Luminescence spectroscopy of Ln-doped Bi-containing phosphates and molybdates

Radiation Measurements 90 (2016) 314e318

Contents lists available at ScienceDirect

Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas

Luminescence spectroscopy of Ln-doped Bi-containing phosphates and molybdates Yu Hizhnyi a, *, V. Chornii a, S. Nedilko a, M. Slobodyanik a, K. Terebilenko a, V. Boyko b, O. Gomenyuk c, V. Sheludko c a b c

Taras Shevchenko National University of Kyiv, 64 Volodymyrska st., 01601 Kyiv, Ukraine National University of Life and Environmental Sciences of Ukraine, 5 Geroiv Oborony st., 03041 Kyiv, Ukraine Oleksandr Dovzhenko Hlukhiv National Pedagogical University, 24 Kyjevo-Moskovs'ka Street, 41400 Glukhiv, Ukraine

h i g h l i g h t s
PL excitation of K3Bi5(PO4)6 and K2Bi(PO4)(MoO4) is related to Bi3þ ions.
Energy transfer from Bi3þ to Eu3þ exists in K3Bi5(PO4)6:Eu and K2Bi(PO4)(MoO4):Eu.
PL excitation spectra of K2Eu(PO4)(MoO4) are formed by O e Eu CT transitions.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 October 2015 Received in revised form 15 December 2015 Accepted 8 January 2016 Available online 11 January 2016

The photoluminescence (PL) emission and excitation spectra of undoped and doped with rare-earth (RE ¼ Eu, Tb) ions K3Bi5(PO4)6 and K2Bi(PO4)(MoO4) crystals are studied in 3.7e14 eV region of the excitation photon energies at T ¼ 8 and 300 K. The mechanisms of the host-related and RE-related luminescence in 3.7e7 eV region of the excitation photon energies are revealed in comparative analysis of the PL spectra of studied compounds. It is assumed that the excitation mechanisms of host luminescence of K3Bi5(PO4)6 and K2Bi(PO4) (MoO4) crystals below 4.8 eV are related to Bi3þ ions in oxygen surrounding. An efficient energy transfer from the Bi3þ-related luminescence centers to the emitting RE centers exists in crystals with low concentration of the RE dopants (1%). The PL excitation spectra of K3Bi5(PO4)6 crystals with high concentration of Eu dopants are formed by O e Eu CT transitions. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Luminescence Eu Bismuth Phosphate Molybdate

1. Introduction The bismuth-containing compounds can be easily doped with luminescent rare-earth ions since the Bi3þ ionic radius is close to ionic radii of the lanthanides. The photo-luminescence properties of the RE-doped Bi-containing phosphates and molybdates are intensively studied at present from viewpoint of potential application of these materials as the components of white light emission diodes, phosphor and laser host materials (Huang et al., 2014; Sanyasi Naidu et al., 2012; He et al., 2010; Voda et al., 1998, 2001; Canibano et al., 2003; Reshak et al., 2008a, 2008b). The origin of PL emission components of the set of undoped Bi-containing phosphate and molybdate hosts was revealed in our recent

* Corresponding author. E-mail address: [email protected] (Y. Hizhnyi). http://dx.doi.org/10.1016/j.radmeas.2016.01.014 1350-4487/© 2016 Elsevier Ltd. All rights reserved.

papers (Nedilko et al., 2013; Hizhnyi et al., 2014, 2013). However, a mechanisms of the PL excitation for these compounds was not analyzed yet. In this paper, we study the PL properties of two crystals from this set, K2Bi(PO4)(MoO4) and K3Bi5(PO4)6. We consider the influence of RE-doping on the PL spectra of these crystals at different temperatures aiming to clarify the origin of luminescence of the crystal hosts, in particular, formation of peculiarities in the PL excitation spectra. It is a well known phenomenon that doping with RE ions usually suppresses host PL emission of oxide compounds. The RE3þ dopants can create new “channels” the excitation energy relaxation which compete with host-related PL emission. The RE3þ ions can be excited by intra-center excitations, either by inner-shell f-f transitions or by charge-transfer (CT) O e RE transitions. At the same time, the excitation energy can be transferred to the RE3þ ions after band-to-band excitations of the crystal host. Comparison of the

Y. Hizhnyi et al. / Radiation Measurements 90 (2016) 314e318

315

excitation spectra of undoped and RE-doped oxide compounds can provide much information on the excitation mechanisms as for the host-related as well as for the RE3þ-related luminescence. In this paper, we consider only those RE dopants in K2Bi(PO4)(MoO4) and K3Bi5(PO4)6 hosts which provide the most relevant information for clarification of the excitation mechanisms in these compounds. 2. Synthesis and experimental methods Two synthetic pathways have been applied for samples preparation. A flux growth has been used for undoped samples and for doped with small amount of activator. The synthetic procedures for K2Bi(PO4)(MoO4), K2Bi(PO4)(MoO4):Eu(0.1, 1%), K3Bi5(PO4)6, K3Bi5(PO4)6:Eu (1%þ) are described in detail in (Nedilko et al., 2013). In case of K2Bi(PO4)(MoO4):Tb(1%) the same procedure has been applied with addition of Tb4O7 in amount of 0.25 mol. % in the initial melt. For samples with high europium content (K2Eu(PO4)(MoO4), K3Bi4Eu(PO4)6, K3Bi2.5Eu2.5(PO4)6) the solid state approach has been chosen. The reagent-grade raw materials were Bi2O3, Eu2O3, K2MoO4, KPO3 and (NH4)2HPO4 powders with purity of more than 99.99%. Stoichiometric amounts of reagents have been thoroughly mixed with an agate mortar and pestle into fine powders at the first stage. The mixture for K2Eu(PO4)(MoO4) in an alumina crucible was gradually heated to 700  C for 8 h, 850  C for 12 h. For K3Bi4Eu(PO4)6 and K3Bi2.5Eu2.5(PO4)6 synthesis, the mixtures of the stoichiometric reagents have been preheated at 500  C for 2 h to get rid of gaseous co-products, and then they were annealed at 750  C for 6 h, 850  C and 950  C for 12 h with intermediate regrinding. The X-Ray powder diffraction (XRD) data were collected on a SHIMADZU XRD-6000 diffractometer with a linear detector and Cu Ka radiation (l ¼ 1.5418Å). Data were collected over the 2q range of 5e90O with the step 0.02O and 1sec exposition per step. The patterns obtained match well with reference data for K2Bi(PO4)(MoO4) (Zatovsky et al., 2006; Daub et al., 2012) and K3Bi5(PO4)6 (Terebilenko et al., 2007). The PL properties under the VUV synchrotron excitations were studied on SUPERLUMI station at HASYLAB (DESY), Hamburg, Germany (Zimmerer, 2007). The PL spectra were obtained for 3.7e14 eV region of excitation photon energies in 8e300 K temperature range. All PL emission and excitation spectra were corrected on instrumental response.

Fig. 1. The PL emission spectra of undoped, Eu- and Tb-doped K2Bi(PO4)(MoO4) crystals, T ¼ 8 K, excitation photon energies Eex are indicated in the figure.

K2Bi(PO4)(MoO4) crystals are presented in Fig. 2. Vertical dashed line in the upper plot represent the value of the energy gap Eg of K2Bi(PO4)(MoO4) crystal estimated in our earlier paper (Hizhnyi et al., 2014). The excitation spectra of the blueegreen and red PL components of undoped K2Bi(PO4)(MoO4) are alike and reveal four well-distinctive components at energies indicated by arrows with corresponding notations. The same components are manifested in

4.2

4.5

K2Bi(PO4)(MoO4)

λem, nm: 420 600

5.2

3. Results and discussion

K2Bi(PO4)(MoO4):Eu

Intensity

The PL emission spectra of undoped, Eu- and Tb-doped K2Bi(PO4)(MoO4) crystals measured at T ¼ 8 K are presented in Fig. 1. The spectra of undoped crystal reveal several components peaking in the blueegreen and red spectral regions. In our previous studies, we attributed the blueegreen emission components of undoped K2Bi(PO4)(MoO4) to radiative transitions in Bi3þ ions, whereas the red component was related to transitions in MoO2 4 groups of the crystal (Hizhnyi et al., 2014). As the Figure shows, doping with europium leads to suppression of host emission and only Eu3þ-related spectral bands are observed in spectra starting from CEu ¼ 1%. These bands are generated by the inner-shell f-f transitions in Eu3þ ions (their detailed assignment can be found in Nedilko et al. (2013). The spectra of Tb-doped samples reveal Tb3þrelated narrow spectral bands which are observed against a “background” of the low-intensity host luminescence band. These narrow bands originate from the inner-shell f-f transitions in Tb3þ ions (the band assignment can be found e.g. in Souza et al. (2010)). Intensity of the most intense Tb3þ-related band observed near 550 nm exceeds intensity of the host emission at this wavelength almost by a decade. The PL excitation spectra of undoped, Eu- and Tb-doped

6.1

x1.5

K2Bi(PO4)(MoO4):Tb x3 4.7

4

615

CEu = 1 %

5.6

5

6.7

550

CTb = 1 %

K2Eu(PO4)(MoO4)

6 7 8 Photon energy (eV)

9

615

10

Fig. 2. The PL excitation spectra of undoped, Eu- and Tb-doped K2Bi(PO4)(MoO4) crystals. For undoped samples: T ¼ 8 K; for Eu- and Tb-doped: T ¼ 8 (lines) and 300 K (lines with circles), lem are indicated in the figure.

Y. Hizhnyi et al. / Radiation Measurements 90 (2016) 314e318

the excitation spectra of the Eu- and Tb-doped samples. When measured at 8 K, the spectrum of K2Bi(PO4)(MoO4):Eu is practically similar to the spectra of undoped crystal. At 300 K, the spectrum of Eu-doped K2Bi(PO4)(MoO4) is also similar to the undoped case, but only at higher energies (above 4.7 eV). Below 4.7 eV, the excitation spectrum of K2Bi(PO4)(MoO4):Eu is different: the band at 4.5 eV is less intense, whereas the 4.2 eV band disappears at all. Above 4.7 eV, the excitation spectra of Tb-doped K2Bi(PO4)(MoO4) also resembles the spectra of undoped crystal (components at 5.2 and 6.1 eV are well manifested both at 8 and 300 K). Below 4.7 eV, the temperature change in K2Bi(PO4)(MoO4):Tb spectra is the same as for the Eu-doped sample: the 4.5 eV band becomes less intense at 300 K, whereas the 4.2 eV band disappears. The excitation spectra of K2Eu(PO4)(MoO4) reveal narrow f-f excitation bands of Eu3þ ions below 4.4 eV and wide bands at higher energies. Three components can be clearly distinguished in these spectra (see corresponding arrows in Fig 2). Disappearance of the 4.2 eV band in the crystal with no regular Bi cations points to the bismuth-related origin of this band (this origin will be analyzed below in this paper). The PL emission spectra of undoped and Eu-doped K3Bi5(PO4)6 crystals are presented in Fig. 3. The undoped K3Bi5(PO4)6 reveal two PL emission components peaking in the blueegreen and the red spectral regions (the origin of these components was analyzed in our previous paper (Hizhnyi et al., 2014) and it was found, in particular that the blueegreen component is related to radiative transitions in Bi3þ ions). Doping of K3Bi5(PO4)6 with europium at low concentration (1%) leads to appearance of narrow Eu3þ-related emission bands observed together with blueegreen component of the host emission (see Fig. 3). In crystals with high Eu concentration, K3Bi4Eu(PO4)6 and K3Bi2.5Eu2.5(PO4)6, emission of the host is completely suppressed and only Eu3þ-related bands are observed. The excitation spectra of undoped K3Bi5(PO4)6 reveal five components indicated by arrows in Fig. 4 (vertical dashed line represents Eg value of the crystal estimated in Hizhnyi et al. (2014). The low-energy excitation component at 4.0 eV is observed only for lem ¼ 700 nm. At 8 K, the excitation spectrum of K3Bi5(PO4)6:Eu is similar to the excitation spectrum of the red component of undoped crystal. At 300 K, the 4.0 and 4.4 eV components of K3Bi5(PO4)6:Eu spectrum cannot be distinguished (at least, they have the same intensity the high-energy components of this

4.0

4.4

K3Bi5(PO4)6

5.2 4.7

λem, nm:

450 700

6.1

x2

K3Bi5(PO4)6:Eu CEu= 1%

x4

Intensity

316

4.7 4.2

5.3

4.7

K3Bi4Eu(PO4)6

6.2

x2 5.5

6.3

4.3

K3Bi2,5Eu2,5(PO4)6

x70

4

5

6

7

8

Photon energy (eV)

9

10

Fig. 4. The PL excitation spectra of undoped and Eu-doped K3Bi5(PO4)6 crystals. For undoped samples: T ¼ 8 K; for Eu-doped: lem ¼ 614 nm, T ¼ 8 (lines) and 300 K (lines with circles).

spectrum, see Fig. 4). Similarity between the excitation spectra of the host-related and the RE-related PL emission in studied compounds means that both types luminescence are mainly excited via the same mechanisms. The luminescence excitation mechanisms in undoped K3Bi5(PO4)6 and K2Bi(PO4)(MoO4) can include band-to-band transitions (host excitation) with subsequent transfer of the excitation energy by free charge carriers or excitons to the host-related emission centers. In this case, the peaks and dips in the excitation spectra can be formed by the near-surface losses (an example of formation of PL excitation spectra of a molybdate crystal in the region of band-to-band transitions can be found in Spassky et al. (2011)). The bands in PL excitation spectra of studied undoped crystals below the fundamental absorption edges can be formed by some localized transitions, most probably in Bi3þ ions which have defects in the nearest surrounding. As it was shown in our previous paper by the electronic structure calculations, the Bi s and Bi p electronic states substantially contribute to formation of the electronic bands of both crystals at the tops of the valence bands and at the bottoms of the conduction bands respectively (Hizhnyi et al., 2014). So, Bi3þ ions can easily form additional electronic states in the crystal bandgaps if some uncontrolled defects (impurities) are located near them. Such defect-related formations can be responsible for the excitation bands below the fundamental absorption edges and can be the centers of the host luminescence. For explanation of the observed spectral features, it is convenient to consider the following simplified schemes of the RE3þ- related PL processes in doped K3Bi5(PO4)6 and K2Bi(PO4)(MoO4) crystals: 1) Intra-center excitation of Bi3þ ions (PL emission centers related to blueegreen components) with subsequent transfer of excitation energy to RE3þ ions: 3þ 3þ a) (Bi3þ)* / Bi3þ þ hnBi þ hnRE lum / (RE )* / RE lum e excitation energy can be transferred via re-absorption of the PL emission light from Bi3þ centers (PL emission bands of

Fig. 3. The PL emission spectra of undoped and Eu-doped K3Bi5(PO4)6 crystals, T ¼ 8 K, Eex are indicated in the figure.

Y. Hizhnyi et al. / Radiation Measurements 90 (2016) 314e318

studied crystals spectrally overlap with f-f excitation bands of RE3þ ions (Nedilko et al., 2013; Souza et al., 2010)) or b) (Bi3þ)* / (RE3þ)* / RE3þ þ hnRE lum - excitation energy can be transferred without luminescence emission of Bi3þ centers (by resonant energy transfer). In this case, the Bi3þ ions where the excitation takes place must be situated close to emitting RE3þ ions. 2) Excitation of RE3þ centers is mediated by host excitations involving band-to-band transitions: host excitation / (RE3þ)* / RE3þ þ hnRE lum e the transfer of the excitation energy can be mediated, in particular by excitons or free charge carriers. 3) Intra-center excitation of RE3þ centers either by f-f inner-shell, or by O e RE charge transfer (CT) transitions:



 RE3þ /RE3þ þ hnRE lum :

Similarity of PL excitation spectra of the Eu- and Tb-doped samples to corresponding spectra of undoped K2Bi(PO4)(MoO4) obviously points to unfavorable scenario for process 3). The O e Eu and O e Tb CT transitions in oxide crystals usually have too different transition energies (the difference can reach several eV (Dorenbos, 2009)) to form the same excitation spectra in K2Bi(PO4)(MoO4). Process 1a) can take place at 8 K, but is not possible at room temperature since the host-related luminescence is quenched in 50e200 K temperature region (Hizhnyi et al., 2014). Process 1b) is possible at 8 K and also at 300 K. So, at 300 K processes 1b) represent probable scheme of RE3þ ions excitation below the fundamental absorption edges. The components observed in the excitation spectra above the edges (in particular, peaks at 5.2 and 6.1 eV) can be formed by band-to-band electronic transitions (process 2). The excitation component at 4.2 eV that disappeared for the REdoped crystals at 300 K can be related to intra-center excitation of Bi3þ ions, i. e. process 1b). Another origin can be also assumed for this low-energy excitation band. Taking into account its energy position with respect to Eg and disappearance at room temperature, this component can be attributed to the excitonic e type excitations of the crystal host. Considering the mentioned above data on the electronic structure of compounds, we can suppose that the excitons are formed on the base of bismuth e oxygen polyhedrons BiO8. An unambiguous assignment of the excitation band at 4.2 eV band can be done only after analysis of additional supporting experimental and theoretical data. The excitation spectra of K2Eu(PO4)(MoO4) reveal three components above 4.5 eV (see Fig. 2). The energy positions of these components differ from the K2Bi(PO4)(MoO4) and K2Bi(PO4)(MoO4):Eu cases. So, the excitation components of K2Eu(PO4)(MoO4) are probably not related to Bi3þ ions. The similarity of the PL excitation spectra of K3Bi5(PO4)6 and K3Bi5(PO4)6:Eu (1%) resembles the K2Bi(PO4)(MoO4) and K2Bi(PO4)(MoO4):Eu case, and therefore can inspire an analogous inference about origin of the PL excitation spectral components observed at 4.7, 5.2 and 6.1 eV for undoped K3Bi5(PO4)6 (see above). However in K3Bi5(PO4)6:Eu case, host-related emission is not completely suppressed: the Eu3þ centers and the host centers give approximately the same contributions to the luminescence signal at 614 nm (see Fig. 3). This fact in some measure opposes making an unambiguous assumption on similarity of the excitation mechanisms of the Eu-related and the host-related luminescence in K3Bi5(PO4)6:Eu crystal. Three components are also revealed in PL excitation spectra of K3Bi4Eu(PO4)6 and K3Bi2.5Eu2.5(PO4)6 crystals above 4.5 eV. While

317

the most intensive component is observed at the same energy for both crystals (4.7 eV), the other two components are shifted to higher energies with increasing Eu concentration (see notations in Fig. 2). Positions of the components in K3Bi2.5Eu2.5(PO4)6 spectra become close to the K2Eu(PO4)(MoO4) case (compare corresponding curves in Figs. 2 and 4). Taking into account analogous coordination of Eu3þ ions in K2Bi(PO4)(MoO4) and K3Bi4Eu(PO4)6 hosts (Bi ions form BiO8 polyhedra in both crystals (Hizhnyi et al., 2014), K2Bi(PO4)(MoO4) and K2Eu(PO4)(MoO4) are isostructural (Zatovsky et al., 2006; Ryumin et al., 2010)), we presume that the mentioned three components in K3Bi4Eu(PO4)6 and K3Bi2.5Eu2.5(PO4)6 spectra are formed by the same mechanisms as in K2Eu(PO4)(MoO4), most probably by the intra-center O e Eu CT transitions. The 4.2 eV band in excitation spectra of K3Bi4Eu(PO4)6 may arise due to excitations of some Bi-related centers since this band disappears in crystals with lower content of Bi ions, K3Bi2.5Eu2.5(PO4)6. More substantiated inferences on formation of the excitation spectra of Eu-rich crystals require additional studies. Comparison of the blueegreen PL excitation spectra of the undoped K3Bi5(PO4)6 and K2Bi(PO4)(MoO4) crystals (see upper parts of Figs. 2 and 4) can provide some additional information on formation of the spectra in both compounds. The Figures indicate a high degree of similarity between the spectra of two crystals. The 5.2 and 6.1 eV components are manifested in spectra of both compounds regardless of the lem value. Such similarity indicates that 5.2 and 6.1 eV components are formed in K3Bi5(PO4)6 and K2Bi(PO4)(MoO4) by analogous mechanisms. The crystals have different Eg values and differ in composition. The K2Bi(PO4)(MoO4) crystal contains MoO4 groups and the electronic states of the groups contribute to the upper part of the valence band and the lower part of conduction band of the crystal (Hizhnyi et al., 2014). So, if the excitation of luminescence in undoped K2Bi(PO4)(MoO4) was mainly related to MoO4 groups, one should expect different components (in particular, peaking at different energies) in the excitation spectra of K3Bi5(PO4)6 and K2Bi(PO4)(MoO4). The excitation spectra should demonstrate different components also if the peaks and gaps in these spectra were formed by the near-surface losses, since in this case the spectra depend on peculiarities of absorbance and reflectance spectra of the crystals. The 0.4 eV difference in the Eg values guarantees much difference between absorbance (reflectance) spectra of K3Bi5(PO4)6 and K2Bi(PO4)(MoO4). Taking into account these speculations we suppose that the excitation mechanism of host luminescence in K3Bi5(PO4)6 and K2Bi(PO4)(MoO4) in 4.8e7 eV region of the excitation photon energies can be related to common structural elements of both compounds, most probably PO3 phosphate groups. Further 4 research is necessary for reliable clarification of the origin of the 5.2 and 6.1 eV excitation bands in both compounds. The red PL emission component of K3Bi5(PO4)6 is probably defect-related since it is effectively excited below the bandgap energy (see Fig. 2, curve for lem ¼ 700 nm). The excitation spectrum of this component is similar to both spectra of K2Bi(PO4)(MoO4) at energies above 4.2 eV. So taking into account analogous coordination of Bi3þ ions in both compounds, there is a full reason to relate the red PL emission component of K3Bi5(PO4)6 to luminescence of Bi3þ ions in oxygen polyhedron (in our previous work (Hizhnyi et al., 2014) the origin of this component was not identified due to lack of experimental data). In this case, the 4.0 and 4.4 eV excitation components can be attributed to the intra-center excitation of such defect centers. 4. Conclusions Comparative analysis of the PL spectroscopic properties of undoped and RE-doped (RE ¼ Eu, Tb) K3Bi5(PO4)6 and

318

Y. Hizhnyi et al. / Radiation Measurements 90 (2016) 314e318

K2Bi(PO4)(MoO4) crystals allows making the following conclusions on formation of the PL spectra of studied compounds in 3.7e7 eV region of the excitation photon energies: 1. The excitation mechanisms of host luminescence of K3Bi5(PO4)6 and K2Bi(PO4)(MoO4) crystals below 4.8 eV are related to Bi3þ ions in oxygen surrounding. 2. The excitation mechanisms of host luminescence in K3Bi5(PO4)6 and K2Bi(PO4)(MoO4) in 4.8e7 eV region can be related to common structural elements of both compounds, most probably PO3 4 phosphate groups. 3. An efficient energy transfer from the Bi3þ-related luminescence centers to the emitting RE centers exists in K2Bi(PO4)(MoO4):Eu, K2Bi(PO4)(MoO4):Tb and K3Bi5(PO4)6:Eu crystals with low concentration of the dopants (1%). 4. The PL excitation spectra in potassium bismuth phosphate crystals with high concentration of Eu dopants, K3Bi4Eu(PO4)6 and K3Bi2.5Eu2.5(PO4)6 as well as in K2Eu(PO4) (MoO4) are formed by the O e Eu CT transitions. References Canibano, H., Boulon, G., Palatella, L., Guyot, Y., Brenier, A., Voda, M., Balda, R., Fernandez, J., 2003. J. Lumin. 102e103, 318e326. €ppe, H.A., 2012. Dalton Trans. 41, 12121e12128. Daub, M., Lehner, A.J., Ho

Dorenbos, P., 2009. J. Alloys Compd. 488, 568e573. He, X., Guan, M., Lian, N., Sun, J., Shang, T., 2010. J. Alloys Compd. 492, 452e455. Hizhnyi, Yu, Nedilko, S., Chornii, V., Nikolaenko, T., Zatovsky, I., Terebilenko, K., Boiko, R., 2013. Solid State Phenom. 200, 114e122. Hizhnyi, Yu A., Nedilko, S.G., Chornii, V.P., Slobodyanik, M.S., Zatovsky, I.V., Terebilenko, K.V., 2014. J. Alloys Compd. 614, 420e435. Huang, H., Chen, G., Wang, Sh, Kang, L., Lin, Zh, Zhang, Y., 2014. Mater. Res. Bull. 51, 455e459. Nedilko, S., Chornii, V., Hizhnyi, Yu, Scherbatskyi, V., Slobodyanik, M., Terebilenko, K., Boyko, V., Sheludko, V., 2013. Funct. Mater. 20, 29e36. Reshak, A.H., Auluck, S., Kityk, I.V., 2008. J. Solid State Chem. 181, 789e795. Reshak, A.H., Auluck, S., Kityk, I.V., 2008. Appl. Phys. A 91, 451e457. Ryumin, M.A., Pukhkaya, V.V., Komissarova, L.N., 2010. Russ. J. Inorg. Chem. 55, 1010e1013. Sanyasi Naidu, B., Vishwanadh, B., Sudarsan, V., Kumar Vatsa, R., 2012. Dalton Trans. 41, 3194e3203. Souza, E.R., Silva, I.G.N., Teotonio, E.E.S., Felinto, M.C.F.C., Brito, H.F., 2010. J. Lumin. 130, 283e291. Spassky, D.A., Vasil'ev, A.N., Kamenskikh, I.A., Mikhailin, V.V., Savon, A.E., Hizhnyi, Yu.A., Nedilko, S.G., Lykov, P.A., 2011. J. Phys. Condens. Matter 23, 365501 (10 pp). Terebilenko, K.V., Zatovsky, I.V., Slobodyanik, N.S., Domasevitch, K.V., Pushkin, D.V., Baumer, V.N., Sudavtsova, V.S., 2007. J. Solid State Chem. 180, 3351e3359. Voda, M., Balda, R., Saez de Ocariz, I., Lacha, L.M., Illarramendi, M.A., Fernandez, J., 1998. J. Alloys Compd. 275e278, 214e218. Voda, M., Iparraguire, I., Fernandez, J., Balda, R., Al-Saleh, M., Mendioroz, A., Lobera, G., Cano, M., Sanz, M., Azkargorta, J., 2001. Opt. Mater. 16, 227e231. Zatovsky, I.V., Terebilenko, K.V., Slobodyanik, N.S., Baumer, V.N., Shishkin, O.V., 2006. J. Solid State Chem. 179, 3550e3555. Zimmerer, G., 2007. Radiat. Meas. 42, 859e864.