A novel self-activated white-light-emitting phosphor of Na₂TiSiO₅ with two Ti sites of TiO₅ and TiO₆

Abstract

In this work, a novel self-activated light emitting phosphor of Na₂TiSiO₅ (NTSO) has been successfully synthesized in a high temperature solid-state reaction. The photoluminescence (PL) and cathodoluminescence (CL) properties of NTSO have been measured. Under excitation at 260 nm, NTSO emits a white light with CIE chromaticity coordinates (0.275, 0.355) and cyan light can also be observed at low voltage excitation. The crystal structure of NTSO has been investigated using the Rietveld refinement, which shows that Ti ions not only occupy the octahedron center but also connect with five O ions to form a TiO₅ polyhedron. The special polyhedron of TiO₅ broadens the emission band, and the energy transfer between the two sites has also been proved through the time-resolved photoluminescence (TRPL) spectra. Finally, the CL properties of NTSO with the functions of accelerating voltage, filament current and electron radiation time have been measured and the results show that NTSO emits bright light, and has excellent stability and good CIE chromaticity coordinates, which indicates that NTSO has the potential to be applied in FEDs.

---

1. Introduction

In recent years, rare-earth activated phosphors have received considerable attention because they can not only combine with blue InGaN LED chips to form white light emitting diodes (WLEDs), but they also play a important role in field-emission displays (FEDs). As we know, WLEDs have many excellent advantages, such as: high brightness, low energy consumption and being environment friendly,^1–4 and FEDs can provide a display with a thin panel, wide viewing angle, quick response time, high contrast ratio, light weight, and low power consumption.^5,6 In other words, rare-earth activated phosphors play the main part in the field of lighting and displays. However, the recent limited supply and the increased demand for rare-earth elements have restricted their application. Therefore, it is necessary to find novel activators acting as luminescent centers to replace the rare-earth elements.

Similar to rare-earth elements, bright light can also be obtained from transition metals resulting from d–d transitions like in Mn and Cr ions. Recently, Ti activated phosphors have gradually been explored and reported such as: A₂Sn_1−xTi_xO₄ (where A = Mg, Ca, Sr, Ba, and Zn),^7–13 BaM_1−xTi_xSi₃O₉ (where M = Sn and Zr),^14,15 Mg₅Sn_1−xTi_xB₂O₁₀,¹⁶ Mg₃Zr_1−xTi_xB₂O₈,¹⁶ AZr_1−xTi_x(BO₃)₂ (where A = Ca, Sr, and Ba),¹⁷ CaSn_1−xTi_x(BO₃)₂,¹⁸ CaSn_1−xTi_xSiO₅,¹⁹ and Ca₃Sn_1−xTi_xSi₂O₉.¹⁹ And it can be observed that Ti⁴⁺ ions commonly replace the Sn⁴⁺ or Zr⁴⁺ ions to build up TiO₆ octahedra, which emit blue light with the emission band ranging from 430 nm to 480 nm. However, in this work, Ti⁴⁺ ions not only enter into the center of the octahedra but also make up the TiO₅ polyhedra, and this finally broadens their emission band.

A phase diagram of the Na₂O–TiO₂–SiO₂ system has been provided by Glasser and Marr.²⁰ The natural occurrence of natisite, given as Na–Ti orthosilicate (Na₂TiOSiO₄), has only been reported by Bussen et al.²¹ in 1975 as being a post-magmatic hydrothermal product associated with an alkalic granitoid massif in the Kola Peninsula, Russia. It has been shown to exhibit bright blue luminescence under X-ray irradiation.²² However, the PL and CL properties of NTSO have never been reported. In this work, white light and cyan light with a peak at around 500 nm can be detected under excitation at 260 nm and low voltage excitation, respectively. The luminescence properties resulting from the two sites of Ti ions have been analyzed in detail, and the energy transfer between TiO₅ and TiO₆ has also been proven using the TRPL spectra. Finally, various accelerating voltages, filament currents and electron radiation times have been measured to indicate its potential application in FEDs.

2. Experimental section

2.1 Material and synthesis

The phosphor of NTSO was synthesized in a high temperature solid-state reaction. The raw materials used for the prepared phosphor were NaOH (A.R., >99.50%), TiO₂ (A.R., >99.90%) and SiO₂ (A.R., >99.999%). Stoichiometric amounts of the reactants were mixed and ground in an agate mortar, and then the final mixture was placed into an Al₂O₃ crucible. Finally, the mixture was annealed at 850 °C for 4 h. After that, the samples were furnace-cooled to room temperature, and ground again into powders for measurements.

2.2 Measurement and characterization

The photoluminescence (PL) and photoluminescence excitation (PLE) were measured using a FLS-920T fluorescence spectrophotometer equipped with a 450 W Xe light source and double excitation monochromators. The diffuse reflectance ultraviolet-visible (UV-vis) absorption spectra were measured using a Perkin Elmer 950 spectrometer. The crystal structures of the synthesized samples were identified using X-ray powder diffraction (XRD) using a Rigaku D/max-2400 X diffractometer with Ni-filtered Cu Kα radiation. All of the measurements were performed at room temperature. Thermal quenching was tested using heating apparatus (TAP-02) in combination with PL equipment. The morphology of the powder was investigated using scanning electron microscopy (SEM, S-340, Hitachi, Japan).

3. Results and discussion

3.1 Crystal structure and phase identification

In order to investigate the crystal structure of NTSO, especially for the coordination environments of the Ti⁴⁺ ions, the Rietveld structural refinements for NTSO were studied based on the general structure analysis system (GASA) program²³ and the calculated, observed and their difference results for the Rietveld refinement XRD patterns are shown in Fig. 1(a). All of the observed peaks satisfy the reflection conditions and converge to R_wp = 9.4%, R_p = 7.9% and χ² = 1.726. As exhibited in Fig. 1(b), NTSO crystallizes in an orthorhombic crystal system with space group Pmc 21(26) and lattice parameters of a = 9.1222(5) Å, b = 4.8040(3) Å and c = 9.8293(5) Å. The detailed crystal data of NTSO is shown in Table 1. There are two cation sites for the Ti ions to occupy. One cation site connects with five O ions, another is located at the distorted octahedron center with the O_h point group system, which connects with six O atoms. The bond lengths of Ti–O are exhibited in Fig. 1(c), and the average bond lengths of TiO₅ and TiO₆ are calculated as 1.8833 Å and 2.1062 Å, respectively. The shorter bond length of TiO₅ exhibits a stronger crystal field strength compared with that of TiO₆, and this crystal field strength is expected to change the emission spectrum. The two kinds of Ti polyhedra connect with each other through a SiO₄ tetrahedron, and the eight-membered ring made up of the Ti polyhedra and Si tetrahedra is shown in Fig. 1(d), which forms the main framework of NTSO through corner-sharing. And the Na⁺ ions occupy octahedral interstitial positions to realize the charge balance.

Fig. 1 (a) The experimental (crosses) and calculated (red solid line) XRD profiles and their difference (blue line) for the Rietveld refinement of NTSO. (b) The crystal structure of NTSO. (c) The coordination environment of the three Ti⁴⁺ sites with the bond lengths of Ti⁴⁺–O²⁻. (d) The eight-membered ring made up of the Ti polyhedron and Si tetrahedron.

Table 1 The crystal data of NTSO from the Rietveld refinement

Formula	Na2TiSiO5
Crystal system	Orthorhombic
Space group	Pmc 21(26)
Cell parameters	a = 9.1222(5) Å, b = 4.8040(3) Å, c = 9.8293(5) Å
Cell volume	430.75(4) Å^3
Z	4

---

Atom	X/a	Y/b	Z/c
Ti1	1/2	0.46766	0.00603
Ti2	0	0.02073	0.77895
Na1	1/2	−0.00164	0.25105
Na2	0.24828	0.00449	0.01486
Na3	0	0.41036	0.498232
Si1	0.25568	0.51049	0.25147
O1	1/2	0.80371	−0.00516
O2	0.35190	0.35960	0.13340
O3	0.35430	0.64520	0.36430
O4	0	0.86815	0.43466
O5	0.14639	0.75258	0.18840
O6	0.14212	0.27468	0.31707

---

The morphology of NTSO is shown in Fig. 2. The irregular morphology of NTSO has been observed and the sample shows good dispersion. The particle size distribution of NTSO has also been investigated using the Nano Measurer program. The results indicate that the particle size of NTSO ranges from 0.5 μm to 5 μm and the average diameter of NTSO is 1.60 μm.

Fig. 2 SEM micrograph of NTSO (a). The particle size distribution of NTSO (b).

3.2 Luminescence properties

Fig. 3(a) shows the diffuse reflection and adsorption spectra of NTSO, and the excitation spectrum monitored at 500 nm has also been exhibited for comparison. NTSO has strong absorption ranging from the 400 nm to 200 nm. It is obviously observed that the two peaks located at around 290 nm and 390 nm can be separated, which is consistent with the excitation spectrum of NTSO. According to the Rietveld structural refinements for NTSO, we have drawn the energy-level diagram of TiO₆, which is shown in Fig. 3(b). As we know, the 3d level is the outermost electron orbital of the Ti⁴⁺ ions, which are not shielded completely by the outer environment. Therefore, the split levels are determined by the symmetry around the Ti⁴⁺ ions. The 3d level of the Ti⁴⁺ ions contains five electron orbitals, and they are d_xy, d_xz, d_yz, d_x²−y² and d_z². When the Ti⁴⁺ ions occupy the site with O_h symmetry, the 3d orbitals of the Ti⁴⁺ ions will split into two levels, which are T₂ and E_g. The T₂ level contains three orbitals (d_xy, d_xz and d_yz) and the other orbitals (d_x²−y² and d_z²) belong to the E_g energy level. The E_g level and the 2p orbitals of the O²⁻ ions would build a bonding orbital and an antibonding orbital. In this model, the Ti⁴⁺ ions have a tendency to be reduced to Ti³⁺. Therefore, the electrons would transfer from the ground state to the antibonding orbital, which is the charge transfer band of O²⁻–Ti⁴⁺. Meanwhile, the T₂–E_g transition of the electrons can also be observed and it has a little contribution to light emission, which corresponds to the excitation spectrum. According to the above discussion, the two absorption peaks located at 290 nm and 390 nm can be attributed to the charge transfer band of O²⁻–Ti⁴⁺ and the T₂–E_g transition of electrons, respectively.

Fig. 3 (a) The reflectance, absorption and PLE spectra of NTSO. (b) The energy level diagram of NTSO.

Fig. 4(a) exhibits the emission spectrum of NTSO. Under excitation at 260 nm, NTSO emits white light with a broad emission band ranging from 400 nm to 650 nm. The emission band can be deconvoluted into four well-separated Gaussian components with peaks at 450 nm (2.7555 eV), 490 nm (2.5306 eV), 555 nm (2.2342 eV) and 625 (1.9840 eV) nm. From the calculated energy, it can be found that the energies of E₁ (2.5306 eV (490 nm) + 2.2342 eV (555 nm) = 4.7642 eV) and E₂ (2.7555 eV (450 nm) + 1.9840 eV (620 nm) = 4.7395 eV) are close to the energy of the excitation spectrum peak at 260 nm (4.7692 eV). Therefore, we deduce that the emission peaks located at 490 nm and 555 nm and the peaks located at 450 nm and 625 nm belong to different sites of the Ti ions. Considering the coordinate environment of Ti⁴⁺, the energy separation between the T_2g and E_g energy level 10Dq can be calculated using the following equation:²⁴

10Dq = 5Ze2(r⁴)/12a (1)

where Ze is the charge on the ligand ion, (r⁴) is the average value of r⁴ for the 3d orbital and a is the average distance between the central ion and the ligand oxygen ions. Compared with the bond length, the crystal field splitting of TiO₅ is stronger than TiO₆, which leads to the red-shift of the emission band. Therefore, it is reasonably deduced that the emission peaks located at 490 nm and 555 nm belong to TiO₅ and the other peaks belong to TiO₆ (450 nm and 625 nm). The same blue light has also been found in CaZr_1−xTi_xO₃ (431 nm emission),²⁵ Ca₂Sn_1−xTi_xO₄ (445 nm emission),¹¹ Mg₅Sn_1−xTi_xB₂O₁₀ (430 nm emission)¹⁷ and BaZr_1−xTi_xSi₃O₉ (450 nm emission),¹⁴ which exhibit that Ti⁴⁺ connects with six O²⁻ ions to form the TiO₆ octahedron with average bond lengths ranging from 2.013 to 2.417 Å. And the shorter average bond length of TiO₅ (1.8833 Å) shows a stronger crystal field strength, which leads to the emission peak changing from blue to blue-green.

Fig. 4 (a) The PL spectrum of NTSO. (b) The energy level diagram of Ti ions occupying two sites.

Fig. 4(b) shows the energy diagram of Ti⁴⁺ in different sites. According to the above calculation, it can be seen that the electron absorbing the activated energy (4.7692 eV) can transfer from the ground state to the E_g energy level along pathway ➀ and partial electrons would transfer to another site along pathway ②. Then the electrons would return to the ground state along pathways ③ and ④ to emit visible light. In this process, electrons would have a relaxation, which is the reason why the energies E₁ (4.7642 eV) and E₂ (4.7395 eV) are smaller than the excitation energy (4.7692 eV).

In order the further investigate the relationship between the two Ti sites and their contribution to the luminescence properties of NTSO, a series of lifetime decays of NTSO were measured at different wavelengths from 400 nm to 650 nm at 5 nm intervals. The time-resolved photoluminescence (TRPL) spectra and the lifetime decay curves of NTSO are shown in Fig. 5(a) and (b). In this regard, it is clear that P1 comes from the Ti1 site peaks at 490 nm and 555 nm and P2 comes from the Ti2 site peaks at 450 nm and 625 nm. With the increase of the decay time, the trends of the time evolution of P1 and P2 are the same. However, it is obvious that the time evolution of the P1 component was much faster than that of the P2 component, which leads to the ratio of P1/P2 decreasing drastically from 1.27 to 0.57 with the increase of the decay time. According to the TRPL results, the energy transfer from TiO₅ to TiO₆ would take place in the host lattice of NTSO, which corresponds to the above deduction.

Fig. 5 (a) The TRPL spectra of NTSO. (b) The lifetime decay curves of NTSO monitored at different emission peaks.

It can be seen that the curves can be well fitted by a second-order exponential decay curve using the following biexponential equation:^26–28

I = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (2)

where I is the luminescence intensity; t is the time; A₁ and A₂ are fitting constants, τ₁ and τ₂ are short and long lifetimes for exponential components, respectively. Therefore, the average lifetime τ can be obtained using the following formula:^29–31

τ = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (3)

On the basis of the above formula, the average lifetimes τ of the NTSO peaks at 445 nm, 490 nm, 555 nm and 620 nm are 0.785, 0.730, 0.773 and 0.878, respectively.

Fig. 6(a) shows the CL emission spectrum of NTSO with two obvious emission peaks located at 450 nm and 490 nm, and the result is different from the PL properties, which indicates that the emission light of NTSO is dependent on the excitation source. The CL properties of NTSO as the function of the accelerating voltage and the filament current have also been investigated and are exhibited in Fig. 6(b) and (d), respectively. When the filament current is fixed at 50 mA, the CL intensity increases as the accelerating voltage rises from 1 kV to 6 kV. Similarly, with the increase of the filament current from 30 mA to 90 mA, the CL intensity shows an obvious increase with the accelerating voltage fixed at 5 kV. This phenomenon is due to the deeper penetration of the electrons into the phosphor body and the larger electron beam current density. The electron penetration depth can be estimated using the empirical formula:³² L = 250(A/ρ)(E/Z^1/2)ⁿ, where n = 1.2/(1 − 0.29lgZ), ρ is the bulk density, Z is the atomic number or the number of electrons per molecule in the case compound, A is the atomic or molecular weight of the material and E is the accelerating voltage. For NTSO, ρ = 3.11402 g cm⁻³, A = 201.9290 and Z = 36, the estimated electron penetration depth at 1.5 kV is about 3.1269 nm. For CL, the Ti⁴⁺ ions are excited by the plasma produced by the incident electrons. The deeper the electron penetration depth, the more plasma that will be produced, which results in more Ti⁴⁺ ions being excited and thus, the CL intensity increases.

Fig. 6 (a) The cathodoluminescence spectra of NTSO. The cathodoluminescence intensities of NTSO as a function accelerating voltage (b) and filament current (d). (c) The aging properties of NTSO were measured under V = 5 kV and I = 50 mA.

The degradation properties of NTSO have also been investigated to assess its application in FEDs. As shown in Fig. 6(c), when the accelerating voltage and filament current are fixed at 5 kV and 50 mA, respectively, in the first 10 min, the CL intensity obviously decreases and then shows a nearly parabolic decrease with continuous electron bombardment. After radiation for 1 h, the intensity of NTSO falls to 82% of the initial value, which indicates that continuous electron radiation has little influence on NTSO.

The temperature dependence of the PL spectra of NTSO has also been investigated and is exhibited in Fig. 7(a). With the increase of the temperature, the emission intensity of NTSO shows an obvious decrease. At 150 °C, the integral intensity of NTSO is about 20% of the initial intensity at room temperature. Meanwhile, the blue shift of the emission peak changing from 500 nm to 455 nm can be detected with the increase of the temperature. In order to explain this phenomenon, the configurational coordinate diagram of the ground states and the excited states of Ti⁴⁺ located at two sites has been drawn and is shown in Fig. 7(b). The electrons absorbing the activation energy would be excited to the high energy level from the ground state and then would relax to the lowest level of the excited energy level, and finally return back to the ground state along pathway ➀ to emit light. In this process, partial electrons may transfer to the TiO₆ site and then return to the ground state along pathway ➁. With the increase of the temperature, the electrons obtaining the thermal activation energy ΔE₁ would show a non-radiative transition to the ground state along pathway ➂, which is the reason for temperature quenching. On the other hand, when partial electrons obtain the thermal activation energy ΔE₂, they would also transfer to the high energy level of TiO₅, which leads to the blue shift of the emission peak resulting from the increase of the temperature. This phenomenon also proves that energy transfer between the two sites would happen.

Fig. 7 (a) The temperature dependence of the emission spectra of NTSO, and the inset shows the temperature dependence of the emission intensity and emission peak of NTSO. (b) The configurational coordinate diagram of the ground states and the excited states of Ti⁴⁺ ions located at two sites.

The CIE XYZ color space was deliberately designed so that the Y parameter was a measure of the brightness or luminance of a color. The chromaticity of a color was then specified by the two derived parameters x and y, two of the three normalized values which are functions of all three tristimulus values X, Y, and Z. The CIE chromaticity coordinates of NTSO with different excitation pathways are displayed in Fig. 8. Under excitation at 260 nm, white light with chromaticity coordinates of (0.274, 0.355) can be observed, which is in the white region. However, at low excitation voltage, NTSO emits cyan light with chromaticity coordinates of (0.198, 0.308), which is out of the triangle area formed by red light Y₂O₂S:Eu³⁺ with CIE chromaticity coordinate point (0.647, 0.343), green light ZnS:Cu (0.298, 0.619) and blue light ZnS:Ag, Al (0.146, 0.056).^33–35 The results indicate that the cyan light emitted by NTSO can enlarge the color gamut of tricolor FED phosphors and thus increase the display quality of full-color FEDs.

Fig. 8 CIE chromaticity coordinate diagram of NTSO.

4. Conclusion

In this work, a novel self-activated white-light-emitting phosphor of Na₂TiSiO₅ has been successfully synthesized in a high temperature solid-state reaction. The crystal structure of NTSO has been analyzed using the Rietveld refinement, which shows that there are two sites for Ti ions to occupy. Under excitation at 260 nm, the broad emission band ranging from 400 nm to 650 nm can be observed, which can be successfully deconvoluted into four well-separated Gaussian components with peaks at 450 nm, 490 nm, 555 nm and 625 nm. And the four peaks belong to the emission of TiO₅ (490 nm and 555 nm) and TiO₆ (450 nm and 625 nm), which has been proven using the TRPL spectra, this also proves that the energy transfer from TiO₅ to TiO₆ would happen. The CL properties of NTSO with functions of accelerating voltage, filament current and electron radiation time have also been measured. The results indicate that NTSO emits bright cyan light and the intensity of NTSO still retains 82% of the initial value after radiation for 1 h, which indicates that NTSO has the potential to be applied in FEDs to enhance the color gamut and improve the display quality.

Acknowledgements

This work was supported by the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20120211130003), the National Natural Science Funds of China (Grant No. 51372105), State Key Laboratory on Integrated Optoelectronics (No. IOSKL2013KF15) and the Fundamental Research Funds for the Central Universities of Lanzhou University (No. lzujbky-2015-205).

Notes and references

1. T. Suehiro, N. Hirosaki and R.-J. Xie, ACS Appl. Mater. Interfaces, 2011, 3, 811.
2. S.-K. Kim, J. W. Lee, H.-S. Ee, Y.-T. Moon, S.-H. Kwon, H. Kwon and H.-G. Park, Opt. Express, 2010, 18, 11025.
3. T.-C. Liu, B.-M. Cheng, S.-F. Hu and R.-S. Liu, Chem. Mater., 2011, 23, 3698.
4. Z.-Y. Mao and D.-J. Wang, Inorg. Chem., 2010, 49, 4922.
5. T. Jüstel, H. Nikol and C. Ronda, Angew. Chem., Int. Ed., 1998, 37, 3084.
6. N. Hirosaki, R. Xie, K. Inoue, T. Sekiguchi, B. Dierre and K. Tamura, Appl. Phys. Lett., 2007, 91, 061101.
7. F. A. Kröger, Some Aspects of the Luminescence of Solids, Elsevier, Amsterdam, 1948, pp. 159–173.
8. Y. Kotera, T. Sekine and M. B. Yonemura, Bull. Chem. Soc. Jpn., 1956, 29, 616–619.
9. A. J. H. Macke, J. Solid State Chem., 1976, 18, 337–346.
10. G. Blasse, Philips Res. Rep., 1968, 23, 344–361.
11. T. Yamashita and K. Ueda, J. Solid State Chem., 2007, 180, 1410–1413.
12. G. Blasse, G. A. M. Dalhoeven, J. Choisnet and F. F. Studer, J. Solid State Chem., 1981, 39, 195–198.
13. L. P. Benderskaya, V. G. Krongauz and M. D. Khalupovskii, J. Appl. Spectrosc., 1974, 20, 238–240.
14. W. L. Konijnendijk, Inorg. Nucl. Chem. Lett., 1981, 17, 129–132.
15. K. Iwasaki, Y. Takahashi, H. Masai and T. Fujiwara, Opt. Express, 2009, 17, 18054–18062.
16. W. L. Konijnendijk and G. Blasse, Mater. Chem. Phys., 1985, 12, 591–599.
17. G. Blasse, S. J. M. Sas, W. M. A. Smit and W. L. Konijnendijk, Mater. Chem. Phys., 1986, 14, 253–258.
18. T. Kawano and H. Yamane, J. Alloys Compd., 2010, 490, 443–447.
19. S. Abe, H. Yamane and H. Yoshida, Mater. Res. Bull., 2010, 45, 367–372.
20. F. P. Glasser and J. Marr, J. Am. Ceram. Soc., 1979, 62, 42.
21. I. V. Bussen, E. M. Es’kova, Y. P. Men’shikov, A. N. Merkov, A. S. Sakharov, E. I. Semenov and A. P. Khomyakov, Mater. Mineral., Geokhim. Petrogr. Zabaik., 1975, 102 , in Russian.
22. A. V. Nikitin, V. V. Ilyukhin, B. N. Litvin, O. K. Mel’nikov and N. V. Belov, Phys.-Dokl., 1964, 1.5, 625.
23. A. C. Larson and R. B. Von Dreele, “GSAS”, General Structure Analysis System, LANSCE, MS-H805, Los Alamos, NewMexico, 1994.
24. P. D. Rack and P. H. Holloway, Mater. Sci. Eng., 1998, 21, 171–219.
25. A. J. H. Macke, J. Solid State Chem., 1976, 18, 337–346.
26. C. H. Huang and T. M. Chen, J. Phys. Chem. C, 2011, 115, 2349.
27. C. H. Huang, T. M. Chen, W. R. Liu, Y. C. Chiu, Y. T. Yeh and S. M. Jang, ACS Appl. Mater. Interfaces, 2010, 2, 259.
28. W. Lv, W. Lü, N. Guo, Y. Jia, Q. Zhao, M. Jiao, B. Shao and H. You, Dalton Trans., 2013, 42, 13071.
29. R. Yu, H. M. Noh, B. K. Moon, B. C. Choi, J. H. Jeong, K. Jang, H. S. Lee and S. S. Yi, Mater. Res. Bull., 2014, 51, 361.
30. Z. Xia, J. Zhuang and L. Liao, Inorg. Chem., 2012, 51, 7202.
31. Z. Xia, J. Zhuang, A. Meijerink and X. Jing, Dalton Trans., 2013, 42, 6327.
32. C. Feldman, Phys. Rev., 1960, 117, 455.
33. F. L. Zhang, S. Yang, C. Stoffers, J. Penczek, P. N. Yocom, D. Zaremba, B. K. Wagner and C. J. Summers, Appl. Phys. Lett., 1998, 72, 2226.
34. M. Kottaisamy, K. Horikawa, H. Kominami, T. Aoki, N. Azuma, T. Nakamura, Y. Nakanishi and Y. Hatanaka, J. Electrochem. Soc., 2000, 147, 1612.
35. M. K. Patra, K. Manzoor, M. Manoth, S. R. Vadera and N. Kumar, J. Lumin., 2008, 128, 267.

---