[7F0]5d1 excited level in the free ion (34,036 cm1; 4.22 eV). The red shift in Cs2CaP2O7 exceeds that in Cs2SrP2O7 (Table 2) mainly because of the larger crystal field splitting of the Eu2+ 4f65d1 electronic configuration. The larger red shift is responsible for the energetically lower Eu2+ emission in Cs2CaP2O7 (16,637 cm1) relative to that in Cs2SrP2O7 (17,635 cm1). 3.5. Temperature dependence of the Eu2+ lifetime The activation energy (DE) for thermal quenching of the Eu2+ emission in Cs2M2+P2O7 was determined by measuring the temperature dependence of the Eu2+ emission lifetime (Figs. 4 and 5). At all temperatures the decay curves were found to be single-exponential. The rapid shortening of the Eu2+ emission lifetime for T4500 K indicates the onset of non-radiative transitions. The drawn line in the figures is the fit to the equation as follows:

t¼

tr 1 þ ½tr =tnr  expðDE=kTÞ

Host

1/tr (s1)

1/tnr (s1)

DE (cm1)

Cs2CaP2O7 Cs2SrP2O7

7.8  105 8.3  105

3.8  1013 1.7  1014

7394 7900

occurs near 700 K. Note that both DE and T0.5 are quite high in Cs2M2+P2O7:Eu2+ (Table 3; Figs. 4 and 5). According to Dorenbos, the main mechanism that is responsible for the thermal quenching of Eu2+ luminescence in solids is 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]. It is determined that thermal quenching due to level crossing in the single configuration coordinate model cannot satisfactorily account for the variation in the thermal quenching temperature of the Eu2+ emission in solids. Hence, the activation energy is the energy required to raise the electron from the relaxed excited level into the host lattice conduction band. Consequently, a relationship between T0.5 and the Stokes shift of emission may not necessarily exist and this was one of the conclusions reached in the Dorenbos analysis [16]. The high T0.5 of the Eu2+ emission in Cs2M2+P2O7, despite the large Stokes shift of emission, is in accordance with the findings of Dorenbos [16]. A high T0.5 is anticipated when the lowest energy level of the relaxed Eu2+ 4f65d1 electronic configuration is well isolated from the host lattice conduction band. This condition is met for the Eu2+ emission in phosphates such as Sr3(PO4)2 and Ba3(PO4)2 (T0.54550 K) [17] and in the oxynitride SrSi2O2N2 (T0.5600 K) [10]. Hence, the high T0.5 (600 K) for the Eu2+ emission in Cs2M2+P2O7 implies that the lowest energy level of the Eu2+ 4f65d1 electronic configuration is well isolated from the host lattice conduction band.

(4)

The parameters of the fits are collected in Table 3. The radiative lifetime of the Eu2+ excited state is found to be 1.2 ms in these materials. The temperature at which the lifetime/emission intensity has decreased to 50% of the value at low temperatures (T0.5) is about 600 K and complete quenching of the Eu2+ emission

3.6. The nature of the Eu2+ luminescence in Cs2M2+P2O7 (M2+ ¼ Ca2+, Sr2) The remarkable features of the Eu2+ luminescence in Cs2M2+P2O7 are the very large Stokes shift of emission (on the

ARTICLE IN PRESS A.M. Srivastava et al. / Journal of Luminescence 129 (2009) 919–925

order of an electron volt) and the very high luminescence quenching temperature (T0.5600 K). As far as the Stokes shift is concerned, and as previously stated, it is uncharacteristic of the normal Eu2+ 4f65d1-4f7 (8S7/2) emission transition in solids. The possibility that the Eu2+ luminescence in Cs2M2+P2O7 is due to the anomalous emission, which has been identified as resulting from an impurity-trapped exciton level, has to be considered [18,19]. The anomalous emission occurs when the levels of the Eu2+ 4f65d1 electronic configuration are near or are situated within the host lattice conduction band. The anomalous emission is typically characterized by a large Stokes shift (approaching 1 eV) of emission and large width of the emission band. The evaluation of the literature data led Dorenbos to propose that anomalous emission of the Eu2+ ion is suggested when the Stokes shift of emission exceeds 4000 cm1 (0.50 eV) and/or Gem is larger than 3000 cm1 (0.37 eV) [1]. Dorenbos [20] identifies host lattices in which the anomalous emission of the Eu2+ ion is presumed. Taking the Stokes shift of emission as a gauge would immediately connect the Eu2+ luminescence in Cs2M2+P2O7 with the anomalous emission. Further, Gem2200 cm1 (see Table 2), which is to some extent larger than the typical emission bandwidth established for the normal Eu2+ luminescence in a large number of solids that cluster around 1600 cm1 [1]. Consequently, the Stokes shift and Gem values would be indicative of the anomalous emission for the Eu2+ ion in Cs2M2+P2O7. However, arguments presented below lead us to conclude otherwise. In the following, we query if other properties of the anomalous emission (apart from the Stokes shift and Gem) that can be identified and which differs from the normal emission of the Eu2+ ion in solids. For example, since the composition of the wavefunction of the emitting states is different, the lifetime of the excited state responsible for the anomalous and normal emission transitions may be dissimilar. Further, there may be differences between the thermal quenching behavior of the anomalous and the normal emission of the Eu2+ ion. Table 4 is a compilation of the characteristics of Eu2+ luminescence in materials that have been associated with the anomalous emission. In this table, Eanem and Ganem are the peak energy and the full width at half peak intensity of the anomalous emission band. In the following subsection, an attempt is made to distinguish between the anomalous and the normal emission of the Eu2+ ion that is based on the lifetime and the thermal quenching parameters. Before proceeding further, it is necessary to remark that of all the materials listed in Table 4, only in the case of BaF2:Eu2+ has photoconductivity measurements established the internal photoionization of the Eu2+ ion [19].

923

3.6.1. Effect on lifetime The characteristic lifetime of the normal Eu2+ 4f65d1-4f7[8S7/ 2] emission transition is around 1.1 ms, although radiative lifetimes as low as 0.4 ms have been reported [17]. The data complied in Table 4, with the exception of Sr2LiSiO4F, shows that lifetime longer than 1.1 ms is encountered in all cases. Specifically, the case of Eu2+ activated Ba2Mg(BO3)2 (t4.2 K ¼ 12.6 ms) is instructive since it exhibits an unusually long lifetime [17]. This is taken to indicate that the emission transition is from the impurity-trapped exciton state to the ground state of the Eu2+ ion. From the data presented in Table 4, it is tentatively concluded that the anomalous emission of the Eu2+ ion in solids is characterized by a radiative lifetime that is longer than about 2 ms. Based on the lifetime parameter, the luminescence of Cs2M2+P2O7:Eu2+ can be associated with the normal Eu2+ 4f65d1-4f7(8S7/2) emission transition, despite the large Stokes shift of emission since the radiative lifetime of the Eu2+ excited state is 1.2 ms. The luminescence of Eu2+ in Sr2LiSiO4F has been identified as being anomalous [17,20]. This conclusion was based on the following observations: (1) the large Stokes shift of emission (6210 cm1; 0.77 eV) (2) the low energy of the emitted photon (18,762 cm1; 2.33 eV) and, (3) Ganem3952 cm1 (0.49 eV). First, as revealed in the case of Cs2M2+P2O7:Eu2+, the large Stoke shift of the Eu2+ emission in Sr2LiSiO4F may not necessarily be a signpost for an anomalous emission. Second, the radiative lifetime of 1.2 ms is characteristic of the normal Eu2+ 4f65d1-4f7 (8S7/2) emission transition. The full width at half peak intensity of the Eu2+ emission band in Sr2LiSiO4F is indeed large and comparable with the bandwidth of the anomalous emission in BaS [21] (Table 4). The possible reason for this is examined below. In the crystal structure of Sr2LiSiO4F there are two nonequivalent sites for the Sr2+ ion. According to Akella and Keszler [22], both the Sr2+ sites are ten coordinated with eight oxygen ions and two fluorine ions. However,Yanga and Zhang [23] dispute this and argue for eight-fold coordination for one of the Sr2+ sites. Further, it has been noticed that the Eu2+ emission band in Sr2LiSiO4F is asymmetrical on an energy scale [24]. Therefore, it is possible that the Eu2+ emission in Sr2LiSiO4F is inhomogenously broadened. Based on the lifetime parameter it is now suggested that the Eu2+ emission in Sr2LiSiO4F is of the normal type, despite the large Stoke shift and the large emission bandwidth. The large variation in the lifetime of the anomalous emission in materials that are listed in Table 4 is not easily understood. The lifetime varies from 1.8 ms (BaF2) to 12.6 ms (Ba2Mg(BO3)2) at T ¼ 4.2 K. This variation may be a sensitive function of the overlap of the electron and hole wave functions as suggested in Ref. [17], where an attempt has also been made to correlate the lifetime variation with the crystal structure and the coordination polyhedra around the Eu2+ ion.

Table 4 The optical properties of the Eu2+ ion in materials suspected of supporting anomalous emission; Eanem is the peak energy of emission band (in cm1); Gem is the full width at half peak intensity of the emission band (in cm1; all values at room temperature unless otherwise noted); t (in ms) is the lifetime at T ¼ 4.2 K and at room temperature (except for Ba3SiO5). Host

Gex

Eanem

DS

Ganem

T0.5 (K)

t (ms) 4.2 K/RT

Refs.

BaS Ba2LiB5O10 Ba2Mg(BO3)2 Ba3SiO5 BaF2 Sr3(BO3)2 Sr2LiSiO4F BaSi2O5

18,315 26,666 24,213 – 26,178 20,620 25,000 –

11,389 15,873 16,447 16,949 16,949 17,301 18,762 19,196

6925 10,807 7743 – 9195 3307 6210 –

3980 2419 (4 K) 3791 2581 4113 (77 K) 4033 (4 K) 3952 –

200 320 340 420 80 4150 450 460

Afterglow 2.9/1.1 12.6/5.4 2.7(100 K) 1.8/– 2.5/– 1.2/1.1 3.3/3.1

[21] [1,16,17] [1,16,17] [27] [1,16,17] [20,28] [1,16,17] [17]

ARTICLE IN PRESS 924

A.M. Srivastava et al. / Journal of Luminescence 129 (2009) 919–925

The lifetime of the anomalous emission in BaF2:Eu2+ (t4.2 K ¼ 1.8 ms) represents the shortest lifetime for the anomalous emission among the materials that are listed in Table 4. The possible reasons for this are now discussed. Mahlik et al. [25] reports that for T4200 K the exciton level thermally populates the levels of the 4f65d1 electronic configuration. This indicates that the exciton level and the localized levels of the Eu2+ 4f65d1 electronic configuration are energetically in close proximity. The possible mixing of the Eu2+ 4f65d1 wave function in the exciton state may explain the short lifetime of the anomalous emission in BaF2. Alternatively, it is possible that the anomalous emission in BaF2:Eu2+ is partially quenched even at low temperatures so that the lifetime of 1.8 ms (T ¼ 4.2 K) does not represent the radiative lifetime of the exciton emission.

3.6.2. Thermal quenching of the Eu2+ emission From Table 4, it may be seen that the listed materials have a relatively low quenching temperature of emission with T0.5 ranging from 80 K for BaF2 to 460 K for BaSi2O5. The T0.5 of the Eu2+ emission in Cs2M2+P2O7 is a great deal higher (600 K). As previously pointed out, the T0.5 for the normal Eu2+ emission in phosphates such as Sr3(PO4)2 and Ba3(PO4)2 (T0.54550 K) [17] and in the oxynitride SrSi2O2N2 (T0.5600 K) [10] are also very high. Both the T0.5 and lifetime of 1.2 ms for the Eu2+ emission in Cs2M2+P2O7 indicate that the states of the Eu2+ 4f65d1 electronic configuration are well isolated from the host lattice conduction band. This suggests that the Eu2+ luminescence in Cs2M2+P2O7 corresponds with the normal emission transition. It should also be remarked that unlike the case of BaF2:Eu2+, where the normal emission of Eu2+ emerges in the emission spectrum for T4200 K, the emission spectrum of Eu2+ in Cs2M2+P2O7 does not show signs of any new emission band when the temperature is increased from T ¼ 77 to 573 K. Let us return to the data presented in Table 4. If attention is focused on the columns pertaining to T0.5 and Eanem, we note that with the exception of BaF2, all the other materials exhibit a rather steady increase in T0.5 with increasing energy of the emitted photon (Eanem). Assuming that the corresponding anomalous excitation/absorption band also increases in energy with increasing Eanem, then the variation in T0.5 can be explained on the basis of level crossing between the configuration coordinate curve of the trapped exciton state and the configuration coordinate curve of the Eu2+ ground state [26]. This is in contrast with the thermal quenching due to photoionization of the normal Eu2+ emission in solids [16]. If the thermal quenching of the anomalous emission can be represented in terms of the configuration coordinate model, then a relationship between DS and T0.5 can be expected. Examination of the data presented in Table 4 shows that there is a general lack of correlation between DS and T0.5. This is not too surprising since as pointed out by Poort et al. [17], the DS values listed in Table 4 cannot be considered as the real Stokes shift of the anomalous emission. This is because Eex corresponds with the energy of the Eu2+ 4f7[8S7/2]-4f6[7F0]5d1 transition and, therefore does not characterize the energy of the anomalous excitation band. Therefore, the lack of general correlation between DS and T0.5 and also between DS and Ganem is not too surprising. We would like to stress that a low T0.5 does not necessarily indicate that the Eu2+ emission is from the impurity-trapped exciton level. The normal Eu2+ emission may quench at relatively low temperatures if the lowest energy level of the 4f6[7F0]5d1 electronic configuration is located close to the conduction band. This situation is encountered in Eu2+ activated SrSiO3 and CaSiO3 in which T0.5 is near 100 K [17]. The Stokes shift is small (1500–1900 cm1) and t4.2 K ¼ 0.7 ms. Since the lifetime is sugges-

tive of the normal emission, the impurity-trapped exciton is not the lowest energy state in SrSiO3 and CaSiO3. Then the thermal quenching in these materials is due to photoionization. 3.6.3. Emission of Eu2+ in phosphates versus emission of Eu2+ in Cs2M2+P2O7(M2+ ¼ Sr, Ca) Referring to the data presented in Ref. [1] for the normal emission of Eu2+ in phosphates, it can be readily seen that the emission band in Cs2M2+P2O7 peaks at exceeding low energies. In fact, the energy of the emitted photon in Eu2+ activated Cs2M2+P2O7 is the lowest ever observed in a phosphate-based materials. First, the rationale for the (normal) Eu2+ 4f7 [8S7/2]24f65d1 optical transitions generally occurring at high energy in phosphate-based materials are discussed. The energy of the Eu2+ 4f65d1-4f7 [8S7/2] emission transition (Eem Eu2þ ; in eV) can be written as [1]: Eem Eu2þ ¼ 4:22  Dð2þ; AÞ  DS

(5)

As previously discussed, the strong polarization of the electron density around the O2 ions by the P5+ ion results in strong covalent bonding within the phosphate moieties, which renders the Eu2+–O2 bond more ionic by the inductive effect. This bonding effect weakens the centroid shift and also to a certain extent the strength of the crystalline field acting on the Eu2+ 4f65d1 electronic configuration. The average red shift found in the phosphate materials is 886271015 cm1 (1.0970.12 eV) [1]. Further, the most frequent Stokes shift value for the normal Eu2+ 4f65d1-4f7 emission transition in solids occurs at 1350 cm1 (0.16 eV) [1]. Hence, in the phosphate family of materials, the Eu2+ emission can be expected in the 390–450 nm wavelength range, which with a few exception, is what is found experimentally [1]. The red shift of the Eu2+ 4f65d1 electronic configuration in Cs2M2+P2O7 is comparable with those found in phosphate-based materials (Table 2). Therefore, the reason for the low energy of the emitted photon in Eu2+ activated Cs2M2+P2O7 relative to the other phosphate-based materials, is contained in the large Stokes shift of emission. This large Stokes shift is singularly responsible for the much longer wavelength of Eu2+ emission in Cs2M2+P2O7 (M2+ ¼ Ca2+, Sr2+) relative to other phosphate-based materials. We have recently measured the room temperature emission spectrum of Rb2SrP2O7: Eu2+. The emission is a broad band peaking near 576 nm (17,360 cm1). As expected for a normal Eu2+ 4f65d1-4f7 emission transition, the emission occurs at lower energy relative to Cs2SrP2O7:Eu2+ (17,762 cm1) due to the stronger crystal field. This observation also suggests that the Eu2+ emission in A+2M2+P2O7 (where A+ ¼ Cs+, Rb+ and M2+ ¼ Ca2+, Sr2+) is of the normal type.

4. Conclusions There is at least one novel feature of the Eu2+ emission in Cs2M2+P2O7 (M2+ ¼ Ca2+, Sr2+) that is in conflict with the general experience of the normal Eu2+ luminescence in solids. This pertains to the very large Stokes shift (approaching one electron volt) of emission. Generally, the very large Stokes shift of the Eu2+ luminescence is associated with the anomalous emission. However, based on the radiative lifetime of the excited state (1.2 ms) and the high quenching temperature of luminescence, it is concluded that the Eu2+ emission in Cs2M2+P2O7 originates from the lowest level of the 4f65d1 electronic configuration. Consequently, the validity of the prediction that the Eu2+ luminescence in solids is due to the anomalous emission when the Stokes shift

ARTICLE IN PRESS A.M. Srivastava et al. / Journal of Luminescence 129 (2009) 919–925

exceeds 4000 cm1 [1] is not found in the luminescence of Cs2M2+P2O7:Eu2+. The data presented for the anomalous emission of the Eu2+ ion in solids further indicate that (1) the anomalous emission can be further characterized by a lifetime which is longer than about 2 ms, (2) the quenching temperature of the anomalous emission is generally low, and (3) the quenching temperature generally increases with increasing energy of the emitted photon (Eanem). The discussion presented in the paper also raises question on the nature of the emitting state in solids such as Sr2LiSiO4F:Eu2+. We realize that in the absence of any photoconductivity data, it is difficult to ascertain if the emission is due to the ionization of the Eu2+ ion. If proven with further experimentation that the Eu2+ emission in Sr2LiSiO4F is anomalous, then this particular example would imply the inability to distinguish between the normal and anomalous emission on the basis of the lifetime parameter only. In our opinion there is still a great need for further experimentation to obtain quantitative information on the nature of the Eu2+ excited state when the Stokes shift and/or the width of emission band is large. References [1] P. Dorenbos, J. Lumin. 104 (2003) 239. [2] M.J. Freiser, S. Methfesskl, F. Holtzberg, J. Appl. Phys. 39 (1968) 900. [3] F.M. Ryan, W. Lehmann, D.W. Feldman, J. Murphy, J. Electrochem. Soc. 121 (1974) 1475. [4] P. Dorenbos, J. Phys.: Condens. Matter 15 (2003) 4797.

925

[5] V.K. Trunov, Yu.V. Oboznenko, S.P. Sirotinkin and N.B. Tskhelashvili, Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy, 27 (2991) 2370. [6] R.D. Shannon, C.T. Prewitt, Acta. Crystallogr. B 25 (1969) 925. [7] A.M. Srivastava, A.A. Setlur, H.A. Comanzo, Y. Gao, M.E. Hannah, J.A. Hughes, U. Happek, Opt. Mater. 30 (2008) 1499. [8] N.C. Chang, J.B. Gruber, J. Chem. Phys. 10 (1964) 3227. [9] F. Ruipe´rez, L. Seijo, Z. Barandiar´an, J. Chem. Phys. 122 (2005) 234507. [10] V. Bachmann, T. Ju¨stel, A. Meijerink, C. Ronda, P.J. Schmidt, J. Lumin. 121 (2006) 441. [11] C. Chartier, C. Barthou, P. Benalloul, J.M. Frigerio, J. Lumin. 111 (2005) 145. [12] J. Andriessen, E. van der Kolk, P. Dorenbos, Phys. Rev. B 76 (2007) 075124. [13] A.A. Bagatur’yants, I.M. Iskandarova, A.A. Knizhnik, V.S. Mironov, B.V. Potapkin, A.M. Srivastava, T.J. Sommerer, Phys. Rev. B 78 (2008) 165125. [14] P. Dorenbos, J. Lumin. 91 (2000) 155. [15] P. Dorenbos, J. Andriessen, M. Marsman, C.W.E. Van Eijk, Radiat. Eff. Defects Solids 154 (2001) 237. [16] P. Dorenbos, J. Phys.: Condens. Matter 17 (2005) 8103. [17] S.H.M. Poort, A. Meijerink, G. Blasse, J. Phys. Chem. Solids 58 (1997) 1451. [18] D.S. McClure, C. Pedrini, Phys. Rev. B 32 (1985) 8465. [19] B. Moine, C. Pedrini, B. Courtois, J. Lumin. 50 (1991) 31. [20] P. Dorenbos, J. Phys.: Condens. Matter 15 (2003) 2645. [21] P.E. Smet, J.E. Van Haecke, F. Loncke, H. Vrielinck, F. Callens, D. Poelman, Phys. Rev. B 74 (2006) 035207. [22] A. Akella, D.A. Keszler, Chem. Mater. 7 (1995) 1299. [23] M. Yanga, S. Zhang, J. Phys. Chem. Solids 64 (2003) 213. [24] S.H.M. Poort, H.M. Reijnhoudta, H.O.T. van der Kuipa, G. Blasse, J. Alloys Compds 241 (1996) 75. [25] S. Mahlik, B. Kuklin´ski, W.M. Yen, R.S. Meltzer, M. Grinberg, J. Lumin. 128 (2008) 715. [26] G. Blasse, J. Chem. Phys. 48 (1968) 3108. [27] M. Yamaga, Y. Masui, S. Sakuta, N. Kodama, K. Kaminaga, Phys. Rev. B 71 (2005) 205102. [28] W.J. Schipper, D. van der Voort, P. van den Berg, Z.A.E.P. Vroon, G. Blasse, Mater. Chem. Phys. 37 (1993) 311.