Preparation, crystal structure and up-conversion luminescence of Er³⁺, Yb³⁺ co-doped Gd₂(WO₄)₃

Abstract

Up-conversion (UC) phosphors Gd₂(WO₄)₃:Er³⁺/Yb³⁺ were synthesized by a high temperature solid-state reaction method. The crystal structure of Gd₂(WO₄)₃:3% Er³⁺/10% Yb³⁺ was refined by Rietveld method and it was showed that Er³⁺/Yb³⁺ were successfully doped into the host lattice replacing Gd³⁺. Under 980 nm laser excitation, intense green and weak red emissions centered at around 532 nm, 553 nm, and 669 nm were observed, which were assigned to the Er³⁺ ion transitions of ⁴H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2, respectively. The optimum Er³⁺ doping concentration was determined as 3 mol% when the Yb³⁺ concentration was fixed at 10 mol%. The pump power study indicated that the energy transfer from Yb³⁺ to Er³⁺ in Er³⁺, Yb³⁺ co-doped Gd₂(WO₄)₃ was a two-photon process, and the related UC mechanism of energy transfer was discussed in detail.

---

Introduction

Near-Infrared-to-visible (NIR) up-conversion is an optical process, when near infrared photons are converted into visible photons by multiphoton absorption, which is governed with the anti-Stokes rule.^1,2 The UC luminescence has attracted a lot of interest from researchers because of its wide potential applications, such as bio-labels, solar cells, optical data storage, displays and so on.^3–9 For UC phosphors, excellent fluorescence is always achieved by doping rare earth (RE) ions into luminescent hosts. Among various ions, Er³⁺ is the one of the most studied active ions. Since Er³⁺ has a long life of intermediates level and abundant 4f level structure, it can continuously emit green or red photons when excited by high energy photons.¹⁰ In contrast, Yb³⁺ ions are doped as a sensitizer, and it can enhance the near-infrared (around 980 nm) absorption and improve the UC luminescent efficiency of Er³⁺.

The host matrixes play a crucial role in the UC process. The good hosts can support doped ions in a fine crystalline field, in which energy transfer (ET) can easily take place in the host, and, thus, improve significantly the luminescence properties. To be more specific, host materials should be chemically stable and have low phonon energy to avoid efficiency loss via non-radiative transfer.¹¹ Tungstate crystals are among classic inorganic luminescent materials.¹² In 1896, the X-ray luminescence of CaWO₄ was discovered by Pupin.¹³ The tungstate crystals provide great mechanical strength and chemical and thermal stability.^14,15 Because of the strong covalent W–O bond in WO₄²⁻ groups, the tungstate can improve the averaged covalency of crystal, and, respectively, the solubility of rare earth ions can be enhanced.^16–19 Of those researches of tungstate doped with rare earth phosphors, most are single tungstate crystals similar to scheelite and double tungstate crystals ALn(WO₄)₂ (A = alkali metal ions, Ln = rare earth ions). While, there is less study on poly-tungstate.

Gadolinium tungstate, Gd₂(WO₄)₃, can be doped easily doped by Er³⁺/Yb³⁺ at Gd³⁺ position because of the similar ionic radius and electrovalence of Gd³⁺ and Er³⁺/Yb³⁺. Besides, the ionic radius of Gd³⁺ is larger than that of Er³⁺ and Yb³⁺, and it can be easily substituted by Er³⁺ and Yb³⁺.^20,21 Li et al.²² prepared Er³⁺/Yb³⁺ co-doped Gd₂(WO₄)₃ via a co-precipitation method, and investigated pumping-route-dependent concentration quenching and temperature effect on the phosphors. Sun et al.²³ prepared Er³⁺/Yb³⁺ co-doped Gd₂(WO₄)₃ and Gd₂WO₆ using co-precipitation method, and reported the up-converted emission differences between those two phosphors. However, the crystal structure of Gd₂(WO₄)₃ has not been reported, and how the doped ions affect the lattice parameters and structure of Gd₂(WO₄)₃ host has not been discussed.

In this research, Er³⁺/Yb³⁺ co-doped Gd₂(WO₄)₃ phosphors have been synthesized by a conventional high temperature solid-state reaction method. The crystal structure of Gd₂(WO₄)₃:3% Er³⁺/10% Yb³⁺ is refined by Rietveld method. And the structural characteristics and UC luminescent characteristics of phosphors are investigated. Also, the rare earth ion doping concentration effect on the UC luminescence and the UC mechanism of energy transfer are discussed.

Experimental

The Gd₂(WO₄)₃:xEr³⁺/0.1Yb³⁺ (x = 0, 0.001, 0.01, 0.02, 0.03, 0.04, 0.05) phosphors and Gd₂(WO₄)₃:0.03Er³⁺/yYb³⁺ (y = 0, 0.05, 0.1, 0.015, 0.2, 0.025) phosphors were synthesized by the high temperature solid-state reaction method. The starting materials of Gd₂O₃ (99.99%), Er₂O₃ (99.99%), Yb₂O₃ (99.99%), and WO₃ (A.R.) were mixed based on stoichiometric ratio and ground in an agate mortar. The mixtures were reacted in an aluminum crucible at 1050 °C for 12 hours in resistance furnace in the air. After the furnace cooled down naturally, the samples were ground for further test.

The phase composition of as-prepared phosphors was examined by X-ray diffraction measurement (XRD, D8 Advance diffractometer, Bruker, Germany, with Cu-Kα and linear VANTEC detector, λ = 0.15406 nm, 40 kV, 30 mA). The powder diffraction data of Gd₂(WO₄)₃:3% Er³⁺/10% Yb³⁺ for Rietveld analysis was collected at room temperature by the step size of 0.02° (2θ), and the counting time was 3 s per step. The Rietveld refinement was performed using package TOPAS 4.2.²⁴ The Fourier transform infrared spectrum (FT-IR) was recorded over the range of 4000–400 cm⁻¹ by an Excalibur 3100 (USA) device. The diffuse reflection spectra were measured by UV-NIR spectrophotometer (Cary 5000, USA). The UC luminescence spectra were collected at room temperature with a Hitachi F-4600 spectrophotometer equipped with an external power-controllable 980 nm semiconductor laser (Beijing Viasho Technology Company, China) as the excitation source.

Results and discussion

The XRD patterns of the as-prepared pure Gd₂(WO₄)₃, Gd₂(WO₄)₃:Er³⁺/Yb³⁺ and the standard PDF diffraction pattern of Gd₂(WO₄)₃ are shown in Fig. 1(a). All the diffraction peaks of the samples fitted well with the standard data of Gd₂(WO₄)₃ (JCPDS no. 23-1076), indicating that pure Gd₂(WO₄)₃ has been successfully synthesized by the high temperature solid-state reaction method at 1050 °C for 12 h. Besides, the Er³⁺ and Yb³⁺ cannot be detected, which shows that the Er³⁺ and Yb³⁺ are completely doped in the host lattice,²⁵ and did not change the crystal structure of Gd₂(WO₄)₃.^26,27 The measurement of the FT-IR spectra of pure Gd₂(WO₄)₃ is shown in Fig. 1(b). In pure Gd₂(WO₄)₃, the strong absorption peaks corresponding to the vibrations of WO₄²⁻ groups are between 400 and 1000 cm⁻¹. The absorption bands at around 734 and 837 cm⁻¹ are related to O–W–O stretch vibrations of WO₄²⁻ tetrahedron.^28,29 The 418 and 460 cm⁻¹ band can be attributed to the stretching vibration of W–O.³⁰

Fig. 1 (a) XRD patterns of Gd₂(WO₄)₃ and Er³⁺/Yb³⁺ co-doped Gd₂(WO₄)₃ and the standard data of Gd₂(WO₄)₃ (JCPDS 23-1076) as a reference; (b) FT-IR spectra of pure Gd₂(WO₄)₃.

The structure of Gd₂(WO₄)₃:3% Er³⁺, 10% Yb³⁺ is unknown, and, for the Rietveld refinement, the XRD pattern is indexed by monoclinic cell (C2/c) with parameters close to those of Eu₂(WO₄)₃: (ICSD #15877).^31,32 The refinement is stable and gives low R-factors (Table 1, Fig. 2). The atom coordinates and main bond lengths are listed in Tables 2 and 3 respectively. The crystal structure of Gd₂(WO₄)₃:3% Er, 10% Yb is depicted in Fig. 3. As seen from Fig. 3, the coordination number of Gd³⁺ ions in Gd₂(WO₄)₃ is eight. The ionic radii of Gd³⁺(CN = 8) = 1.053, while the ionic radii of dopants are IR(Yb³⁺, CN = 8) = 0.985, IR(Er³⁺, CN = 8) = 1.004, which are smaller and closer than IR(Gd³⁺, CN = 8).³³ Thus the Gd³⁺ ions are successfully replaced by Yb³⁺ and Er³⁺ ions, which should lead to the host cell parameters shrinkage. The crystallographic data and refinement parameters are shown in Table 1. The cell volume of Gd₂(WO₄)₃:Er³⁺, Yb³⁺ is V = 936.65(5) ų, compared with the stand cell volume of Gd₂(WO₄)₃, V = 938.15 ų,³⁴ which indicates that the cell volume decrease on the doping by Er³⁺ and Yb³⁺.

Table 1 Main parameters of processing and refinement of the Gd₂(WO₄)₃:3% Er, 10% Yb sample

Compound		Gd2(WO4)3:3% Er, 10% Yb
Sp. Gr.	C2/c	No. of reflections	492
a, Å	7.6541 (2)	No. of refined parameters	66
b, Å	11.4140 (3)	Rwp, %	12.66
c, Å	11.3909 (3)	Rp, %	8.67
β, °	109.744 (2)	Rexp, %	4.83
V, Å^3	936.65 (5)	χ^2	2.62
Z	1	RB, %	3.35
2θ-interval, °	5–100

---

Fig. 2 Difference Rietveld plot of Gd₂(WO₄)₃:3% Er, 10% Yb.

Table 2 Fractional atomic coordinates and isotropic displacement parameters (Ų) of Gd₂(WO₄)₃:3% Er, 10% Yb

Atom	x	y	z	Biso	Occ.
Gd	0.3272 (10)	0.3786 (4)	0.4069 (4)	0.3 (3)	0.87
Yb1	0.3272 (10)	0.3786 (4)	0.4069 (4)	0.3 (3)	0.1
Er1	0.3272 (10)	0.3786 (4)	0.4069 (4)	0.3 (3)	0.03
W1	0	0.1327 (3)	0.25	0.6 (3)	1
W2	0.1536 (7)	0.3921 (2)	0.0516 (2)	0.3 (3)	1
O1	0.168 (6)	0.047 (2)	0.223 (4)	1.5 (5)	1
O2	0.126 (6)	0.212 (3)	0.384 (4)	1.5 (5)	1
O3	0.223 (6)	0.324 (3)	0.199 (3)	1.5 (5)	1
O4	0.366 (7)	0.449 (3)	0.040 (3)	1.5 (5)	1
O5	0.060 (7)	0.463 (3)	0.423 (4)	1.5 (5)	1
O6	0.442 (6)	0.210 (3)	0.060 (4)	1.5 (5)	1

---

Table 3 Main bond lengths (Å) of Gd₂(WO₄)₃:3% Er, 10% Yb^a

a Symmetry codes: (i) −x + 1/2, y + 1/2, −z + 1/2; (ii) −x + 1/2, −y + 1/2, −z + 1; (iii) −x + 1, y, −z + 1/2; (iv) x, −y + 1, z + 1/2; (v) −x + 1/2, y − 1/2, −z + 1/2; (vi) −x, y, −z + 1/2.
(Gd, Yb, Er)–O1^i	2.43 (3)	W1–O1	1.72 (3)
(Gd, Yb, Er)–O2	2.40 (3)	W1–O2	1.75 (4)
(Gd, Yb, Er)–O2^ii	2.51 (4)	W2–O3	1.76 (3)
(Gd, Yb, Er)–O3	2.31 (4)	W2–O4	1.79 (4)
(Gd, Yb, Er)–O4^iii	2.36 (4)	W2–O5^v	1.93 (4)
(Gd, Yb, Er)–O4^iv	2.44 (3)	W2–O6^vi	1.70 (4)
(Gd, Yb, Er)–O5	2.33 (4)
(Gd, Yb, Er)–O6^iii	2.56 (3)

---

Fig. 3 The crystal structure of Gd₂(WO₄)₃:3% Er, 10% Yb.

The diffuse reflection of pure Gd₂(WO₄)₃, Yb³⁺ doped Gd₂(WO₄)₃, Er³⁺ doped Gd₂(WO₄)₃ and Yb³⁺/Er³⁺ co-doped Gd₂(WO₄)₃ are shown in Fig. 4. From the figure, pure Gd₂(WO₄)₃ does not show apparent absorption in the range of 300–1200 nm. The Er³⁺-doped Gd₂(WO₄)₃ possesses strong absorption at 520, 655, 795 nm, while weak absorption at 974 nm is observed. The absorption at 974 nm originated from ²F_7/2 → ²F_5/2 transition of Yb³⁺ ions is observed in Yb³⁺-doped Gd₂(WO₄)₃. Yb³⁺ and Er³⁺ co-doped Gd₂(WO₄)₃ sample is characterized by the apparent absorption at 520, 655, 795 and 974 nm. From the insets, it is obvious that Yb³⁺/Er³⁺ co-doped Gd₂(WO₄)₃ exhibits higher UC efficiency than that of Er³⁺-doped Gd₂(WO₄)₃, which demonstrates that the increasing absorption at 980 nm of Er³⁺ is mainly caused by the energy transition of Yb³⁺ to Er³⁺.³⁵

Fig. 4 Diffuse reflection spectra of pure Gd₂(WO₄)₃, Yb³⁺ doped Gd₂(WO₄)₃, Er³⁺ doped Gd₂(WO₄)₃ and Yb³⁺/Er³⁺ co-doped Gd₂(WO₄)₃, and the inset shows the green luminescence images of the phosphors when irradiated by a 980 nm diode ((a) Er³⁺ doped Gd₂(WO₄)₃, (b) Yb³⁺/Er³⁺ co-doped Gd₂(WO₄)₃).

The UC luminescence spectra of single Yb³⁺ doped and xEr³⁺/0.1Yb³⁺ (x = 0–0.05) co-doped Gd₂(WO₄)₃ under 980 nm near-infrared laser excitation at room temperature is shown in Fig. 5, and the inset shows the dependence of green UC emission intensity (at 532 nm and 553 nm) on Er³⁺ concentration. First of all, Yb³⁺-doped Gd₂(WO₄)₃ does not show the UC luminescence because of activator Er³⁺ ions absence. Due to concentration quenching effect,³⁶ UC luminescence green emission intensities increase firstly and, then, decrease approaching the maximum at 3 mol% of Er³⁺ content (Yb³⁺ concentration was fixed at 10 mol%). In the spectra, two strong green emission bands centered at 532 nm and 553 nm and a weak red emission band centered at around 669 nm are observed, which are assigned to the Er³⁺ ion transitions of ⁴H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2, respectively.^20,35,37

Fig. 5 Comparison of UC luminescence spectra of Gd₂(WO₄)₃:xEr³⁺/0.1Yb³⁺ under 980 nm laser excitation and the inset shows the intensity of the green emission as a function of Er³⁺ doping concentration and the enlarged red emission band.

The UC luminescence spectra of single Er³⁺ doped and 0.03 Er³⁺/yYb³⁺ (y = 0–0.25) co-doped Gd₂(WO₄)₃ under 980 nm near-infrared laser excitation are presented in Fig. 6, and the inset shows the variation of the UC luminescent intensity at 532 nm and 553 nm on the Yb³⁺ concentration increase. Similarly, when singly doped with Er³⁺ ion, the UC spectrum includes very weak UC emission, because Er³⁺ solely can hardly absorb the near-infrared excitation energy in the absence of sensitizer Yb³⁺ ions. With increasing concentration of Yb³⁺, the UC emission regions are enhanced. Moreover, it is also found that the two strong green emission bands and one weak red emission band are assigned to the Er³⁺ ion transitions.

Fig. 6 Comparison of UC luminescence spectra of Gd₂(WO₄)₃:0.03Er³⁺/yYb³⁺ under 980 nm laser excitation. The inset shows the intensity of the green emission as a function of Yb³⁺ doping concentration.

The UC mechanism can be explained by the dependence of UC emission intensity (I) on pump power (P), which follows the relation: I ∝ Pⁿ,^20,25,38 where n is the pump photon number required for the transition from ground state to upper emitting state. The number n is obtained from the slope of the fitting line of log I versus log P. Fig. 7 shows the UC luminescence spectrum of Gd₂(WO₄)₃:0.03Er³⁺/0.1Yb³⁺ with different pump powers, and the double-logarithmic plot of green and red UC emission intensities upon pump powers is shown in the inset. It is obvious that the UC luminescence intensity of the phosphors increases as the pump power increased. The calculated slopes are 2.12, 2.12 for the green emission (532 nm: ⁴H_11/2 → ⁴I_15/2, 553 nm: ⁴S_3/2 → ⁴I_15/2), and 1.48 for the red emission (669 nm: ⁴F_9/2 → ⁴I_15/2), indicating that both green emission and red emission are two-photon process.^39–41

Fig. 7 UC emission spectra of Gd₂(WO₄)₃:0.03Er³⁺/0.1Yb³⁺ with different pump power, and the inset shows the dependence of green and red UC emission intensities upon pump power.

The deviation from the theoretical value 2 for those special two-photon process may be caused by the crystal structure and the defect states inside the bandgap in the energy transition.^20,40 The n value lower than 2 for red emission at 669 nm could be due to the large UC rate for the depletion of the intermediate excited states, competition between linear decay, and the local thermal effect as well.²⁵ In the limit of infinitely small UC rates, the UC luminescence intensity for a special n-photon energy transfer tends to be proportional to n-th power of pump power (Pⁿ); while in the limit of infinitely large UC rate, the intensity is proportional to the pump power (P¹). Thus, the UC intensity which excited by the sequential absorption of n photons has a dependence of P^β on pump power, with β ranges from 1 to n.⁴² Besides, as the excitation power is increased, the slope may decrease, which may due to the self-focusing,⁴³ or the high excitation densities may lead to high non-radiative rates and high temperature in the internal samples, and then, the thermal effect causes the quenching of UC intensity.⁴²

According to the above results, the energy level diagram of the Er³⁺ and Yb³⁺ ions and the proposed UC mechanism in Er³⁺/Yb³⁺ co-doped Gd₂(WO₄)₃ are illustrated in Fig. 8. Under the 980 nm excitation, Yb³⁺ ion can be excited by one photon and transferred from the ground state ²F_7/2 to the excited state ²F_5/2. The Er³⁺ ion may be excited and transferred from the ground state ⁴I_15/2 to the excited state ⁴I_11/2 through the ground state absorption (GSA), or may be excited though the energy transfer (ET1) from Yb³⁺ ion. The second step of ET2₁ can promote an excited state absorption (ESA) of Er³⁺ from ⁴I_11/2 to the ⁴F_7/2 level.³⁸

Fig. 8 The energy level diagrams of Er³⁺ and Yb³⁺ ions, and the proposed UC mechanism in Gd₂(WO₄)₃:Er³⁺/Yb³⁺ phosphors.

Because of the small energy gap between the ⁴F_7/2, ⁴H_11/2 and ⁴S_3/2, the transition of Er³⁺ occurred rapidly from the ⁴F_7/2 to the ⁴H_11/2 and ⁴S_3/2 by non-radiative relaxation (NR).³⁵ Finally, the green emissions centered at 532 nm and 553 nm were produced through radiative transitions of ⁴H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2. The red emission centered at 669 nm was associated with ⁴F_9/2 → ⁴I_15/2 due to the NR from ⁴S_3/2 to the ⁴F_9/2 level or the ET2₂: ²F_5/2 (Yb³⁺) + ⁴I_13/2(Er³⁺) → ²F_7/2(Yb³⁺) + ⁴F_9/2(Er³⁺).²⁹

Conclusions

Er³⁺, Yb³⁺ single-doped and Er³⁺/Yb³⁺ co-doped Gd₂(WO₄)₃ phosphors were successfully synthesized by the high temperature solid-state method at 1050 °C for 12 h. The Er³⁺ and Yb³⁺ doping induced the lattice parameters decrease which proved the Gd³⁺ replacement by Er³⁺ and Yb³⁺ ions. Under the 980 nm laser excitation, the Er³⁺ or Yb³⁺ single-doped Gd₂(WO₄)₃ samples hardly show the UC luminescence. While the Er³⁺/Yb³⁺ co-doped Gd₂(WO₄)₃ phosphors exhibited remarkable green UC emission at 532 and 553 nm and the low-intensity red UC emission at around 669 nm, which were assigned to the characteristic level transition of ⁴H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 of Er³⁺ ion, respectively. The optimum Er³⁺ doping concentration was determined as 3 mol%. With the Yb³⁺ doping concentration increase, the UC emission intensities of Er³⁺/Yb³⁺ co-doped Gd₂(WO₄)₃ enhanced gradually. The power-dependent luminescent properties indicated that the energy transfer existed in both green and red emissions was a two-photon process. Generally, the results of the present study demonstrates that the Er³⁺/Yb³⁺ co-doped Gd₂(WO₄)₃ is an efficient green emission UC phosphor.

Acknowledgements

The present work was supported by the National Natural Science Foundations of China (Grant No. 51472223), the Fundamental Research Funds for the Central Universities (Grant No. 2652015008), and New Century Excellent Talents in University of Ministry of Education of China (Grant No. NCET-12-0951).

Notes and references

1. F. Auzel, Chem. Rev., 2004, 104, 139.
2. F. Wang and X. Liu, Chem. Soc. Rev., 2009, 38, 976–989.
3. L. Shi, C. Li, Q. Shen and Z. Qiu, J. Alloys Compd., 2014, 591, 105–109.
4. A. Patra, C. S. Friend, R. Kapoor and P. N. Prasad, J. Phys. Chem. B, 2002, 106, 1909.
5. E. A. Ferreira, F. C. Cassanjes and G. Poirier, Opt. Mater., 2013, 35, 1141–1145.
6. K. Teshima, S. Lee, N. Shikine, T. Wakabayashi, K. Yubuta, T. Shishido and S. Oishi, Cryst. Growth Des., 2011, 11, 995–999.
7. A. Shalav, B. S. Richards and M. A. Green, Sol. Energy Mater. Sol. Cells, 2007, 91, 829–842.
8. X. Li, D. Zhao and F. Zhang, Theranostics, 2013, 3, 292–305.
9. C. Guo, J. Yu, J.-H. Jeong, Z. Ren and J. Bai, Phys. B, 2011, 406, 916–920.
10. T. Li, C.-F. Guo, Y.-M. Yang, L. Li and N. Zhang, Acta Mater., 2013, 61, 7481–7487.
11. D. Vennerberg and Z. Lin, Sci. Adv. Mater., 2011, 3, 26–40.
12. Q.-H. Zeng, X.-G. Zhang, P. He, H.-B. Liang and M.-L. Gong, J. Inorg. Organomet. Polym. Mater., 2010, 25, 1009–1014.
13. G. Blasse, J. Lumin., 1997, 72–74, 129.
14. V. A. Morozov, A. Bertha, K. W. Meert, S. van Rompaey, D. Batuk, G. T. Martinez, S. van Aert, P. F. Smet, M. V. Raskina, D. Poelman, A. M. Abakumov and J. Hadermann, Chem. Mater., 2013, 25, 4387–4395.
15. C. Sung Lim, J. Phys. Chem. Solids, 2015, 78, 65–69.
16. Y. Liu, Y. Y. Jiang, G. X. Liu, J. X. Wang, X. T. Dong and W. S. Yu, Chin. J. Inorg. Chem., 2013, 2, 277.
17. S. Han, B. Song, L. Liu, C. He and K. S. Yang, Chin. J. Lumin., 2013, 9, 1183.
18. Y. Lin, S. K. Gao, G. M. Wang and W. M. Shi, J. Fuzhou Univ., Nat. Sci. Ed., 2008, 1, 134.
19. V. V. Atuchin, E. N. Galashov, O. Y. Khyzhun, A. S. Kozhukhov, L. D. Pokrovsky and V. N. Shlegel, Cryst. Growth Des., 2011, 11, 2479–2484.
20. Z. Xia, J. Li, Y. Luo, L. Liao and J. Varela, J. Am. Ceram. Soc., 2012, 95, 3229–3234.
21. K. R. Kort and S. Banerjee, Inorg. Chem., 2011, 50, 5539–5544.
22. J. Li, J. Sun, J. Liu, X. Li, J. Zhang, Y. Tian, S. Fu, L. Cheng, H. Zhong, H. Xia and B. Chen, Mater. Res. Bull., 2013, 48, 2159–2165.
23. M. Sun, L. Ma, B. J. Chen, F. Stepongzi, F. Liu, Z. W. Pan, M. K. Lei and X. J. Wang, J. Lumin., 2014, 152, 218–221.
24. A. X. S. Bruker, TOPAS V4 – User's Manual, Bruker AXS, Karlsruhe, Germany, 2008.
25. M. Pollnau, D. R. Gamelin, S. R. Luthi and H. U. Gudel, Phys. Rev. B: Condens. Matter Mater. Phys., 2000, 61, 3337.
26. J. Chen, Y.-g. Liu, H. Liu, D. Yang, H. Ding, M. Fang and Z. Huang, RSC Adv., 2014, 4, 18234.
27. C. Guo, Y. Xu, X. Ding, M. Li, J. Yu, Z. Ren and J. Bai, J. Alloys Compd., 2011, 509, L38–L41.
28. A. Durairajan, D. Thangaraju, D. Balaji and S. Moorthy Babu, Opt. Mater., 2013, 35, 740–743.
29. X. Yu, M. Gao, J. Li, L. Duan, N. Cao, Z. Jiang, A. Hao, P. Zhao and J. Fan, J. Lumin., 2014, 154, 111–115.
30. F. Lei and B. Yan, J. Solid State Chem., 2008, 181, 855–862.
31. D. H. Templeton and A. Zalkin, Acta Crystallogr., 1963, 16, 762.
32. V. V. Atuchin, A. S. Aleksandrovsky, O. D. Chimitova, T. A. Gavrilova, A. S. Krylov, M. S. Molokeev, A. S. Oreshonkov, B. G. Bazarov and J. G. Bazarova, J. Phys. Chem. C, 2014, 118, 15404–15411.
33. R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr., 1976, 32, 751.
34. K. Nassau, H. J. Levinstein and G. M. Loiacono, J. Phys. Chem. Solids, 1965, 26, 1805.
35. M. Guan, H. Zheng, L. Mei, M. S. Molokeev, J. Xie, T. Yang, X. Wu, S. Huang, Z. Huang and A. Setlur, J. Am. Ceram. Soc., 2015, 98, 1182–1187.
36. M. Y. Peng, N. Zhang, L. Wondraczek, J. R. Qiu, Z. M. Yang and Q. Y. Zhang, Opt. Express, 2011, 19, 20799.
37. Y. Wang and J. Ohwaki, Appl. Phys. Lett., 1993, 63, 3268.
38. P. Du, Z. Xia and L. Liao, J. Lumin., 2013, 133, 226–229.
39. D. Dai, S. Xu, S. Shi, M. Xie and C. Che, Opt. Lett., 2005, 30, 3377–3379.
40. J. Zhang, Y. Wang, L. Guo and P. Dong, Dalton Trans., 2013, 42, 3542–3551.
41. L. Xing, Y. Xu, R. Wang, W. Xu, S. Gu and X. Wu, Chem. Phys. Lett., 2013, 577, 53–57.
42. J. Zhang, Y. Wang, L. Guo, F. Zhang, Y. Wen, B. Liu and Y. Huang, J. Solid State Chem., 2011, 184, 2178–2183.
43. H. Yang, S. Xu, C.-H. Tao, V. W.-W. Yam and J. Zhang, J. Appl. Phys., 2011, 110, 043105.

---