Growth and spectroscopic properties of Tb3+ doped La2CaB10O19 crystal

Optical Materials 49 (2015) 27–31

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Growth and spectroscopic properties of Tb3+ doped La2CaB10O19 crystal Faxian Shan a,b, Ying Fu a,b, Guochun Zhang a,⇑, Tianxiang Xu c, Xinyuan Zhang a,b, Yicheng Wu a a Beijing Center for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China b University of Chinese Academy of Sciences, Beijing 100049, China c State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan 250100, China

a r t i c l e

i n f o

Article history: Received 31 May 2015 Received in revised form 16 August 2015 Accepted 24 August 2015

Keywords: Tb:La2CaB10O19 Crystal growth Refractive index Spectroscopy

a b s t r a c t Tb3+ doped La2CaB10O19 (Tb:LCB) crystal with dimensions of 60
26
22 mm3 has been grown by the top-seeded solution growth (TSSG) method. X-ray powder diffraction (XRPD) analysis shows that the introduction of Tb3+ does not change the structure of LCB crystal. Tb3+ content was determined by inductively coupled plasma optical emission spectrometry to be 3.76
1020 ions/cm3. The principal refractive indices of Tb:LCB crystal were accurately measured at room temperature by using the minimum deviation method, and Sellmeier equations were fitted. The unpolarized absorption and emission spectra were measured at room temperature by using X-, Y-, and Z-cut crystals. According to the Judd–Ofelt theory, the spontaneous transition probabilities, fluorescence branch ratio, and radiation lifetime of 5D4 state were calculated. The unpolarized and polarized emission properties under the 368 nm excitation were also evaluated. The decay time of the 543 nm emission corresponding to 5D4–7F5 transition was measured. The experimental results agree well with the calculated ones. Ó 2015 Published by Elsevier B.V.

1. Introduction In the past decades, much attention has been paid to rare earth borates for their excellent nonlinear optical properties. These materials include YAl3(BO3)4 (YAB) [1], Ca4YO(BO3)3 (YCOB) [2], Ca4GdO(BO3)3 (GdCOB) [3], Na3La9O3(BO3)8 (NLBO) [4], and La2CaB10O19 (LCB) [5], etc., which are also excellent luminescent host materials as La3+ and Y3+ ions could be easily replaced by other rare earth active ions without charge imbalance and large structure distortion. In particular, rare earth borates have potential capability in achieving self-frequency doubling [6–9]. Most studies about rare earth active ions are focused on Nd3+, but the emphasis on other ions is increasing due to their special optical properties. Tb3+ doped materials could emit the sharp and intense fluorescence in the yellow-green range, which are considered to be promising phosphor materials for practical use [10]. In addition, a recent study [11] has shown that Tb3+ has potential to provide a visible four-level laser system as its energy-level structure is similar to that of Nd3+. However, there are very few reports on the spectroscopic properties of Tb3+ doped rare earth borates, except for some primary studies on Tb:GdCOB [12] and Tb:YAB [13] crystals. The exploration and use of Tb3+ doped materials demand further research on their spectroscopic properties. Since the ⇑ Corresponding author. E-mail address: [email protected] (G. Zhang). http://dx.doi.org/10.1016/j.optmat.2015.08.020 0925-3467/Ó 2015 Published by Elsevier B.V.

spectroscopic characteristic is sensitive to host lattice [10], it is also significant to investigate Tb3+ luminescence in other promising host materials. LCB is a promising nonlinear optical crystal with excellent optical properties, such as a large nonlinear optical coefficient, a wide transparency range, a high laser damage threshold, and a moderate birefringence [14,15]. It also exhibits good mechanical properties and high chemical stability [16], which can be kept for many years in air. To our knowledge, Tb3+ doped LCB crystal are not available in previous literatures. In this work, we report on the crystal growth and optical properties of Tb:LCB. The refractive indices were measured at room temperature, and Sellmeier equations were fitted. Besides, the spectral properties in different direction were characterized by absorption and emission spectra, and spectroscopic parameters were calculated on the basis of the Judd–Ofelt theory [17,18]. The fluorescence decay time of 5D4–7F5 transition in Tb:LCB crystal has also been reported. 2. Experimental 2.1. Crystal growth The following reagents were used as obtained: Tb4O7 (Changchun Haipurui Rare Earth Material Technology Co., Ltd., 99.99%), La2O3 (Shanghai Yuelong New Material Co., Ltd., 99.99%), Li2CO3 (Xinjiang Research Institute of Nonferrous Metals, 99.99%), CaCO3

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F. Shan et al. / Optical Materials 49 (2015) 27–31

(Xilong Chemical Co., Ltd., 99%), and H3BO3 (Tianjin Fengchuan Chemical Reagent Co., Ltd., 99.999%). Single crystals were grown by the top-seeded solution growth (TSSG) method from the CaB4O7–Li2B4O7–B2O3 flux system, which was considered to be suitable for growing high quality LCB crystals [19]. Compared with the previously used flux CaB4O7 [5], the introduction of Li2B4O7 could significantly lower the growth temperature and evaporation degree, and eliminate the uncontrolled LaB3O6 crystallization at the bottom of the crucible [19–21]. A mixture (total mass 1940.30 g) of Tb4O7, La2O3, Li2CO3, CaCO3, and H3BO3 in molar ratio of 1:18:46:40:560 were ground in an agate mortar and melted into a platinum crucible with a diameter of 95 mm and a height of 90 mm in several batches, and the height of the melt was up to 80 mm. The crucible was placed in a vertical cylindrical electric furnace having nickel–chrome alloy wire heating and equipped with a Pt–Rh/Pt thermo-couple based Al-708P controller with an accuracy of ±0.1 °C. The sample was heated up to 1000 °C and stirred by a platinum stirrer for 48 h to ensure its homogenization. The saturation temperature (962 °C) was determined by the tentative seed crystal method. A seed with [1–10] orientation was slowly dipped into the solution at 980 °C, and kept for half an hour to dissolve its outer surface. After that the temperature was lowered to 962 °C in half an hour, and then decreased at a rate of 0.3 °C/day until the end of growth. The growing crystal was rotated at 4 rpm. When the growth was finished after about a month, the crystal was drawn out of the solution and cooled to room temperature at a rate of 10 °C/h. A transparent parallelepiped crystal with size of 60
26
22 mm3 is shown in Fig. 1. 2.2. X-ray powder diffraction analysis X-ray powder diffraction (XRPD) analysis of as-grown crystal was performed on an automated Bruker D8 X-ray diffractometer equipped with a diffracted monochromator set for Cu Ka radiation. Powder diffraction data were collected in the 2h range of 5–80° with a scanning step width of 0.02° and a counting time of 0.1 s. The Tb3+ concentration was determined by inductively coupled plasma optical emission spectrometry (Varian 710 ICP-OES). The sample was cut near the growth starting position of the crystal. 2.3. Measurement of refractive indices The refractive index is an essential parameter for optical materials. The principal refractive indices of Tb:LCB crystal at wavelengths of 0.365, 0.4047, 0.4358, 0.48, 0.5461, 0.5876,

Fig. 1. Photograph of Tb:LCB crystal grown by the top-seeded solution growth method.

0.6438, and 0.7065 lm were accurately measured at room temperature by using the minimum deviation method with a refractive index measurement instrument (SpectroMaster UV–vis–SWIR–IR, Trioptics, Germany) with a high accuracy of 2
106. The experimental details about the measurement of refractive index could be found in former works [14–16]. 2.4. Spectral measurements The as-grown crystal was cut into three rectangular wafers with dimensions of 4 mm
4 mm
2.5 mm ([Y
Z
X], [X
Z
Y], [X
Y
Z]), and the 4 mm
4 mm facets were polished on both sides to 2 mm in thickness. (X, Y, and Z are the three spindles of principal refractive index.) The absorption spectra of the crystals were measured at room temperature with a Lambda 900 UV–vis–NIR spectrophotometer over the wavelength range of 270–700 nm. Emission spectra from 450 to 700 nm were measured at room temperature by a FLS920 spectrofluorimeter (Edinburg Instrument). The crystals were excited by a 450 W stable Xenon lamp at wavelength of 368 nm, and emission signal was recorded by an InGaAs solid-state detector with a resolution of <0.09 nm. The fluorescence decay time for 5D4–7F5 transition monitored by 543 nm and excited by 368 nm was also measured at room temperature with the same spectrofluorimeter. 3. Results and discussion 3.1. XRPD Fig. 2 represents the XRPD data of Tb:LCB crystal and standard LCB crystal. It reveals that the diffraction peaks of Tb:LCB are consistent with the ones of LCB (JCPDS Card file NO. 54-0033). No impurity phases were found. The monoclinic unit-cell parameters of Tb:LCB crystal calculated with the software Jade [22] were found to be a = 10.98 Å, b = 6.54 Å, c = 9.07 Å, b = 91.51°, and V = 651.4 Å3. Compared with the parameters of LCB (a = 11.04 Å, b = 6.56 Å, c = 9.13 Å, b = 91.47°, and V = 661.4 Å3), the difference results from the different ionic radius of Tb3+ (109.5 pm) and La3+ (121.6 pm) [23]. The difference in ionic radius might bring thermal stress to the growing crystal and result in slightly cracking of the crystal (Fig. 1). The content of Tb3+ was determined to be 3.76
1020 ions/cm3. The segregation coefficient was calculated to be 0.82 with the formula keff = c1/c2, where c1 and c2 are the concentration of Tb3+ at the growth starting position and in the melt, respectively. Further experiment shows that the crystal is relatively uniform, which might result from the large segregation coefficient, the mild

Fig. 2. X-ray powder diffraction data of Tb:LCB crystal and standard LCB crystal.

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F. Shan et al. / Optical Materials 49 (2015) 27–31

temperature gradient [24,25], the low growth speed, and the large amount of flux. 3.2. Refractive indices and Sellmeier equations The measured principal refractive indices are listed in Table 1 (k is the wavelength in micrometer, and nx, ny, and nz are three principal refractive indices.). The refractive index of Tb3+ doped crystal is comparable to that of LCB crystal [14–16]. The Sellmeier equations (Eq. (1)) were obtained with the least-square-fit method, where k is the wavelength in micrometer. Based on these equations, refractive indices at other wavelengths could be calculated, which would benefit the calculation of spectral parameters.

n2x ¼ 2:77709 þ n2y ¼ 2:78236 þ n2z ¼ 2:95280 þ

0:01764 2

k  0:00923 0:01635 2

k  0:01251 0:02303 k2  0:00524

 0:00915k2

ð1aÞ

 0:01413k2

ð1bÞ

 0:00936k2

ð1cÞ

3.3. Absorption spectra and Judd–Ofelt theory analysis The absorption spectra of X-, Y-, and Z-cut crystals in the wavelength range of 270–700 nm are shown in Fig. 3. Although LCB belongs to monoclinic system, Fig. 3 shows that the positions of dominant absorption in the three absorption curves are nearly isotropic. However, the dominant absorption intensities and the characteristics of shoulder peak are anisotropic. The absorption shows no strong anisotropy, which might result from the very poor absorption of Tb3+ [26]. The dominant absorption bands corresponding to the 4f electronic transitions of Tb3+ from the ground state 7F6 to excited states are very weak when compared with those of other trivalent rare earth ions such as Pr3+, Yb3+, and Dy3+ [27–29]. The main absorption peaks were labeled by the usual SLJ designation [30], as shown in Fig. 3 and Table 2. The strongest absorption occurs at about 368 nm, which was selected as the excitation wavelength. The radioactive transition of 4fn configuration of a rare earth ion can be analyzed by the Judd–Ofelt approach. According to their theory, the calculated line strengths Scal ed of the electric-dipole (ED) transition from an initial state to a final state are given by 0 Scal ed ðJ; J Þ ¼

X

Xt jhðS; LÞJjjU ðtÞ jjðS0 ; L0 ÞJ0 ij2

ð2Þ

t¼2;4;6

where hðS; LÞJjjU ðtÞ jjðS0 ; L0 ÞJ 0 i is the reduced matrix element of the tensorial operator, which is independent of host materials and has been calculated by Carnall [30]. Xt (t = 2, 4, 6) are the three phenomenological intensity parameters. The experimental line strengths of ED transition Sexp ed can be obtained from the absorption spectrum:

Fig. 3. Unpolarized absorption spectra of X-, Y-, and Z-cut Tb:LCB crystals at room temperature.

0 Sexp ed ðJ; J Þ ¼

3hcð2J þ 1Þ 9n 2:3 8p3 e2 N ðn2 þ 2Þ2 kL

Z ODðkÞdk

ð3Þ

where  k is the mean wavelength of the absorption band, L is the crystal thickness, OD(k) is the measured optical density, N is the Tb3+ concentration, n is the corresponding refractive index of Tb: LCB crystal, e is the charge of electron, c is the velocity of light, and h is the Planck constant. The three phenomenological intensity parameters Xt (t = 2, 4, 6) were fitted by a least-square-fit method based on the above equations for X-, Y-, and Z-cut crystals, respectively. Table 2 shows the experimental and calculated line strengths of ED transition from the ground state 7F6 to excited states, and Table 3 shows the fitted results of Xt (t = 2, 4, 6). These spectroscopic parameters are anisotropic. The root-mean-squares deviation (rms-DS) about line strengths for the three crystals are 3.63
1023 cm2, 1.36
1022 cm2, and 9.75
1023 cm2, respectively. They are much less than the values of line strength, indicating that the calculated results agree well with the experimental ones. The spontaneous transition probability A from the excited state 5 D4 to other lower-lying state could be calculated with the following equations:

A ¼ Aed þ Amd

ð4Þ

Aed ðJ; J 0 Þ ¼

64p4 e2 nðn2 þ 2Þ 0cal S 3hk3 9ð2J þ 1Þ ed

ð5Þ

Amd ðJ; J 0 Þ ¼

64p4 e2 n3 S0cal 3hk3 2J þ 1 md

ð6Þ

2

0 S0cal md ðJ; J Þ ¼



h 4pmc

2

jhðS; LÞJjjL þ 2SjjðS0 ; L0 ÞJ 0 ij2

ð7Þ

where Aed is the contribution from ED, Amd is the contribution from Table 1 Principal refractive indices (nx, ny, and nz) of Tb:LCB crystal at room temperature. k (lm)

nx

ny

nz

0.3650 0.4047 0.4358 0.4800 0.5461 0.5876 0.6438 0.7065

1.708264 1.699922 1.694973 1.689627 1.683919 1.681099 1.678394 1.675858

1.707606 1.699460 1.694646 1.689424 1.683860 1.681256 1.678445 1.675979

1.769582 1.759722 1.753736 1.747289 1.740244 1.737001 1.733634 1.730495

magnetic-dipole (MD), S0cal ed was calculated based on Xt and reduced matrix elements [10], S0cal md is the calculated line strength of MD transition, and m is the mass of electron. Eq. (7) shows that S0cal md does not change with the host lattice. Therefore we adopted the values reported in Ref. [10]. The fluorescence branch ratio bc and the radiation lifetime srad of the excited state 5D4 were calculated and summarized in Table 4. From this table, we can see that the 5D4–7F5 emission in all crystals exhibits the largest branch ratio (about 50%), and 5D4 state has a long radiation lifetime about 2 ms.

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F. Shan et al. / Optical Materials 49 (2015) 27–31

Table 2 Experimental and calculated line strengths of ED transition. Transitions 7F6?

X-cut

5

I8 H6 5 H7 5 L7 5 L9 + 5G4 + 5D2 5 L10 + 5G6 + 5D3 5

 k (nm)

Sexp ed

283 301 316 339 351 368

8.737 2.813 5.653 2.872 8.146 14.928

Y-cut 22

(10

2

cm )

Scal ed

22

(10

Sexp ed

2

cm )

8.798 2.692 5.961 1.751 9.137 14.287

Z-cut 22

(10

13.779 3.543 8.712 4.276 10.074 22.073

2

cm )

Scal ed

(10

14.143 3.745 9.215 2.284 12.432 20.337

22

2

cm )

Sexp (1022 cm2) ed

22 cm2) Scal ed (10

12.135 3.959 6.650 3.862 10.967 20.370

12.146 3.286 7.330 2.320 12.523 19.401

Table 3 Phenomenological intensity parameters Xt (t = 2, 4, 6) for X-, Y-, and Z-cut crystals, respectively. Intensity parameters (X
1020 cm2)

X-cut

Y-cut

Z-cut

X2 X4 X6

4.319 9.386 1.211

6.909 16.400 1.492

4.460 13.104 1.652

Table 4 Calculated and experimental luminescence parameters. Transitions 5 D4? 7

F0 7 F1 7 F2 7 F3 7 F4 7 F5 7 F6

X-cut

Y-cut bc (%)

A (s1)

bc (%)

A (s1)

bc (%)

22.240 26.976 19.297 30.977 22.481 196.443 61.725

5.850 7.096 5.076 8.149 5.914 51.677 16.237

38.931 47.222 32.245 46.675 36.118 278.304 101.786

6.697 8.124 5.547 8.030 6.214 47.878 17.511

34.697 42.093 25.936 38.790 32.871 228.265 92.499

7.008 8.501 5.238 7.834 6.639 46.100 18.681

srad = 2.63 ms sexp = 1.96 ms g = 74.7%

AðJ; J 0 Þ bc ¼ P 0 J 0 AðJ; J Þ 1 AðJ; J0 Þ J

srad ¼ P

Z-cut

A (s1)

0

srad = 1.72 ms sexp = 1.52 ms g = 88.3%

Fig. 4. Unpolarized emission spectra of X-, Y-, and Z-cut Tb:LCB crystals under excitation at 368 nm at room temperature.

srad = 2.02 ms sexp = 1.89 ms g = 93.8%

ð8Þ

ð9Þ

3.4. Emission spectra Fig. 4 displays the unpolarized emission spectra of three Tb:LCB crystals under the 368 nm excitation at room temperature. There are four obvious emission bands at about 489 nm, 543 nm, 587 nm, and 623 nm, respectively. They could be assigned to the transitions from 5D4 state to 7F6, 7F5, 7F4, and 7F3 state, respectively. The span of emission peaks is caused by the Stark splitting in the crystal-field. The two main emissions correspond to the 5D4–7F6 and 5D4–7F5 transitions, which is in good agreements with the calculated results in Table 4. In particular, the emission centered at 543 nm exhibits an extremely high intensity, resulting in a strong green emission in our experiment. The unpolarized emission crosssection rem (calculated by the Fuchtbauer-Landenburg (FL) formula [31]) at 543 nm for X-, Y-, and Z-cut crystals are 1.44
1021 cm2, 1.42
1021 cm2, and 1.22
1021 cm2, respectively. In addition, the full-width at half-maximum (FWHM) of the 543 nm emission in three crystals are 5.51 nm, 5.58 nm, and 5.82 nm, respectively. Due to the high similarity of unpolarized emission spectra, polarized emission spectra may help to get a better understanding of the emission characteristics. The polarized emission spectra of

Fig. 5. Polarized emission spectra of X-, Y-, and Z-cut Tb:LCB crystals under excitation at 368 nm at room temperature.

F. Shan et al. / Optical Materials 49 (2015) 27–31 Table 5 Emission intensity of 5D4–7F5 transition and its integral (Sem) along the wavelength in polarized emission spectra. 5

D4–7F5 emission

Intensity (104) Sem (105 nm)

X-cut

Y-cut

Z-cut

E//Y

E//Z

E//X

E//Z

E//X

E//Y

4.152 2.612

10.132 4.852

4.429 2.403

9.095 4.585

8.155 4.677

3.975 2.220

31

room-temperature Sellmeier equations have been fitted. The spectroscopic properties oriented in X, Y, and Z directions have been investigated through absorption and emission spectra at room temperature. Based on the Judd–Ofelt theory, the intensity parameters Xt (t = 2, 4, 6), the spontaneous transition probability A, fluorescence branch ratio bc, radiation lifetime srad, and emission crosssection rem have been calculated. The experimental results agree well with the calculated ones. The experimental emission cross-section rem, FWHM, and florescence lifetime sexp for 5 D4–7F5 transition in X-, Y-, and Z-cut crystals are 1.44
1021 cm2, 5.51 nm, 1.96 ms; 1.42
1021 cm2, 5.58 nm, 1.52 ms; and 1.22
1021 cm2, 5.82 nm, 1.89 ms; respectively. The 5D4–7F5 transition exhibits large branch ratio and high luminescence quantum efficiency. In addition, the polarized emission spectra show that when the applied electric field along the Z direction, the 5D4–7F5 emission has the highest intensity. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Nos. 51132008 and 91422303). References

Fig. 6. Fluorescence decay curves for the 5D4–7F5 transition in X-, Y-, and Z-cut Tb: LCB crystals.

X-, Y-, and Z-cut crystals are displayed in Fig. 5, where E represents the electric field of incident light. The intensity of the most intense emission peak at about 543 nm and its integral (Sem) along the wavelength were calculated and summarized in Table 5. It can be found that when the applied electric field along the Z direction, 5 D4–7F5 transition has the largest intensity and emission crosssection. The results would help to obtain a higher power luminescence because a larger emission cross-section facilitates the increase in output power. The fluorescence decay curves for the 5D4–7F5 transition in three crystals have been recorded, and the experimental lifetime sexp have been fitted, as shown in Fig. 6. The experimental lifetime sexp and luminescence quantum efficiency g are summarized at Table 4. X- and Z-cut crystals have longer life time, but Y- and Z-cut crystals have higher efficiency. Tb:LCB crystal has a florescence lifetime longer than 1.5 ms, comparable to that of Tb:GdCOB (2.7 ms) [12] and Tb:MgAl2O4 (2.0 ms) [32]. In addition, the high quantum efficiency of 5D4–7F5 transition indicates that Tb:LCB crystal is efficient for the generation of green light at 543 nm. 4. Conclusions In TSSG

summary, Tb:LCB crystal has been grown by the method from CaB4O7–Li2B4O7–B2O3 flux system. Its

[1] A. Brenier, D. Jaque, A. Majchrowski, Opt. Mater. 28 (2006) 310–323. [2] J. Luo, S.J. Fan, H.Q. Xie, K.C. Xiao, S.X. Qian, Z.W. Zhong, G.X. Qian, R.Y. Sun, J.Y. Xu, Cryst. Res. Technol. 36 (2001) 1215–1221. [3] G. Aka, E. Reino, P. Loiseau, D. Vivien, B. Ferrand, L. Fulbert, D. Pelenc, G. LucasLeclin, P. Georges, Opt. Mater. 26 (2004) 431–436. [4] P. Peshev, S. Pechev, V. Nikolov, P. Gravereau, J.P. Chaminade, D. Binev, D. Ivanova, J. Solid State Chem. 179 (2006) 2834–2849. [5] Y.C. Wu, P.Z. Fu, F. Zheng, S.M. Wan, X.G. Guan, Opt. Mater. 23 (2003) 373–375. [6] D. Vivien, P. Georges, Opt. Mater. 22 (2003) 81–83. [7] F. Mougel, G. Aka, A. Kahn-Harari, D. Vivien, Opt. Mater. 13 (1999) 293–297. [8] R. Balda, V. Jubera, C. Frayret, S. Pechev, R. Olazcuaga, P. Gravereau, J.P. Chaminade, M. Al-Saleh, J. Fernandez, Opt. Mater. 30 (2007) 122–125. [9] W.P. Chen, L. Li, H.B. Liang, Z.F. Tian, S. Qiang, G.B. Zhang, Opt. Mater. 32 (2009) 115–120. [10] T. Hoshina, Jpn. J. Appl. Phys. 6 (1967) 1203–1211. [11] M. Sekita, Y. Miyazawa, S. Morita, H. Sekiwa, Y. Sato, Appl. Phys. Lett. 65 (1994) 2380–2382. [12] G. Dominiak-Dzik, A. Pajaczkowska, S. Golab, S. Baba, W. Rybe-Romanowski, Sarjono, J. Alloys Compd. 341 (2002) 144–149. [13] J. Li, J.Y. Wang, X.F. Cheng, X.B. Hu, X.Q. Wang, S.R. Zhao, Mater. Lett. 58 (2004) 1096–1099. [14] F.L. Jing, P.Z. Fu, Y.C. Wu, Y.L. Zu, X. Wang, Opt. Mater. 30 (2008) 1867–1872. [15] N.X. Zhai, Y. Li, G.C. Zhang, Y.C. Wu, C.T. Chen, Opt. Express 21 (2013) 20641– 20648. [16] J.X. Zhang, L.R. Wang, Y. Wu, G.L. Wang, P.Z. Fu, Y.C. Wu, Opt. Express 19 (2011) 16722–16729. [17] B.R. Judd, Phys. Rev. 127 (1962) 750–761. [18] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511–521. [19] F.L. Jing, Y.C. Wu, P.Z. Fu, J. Cryst. Growth 292 (2006) 454–457. [20] A. Majchrowski, A. Mandowska, I.V. Kityk, J. Kasperczyk, M.G. Brik, I. Sildos, Curr. Opin. Solid State Mater. Sci. 12 (2008) 32–38. [21] A.M. El-Naggar, N.S. Alzayed, A. Majchrowski, L. Jaroszewicz, M.G. Brik, W. Kuznik, I.V. Kityk, J. Cryst. Growth 334 (2011) 122–125. [22] J.X. Li, D.Y. Song, J.Y. Zhang, C.G. Zheng, Rock Miner. Anal. 27 (2008) 189–192. [23] R.D. Shannon, Acta Crystallogr. Sect. A 32 (1976) 751–767. [24] A. Majchrowski, M.T. Borowiec, E. Michalski, J. Cryst. Growth 264 (2004) 201– 207. [25] D. Kasprowicz, M.G. Brik, A. Majchrowski, E. Michalski, E. Mykowska, J. Lumin. 130 (2010) 623–630. [26] G. Lakshminarayana, J.R. Qiu, M.G. Brik, I.V. Kityk, J. Phys.: Condens. Matter 20 (2008) 335106. [27] L.R. Jaroszewicz, A. Majchrowski, M.G. Brik, N. AlZayed, W. Kuznik, I.V. Kityk, S. Kłosowicz, J. Alloys Compd. 538 (2012) 220–223. [28] A. Majchrowski, S. Klosowicz, L.R. Jaroszewicz, M. Swirkowicz, I.V. Kityk, M. Piasecki, M.G. Brik, J. Alloys Compd. 491 (2010) 26–29. [29] S. Klosowicz, W. Kuznik, M.G. Brik, V. Kiisk, S. Lange, I. Sildos, L. Jaroszewicz, A. Majchrowski, I.V. Kityk, J. Alloys Compd. 520 (2012) 89–92. [30] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4447–4449. [31] B.F. Aull, H.P. Jenssen, IEEE J. Quantum Electron. 18 (1982) 925–930. [32] H. Nakagawa, E. Ebisu, M. Zhang, M. Kitaura, J. Lumin. 102 (2003) 590–596.