Rare-earth free self-activated and rare-earth activated Ca2NaZn2V3O12 vanadate phosphors and their color-tunable luminescence properties

Journal of Physics and Chemistry of Solids 74 (2013) 1439–1443

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Rare-earth free self-activated and rare-earth activated Ca2NaZn2V3O12 vanadate phosphors and their color-tunable luminescence properties Xue Chen, Zhiguo Xia n, Min Yi, Xiachan Wu, Hao Xin School of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, China

art ic l e i nf o

a b s t r a c t

Article history: Received 9 March 2013 Received in revised form 29 April 2013 Accepted 3 May 2013 Available online 13 May 2013

Novel rare-earth free self-activated and Eu3+ or Sm3+ doped vanadate phosphors Ca2NaZn2V3O12 were synthesized via the solid-state reaction route. The pure-phase garnet structures in this system were verified by the X-ray diffraction (XRD) and Rietveld refinement. The self-activated luminescence and rare earth doped luminescence behaviors have been studied in detail. The broad-band green emission can be found from the as-prepared Ca2NaZn2V3O12 compound, originating from the VO43− emission. The energy transfer behaviors from VO43− to Eu3+/Sm3+ ions in Ca2NaZn2V3O12:A (A ¼ Eu3+, Sm3+) phosphors have been demonstrated by photoluminescence (PL) and decay time measurement. All the results indicate that self-activated Ca2NaZn2V3O12 and the Eu3+ or Sm3+ doped Ca2NaZn2V3O12 phosphors show great potential for the application in the near-UV excited white LEDs. & 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Optical materials C. X-ray diffraction D. Luminescence

1. Introduction White light-emitting diodes (w-LEDs) are considered to be one of the most important solid-state light sources, which can be used to take place of conventional light sources such as incandescent lamp or fluorescent lamp, because of their high efficiency, long lifetime, low power requirement, and so on [1,2]. With the appearance of the first commercial w-LED in 1997, made by the combination of the blue LEDs chip and the yellow-emitting phosphor, typically Ce3+-doped yttrium aluminum garnet (YAG: Ce3+) [3]. However, low color-rendering index (Ra o 80) and high correlated color temperature due to the deficiency of red emission seriously affects the quality of the present w-LEDs [4]. Therefore, the development of the new phosphors has attracted more and more attentions. In recent years, vanadates are widely studied as a kind of new phosphor hosts from the commercial red-emitting YVO4:Eu3+ phosphors. Moreover, in the near-UV region of the excitation spectrum, vanadates exhibit a broad absorption band in some hosts owing to the intense charge-transfer (CT) transition, which also show the broad-band emission from 400 to 700 nm in some hosts [5]. Generally, the majority of the commercially available phosphors require radiation to excite the rare-earth activator in the phosphor system to generate multi-color light. However, the luminescent centers (dopants) of the phosphors used in fluorescent lamps or light emitting diodes (LEDs) are rareearth elements that are typically expensive. But some recent

n

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0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.05.002

works indicate that rare-earth free self-activated luminescence can be realized in some vanadates hosts [6]. Furthermore, Eu3+ ions were widely used as the activators of red luminescent materials. Sm3+ ions were also very important red-emitting lanthanide ions, which have the 4f5 electronic configuration exhibiting a strong orange–red fluorescence in the visible region [7]. It is believed that color-tunable luminescence can be obtained in some rare-earth free self-activated and Eu3+ or Sm3+ doped vanadates systems. Rare-earth aluminate garnets (REAG, RE3Al5O12) have been widely studied for optical applications [8]. However, they belong to the rare earth-containing compounds, which possess the high cost during the practical applications. Therefore, Bayer firstly reported LiCa3M2V3O12 (M ¼ Mg, Cu, Zn, Co and Ni) phase in 1965, which is of the same garnet structure as aluminate garnet YAG [9]. Recently, Dhobale et. al. proved that efficient energy transfer takes place in Ca2NaMg2V3O12 from the host vanadate to the dopant RE ions without any cross transfer between the RE ions [10]. In 2004, single crystals of the vanadate garnet Ca2NaZn2V3O12 were synthesized by the floating zone method, and its crystal structure was investigated [6]. However, the intrinsic photoluminescence properties and rare earth ions doped luminescence behavior have not yet been reported in the Ca2NaZn2V3O12 system. In this paper, we firstly report the self-activated and Eu3+ or Sm3+ doped Ca2NaZn2V3O12 phosphor. We have demonstrated the efficient energy transfer from the host lattice to the RE ions in the present vanadate garnet matrix. By fine controlling of the rare earth ion content, color-tunable emission from bluish green to yellow can be realized in the Ca2NaZn2V3O12 system upon excitation of the broad-band near-UV light.

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X. Chen et al. / Journal of Physics and Chemistry of Solids 74 (2013) 1439–1443

2. Experimental 2.1. Synthesis Ca2NaZn2V3O12 host, Ca2NaZn2V3O12:Eu3+ and Ca2NaZn2V3O12: Sm phosphors were all synthesized via the high temperature solid state reaction route. The raw materials were 99.5% pure calcium carbonate (CaCO3), sodium carbonate (Na2CO3), zinc oxide (ZnO), and ammonium metavanadate (NH4VO3) powders, and the 99.995% pure europium oxide (Eu2O3) and samarium oxide (Sm2O3). Firstly, stoichiometric amounts of starting materials were mixed and ground thoroughly in an agate mortar vibration milled for 1 h. Then, the mixture was transferred into an alumina crucible and calcined in a muffle furnace at 800 1C for 5 h. Finally, the samples were furnace-cooled to room temperature, and ground again into powder for measurement. 3+

2.2. Characterizations The phase identification was performed by X-ray powder diffraction (SHIMADZU, XRD-6000, 40 kV and 30 mA, Cu Kα¼ 0.15406 nm). The continuous scanning rate (2θ ranging from 201 to 701) used as phase formation determination was 41 (2θ)/min, and the step scanning rate (2θ ranging from 51 to 1001) used as Rietveld analysis was 8 s/step with a step size of 0.02. Powder diffraction data were obtained using the computer software TOPAS package. Room temperature excitation and emission spectra were measured on a fluorescence spectrophotometer (F-4600, HITACHI, Japan) with a photomultiplier tube operating at 400 V, and a 150 W Xe lamp used as the excitation lamp. The lifetimes were recorded on a spectro-fluorometer (HORIBA, JOBIN YVON FL3-21), and the 370 nm pulse laser radiation (nano-LED) was used as the excitation source. The quantum efficiency (QE) was measured by an absolute photoluminescence quantum yield measurement system (C9920-02, Hamamatsu-Photonics) with an integral sphere at room temperature.

3. Results and discussions 3.1. Structural characterizations

Counts

Rietveld analysis was performed to ensure the phase purity and to obtain the detailed crystal structure information of Ca2NaZn2V3O12. Powder diffraction patterns for as-prepared Ca2NaZn2V3O12 host were performed, as shown in Fig. 1. In this work, Ca2NaZn2V3O12 served as an initial structural model. Structural refinements of the Ca2NaZn2V3O12 were performed by a

cubic space group of Ia-3d. Crystallographic data and details in the data collection and refinement parameters are summarized in Table 1. It can be seen from Table 1 that the residual factors are R-Bragg ¼2.30%, Rwp ¼ 10.15%, and Rp ¼6.24%. The unit cell parameters obtained for Ca2NaZn2V3O12 are a ¼12.45642 Å, b¼12.45654 Å and c¼12.45654 Å. Furthermore, Fig. 2 presents the XRD patterns of the asprepared Ca2NaZn2V3O12 host and samples doped with different rare earth ions, Eu3+ and Sm3+. The diffraction patterns of all the samples match well with the diffraction pattern of the Ca2NaZn2V3O12 single crystal crystallized in a cubic geometry with ICSD no. 99775. All peaks are well indexed to the cubic garnet structure with space group of Ia-3d (230). Therefore, it indicates that single-phase Ca2NaZn2V3O12 powder can be obtained by the solid-state method, and the doping Eu3+ and Sm3+ ions are completely incorporated into the host lattice by substituting for Ca2+ sites, and some excessive Na+ ions can act as the charge compensated agents [6]. Therefore, the charge balance in this system is maintained by Na+, which is located in the large cavities in the framework. A schematic of the crystal structure of Ca2NaZn2V3O12 is presented in Fig. 3a. Ca2NaZn2V3O12 has a garnet structural cubic unit cell with space group Ia-3d, and Ca2+, Na+ and Zn2+ cations are surrounded by various quantitative oxygens. As it is also given in Fig. 3b, there is one calcium atom showing the same eightfold coordination. Ca/Na has eight oxygen neighbors (2.435–2.544 Å), which all belong to the cube.

3.2. Photoluminescence properties The photoluminescence emission (PL) and photoluminescence excitation (PLE) spectra of Ca2NaZn2V3O12 and Ca2NaZn2V3O12:A (A¼Eu3+, Sm3+) are shown in Fig. 4, respectively. It is found that the emission spectrum of Ca2NaZn2V3O12 consists of a broad emission band and it covers the spectral region from 400 to 700 nm with emission peak near 497 nm (Fig. 4a). We can also observe from Fig. 4b and c that the emission spectra of Ca2NaZn2V3O12:Eu3+ and Ca2NaZn2V3O12:Sm3+ consist of a broad emission band and some sharp emission lines, and RE ions play an important role in the sharp emission lines. It is accepted that the broad-band emission in this series of vanadate phosphors is attributed to the charge transfer (CT) of an electron from the oxygen 2p orbital to the vacant 3d orbital of V5+ in tetrahedral VO43− with Td symmetry and originates from the 3T2-1A1 and 3T1-1A1 transition of VO43− group [10]. The energy difference between the two exicited levels of 3T2 and 3T1 is approximately 500 cm−1. Thus the two emission peaks from these two transitions are so close that it is hard to distinguish them in the emission spectrum [11]. Except for the broad band originating from

12,000 11,000 10,000 9,000 8,000 7,000 6,000 5,000 4,000 3,000 2,000 1,000 0 -1,000

Structure 100.00 %

5

10

15

20

25

30 35

40

45

50

55

60

65

70

75

80

85

90

95 100

2Th Degrees Fig. 1. Observed intensities (blue circles), calculated patterns (red lines), Bragg positions (tick marks), and the different plots (gray lines) for the Rietveld refinement of asprepared Ca2NaZn2V3O12 sample. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

X. Chen et al. / Journal of Physics and Chemistry of Solids 74 (2013) 1439–1443

Table 1 Crystallographic data and details in the data collection and refinement parameters. Ca2NaZn2V3O12 Crystal system Space group Z V (Å3) cell parameters

Cubic Ia-3d 8 1932.77(16) a¼ 12.45642(34)Å b¼ 12.45654(34) Å c¼ 12.45654(34) Å 5≤2θ≤100 2.301 10.15 6.91 6.24 1.47

Profile range R-Bragg (%) Rwp (%) Rexp (%) Rp (%) GOF

Ca1.98Na1.01Zn2V3O12:0.01Eu

3+

Intensity (a.u.)

Ca1.98Na1.01Zn2V3O12:0.01Sm

3+

Ca2NaZn2V3O12

ICSD 99775

20

30

40

50

60

70

2 Theta (Degree) Fig. 2. XRD patterns of the as-prepared Ca2NaZn2V3O12, Ca1.98Na1.01Zn2V3O12:0.01Eu3+ and Ca1.98Na1.01Zn2V3O12:0.01Sm3+ phosphors, and the standard card of Ca2NaZn2V3O12 (ICSD 99775) is also given as a comparison.

the VO43− group, the emission spectrum of Eu3+ in Ca2NaZn2V3O12 consist of a well-known 5D0-7FJ (J¼1, 2, 3 and 4) lines of Eu3+ [12]. As shown in Fig. 4b, the most intense emission line of Eu3+ peaks at 613 nm is corresponding to 5D0-7F2 forced electric transition of Eu3+, suggesting that it occupied a non-centrosymmetric site (Ca2+ site) in Ca2NaZn2V3O12 host [13]. However, the emission spectrum of Sm3+ in Ca2NaZn2V3O12 has three peaks observed at 566, 617 and 652 nm attributed to the emission from the excited state 4G5/2 to 6 H5/2, 6H7/2 and 6H9/2, respectively [14]. As shown in the left part of Fig. 4a, the excitation spectrum of Ca2NaZn2V3O12 host gives the broad-band absorption from 250 to 400 nm, which is ascribed to the CT transition of VO43− group. Furthermore, as given in the excitation spectra of Ca2NaZn2V3O12: A (A ¼Eu3+, Sm3+) (Fig. 4b and c), a similar broad absorption band from 250 to 400 nm can be observed, and the characteristic excitation peaks ascribed to Eu3+ (393 nm) or Sm3+ (411 nm) can be also found in Fig. 4b and c. It is more important to note that the excitation spectra give similar spectral profile corresponding to monitoring wavelengths, 497 nm for the emission from VO43− group, 613 nm for Eu3+ and 617 nm for Sm3+, respectively. Such a phenomenon verified that the energy transfer process should exist between VO43− and Eu3+ or Sm3+ in the present system, so that we can only find the characteristic absorption of VO43− group even if the monitoring wavelengths of Eu3+ and Sm3+ emission are used. The emission spectra of Ca2−2xNa1+xZn2V3O12:xEu3+ (x ¼0, 0.01, 0.03, 0.08, 0.10 and 0.15) under a 365 nm excitation at room

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temperature, as given in Fig. 5. The peaks corresponding to the broad-band emission are located near 497 nm, and the emission lines are ascribed to the transitions of Eu3+ from 5D0-7FJ (J ¼1, 2, 3 and 4) with strongest peak at 613 nm. Ca2−2xNa1+xZn2V3O12:xEu3+ (x¼ 0.01, 0.03, 0.08, 0.10 and 0.15) have similar spectral profile with different relative intensities, and the excitation spectra for the selected sample also indicated the existence of energy transfer from VO43− to Eu3+, as mentioned above. Furthermore, the emission intensity from the CT transition of VO43− (at 497 nm) decreases with increasing Eu3+ concentration. At the same time, the characteristic emission peaks of Eu3+ (at 613 nm) increases, proving that the possible energy transfer process could be existed between VO43− and Eu3+. A further increase in the Eu3+ concentration (x40.03) results in a decrease in the emission intensity because of concentration quenching of Eu3+ emission. Fig. 6 exhibits the PL and PLE spectra of Ca2−2y Na1+yZn2V3O12: ySm3+ samples, which are similar as the spectral profile of the Ca2−2xNa1+xZn2V3O12:xEu3+ samples. Characteristic Sm3+ emissions from the 4G5/2 excited state to the ground states (6H5/2, 6 H7/2 and 6H9/2) are clearly observed except for the broad-band emission from CT transition of VO43− (at 497 nm). With increasing Sm3+ concentration, the emission intensity of CT transition from VO43− decreases and the emission intensity of Sm3+ increases, which is similar as that of the Eu3+doped samples. It is found that the emission intensity at 497 nm ascribed to the CT transition of VO43− is equal to the emission of Sm3+ when the concentration of Sm3+ was fixed at 0.03. Based on PLE spectra in Fig. 6, it can also infer that energy transfer happens from VO43− to Sm3+ based on the similar spectral profiles. It can be seen from Fig. 7 that the excitation and emission mechanism, as well as the energy levels of the vanadate group and Eu3+ or Sm3+ are given, and the energy transfer process among them is demonstrated. The energy transfer process taking place in the system can be explained as follows. In case of the undoped vanadate group, the excess energy available with 1T2 state causes decays to the 3T1,2 levels in a non radiative way. As RE ions are doped in this system, new energy levels are created between the ground states and 3T states. Part of the energy available with the 3T states is utilized to pump the upper levels of 5D0 and 4G5/2 in Eu3+ and Sm3+, respectively [8]. Therefore, we can find broad-band emission originated from the CT transition of VO43− group and the characteristic f–f transition of Eu3+ or Sm3+ in a system. The fluorescence decay curves of Ca2NaZn2V3O12 host and the selected Ca1.94Na1.03Zn2V3O12:0.03 Eu3+ and Ca1.94Na1.03Zn2V3O12:0.03Sm3+ samples were measured at room temperature under a 370 nm excitation source, which are monitored by the emission peak at 497 nm. Fig. 8 presents the decay curves and lifetime of the three samples. It is found that the decay curves are well fitted with a second order exponential equation [15] IðtÞ ¼ I 0 þ A1 expð−t=τ1 Þ þ A2 expð−t=τ2 Þ

ð1Þ

Here I is the luminescence intensity at time t, and I0 is the luminescence intensity initially, A1 and A2 are two constants which are related with the initial intensity, τ1 and τ2 denote fast decay constant and slow decay constant, respectively. In addition, the average lifetime constant (τn) can be calculated as [16] τn ¼ ðA1 τ1 2 þ A2 τ2 2 Þ=ðA1 τ1 þ A2 τ2 Þ

ð2Þ

The measured fluorescence lifetimes (τ) for Ca2NaZn2V3O12, Ca1.94Na1.03Zn2V3O12:0.03 Eu3+ and Ca1.94Na1.03Zn2V3O12:0.03Sm3+ phosphors are 9.55, 8.73 and 9.03 ms, respectively. It is found that the lifetime of VO43− emission decreases with the introduction of rare earth ions, further suggesting the existence of the energy transfer of VO43− to Eu3+ or Sm3+. The CIE chromaticity coordinates of Ca2NaZn2V3O12:A (A¼Eu3+, 3+ Sm ) phosphors with different doping contents of Eu3+ and Sm3+

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X. Chen et al. / Journal of Physics and Chemistry of Solids 74 (2013) 1439–1443

Fig. 3. (a) Crystal structure diagram of Ca2NaZn2V3O12 compound with garnet phase and (b) coordination spheres and bond lengths (Å) of the two different Ca2+ sites in Ca2NaZn2V3O12.

Ca2-x Na1+x Zn2 V3 O12 :xSm3+

497

=617nm

em

Intensity (a.u.)

em

PLE

Fig. 4. The PL and PLE spectra of Ca2NaZn2V3O12 (a), Ca1.94Na1.03Zn2V3O12:0.03 Eu (b) and Ca1.94Na1.03Zn2V3O12:0.0.03Sm3+ (c) phosphors.

Ca2-xNa1+xZn2V3O12:xEu

3+

Intensity (a.u.)

λem=497nm

x 4 = 0.08 x 3 = 0.10

300

617 nm

nm

400

500

600

700

Wavelength (nm) Fig. 6. Sm3+ concentration dependent PL spectra of Ca2−xNa1+xZn2V3O12:xSm3+ (x ¼0, 0.01, 0.03, 0.08 and 0.10) phosphors, and the PLE spectra for the selected sample.

613

PLE λem=613nm

x 3 = 0.03

=365nm

ex

PL

Sample (x 3 = 0.03)

200 3+

=497nm

x1 = 0 x 2 = 0.01

PL x1 = 0 x2 = 0.01 x3 = 0.03 x4 = 0.08

λex=365nm 497

x5 = 0.1 Sample 393nm (x3 = 0.03)

200

300

400

500

600

700

Wavelength (nm) Fig. 5. Eu3+ concentration dependent PL spectra of Ca2−xNa1+xZn2V3O12:xEu3+ (x¼ 0, 0.01, 0.03, 0.08 and 0.10) phosphors, and the PLE spectra for the selected sample.

under 365 nm UV excitation are calculated and exhibited in Table 2. The chromaticity coordinates for Ca2NaZn2V3O12:0.08 Eu3+ could be tuned from bluish green (0.2146, 0.3380) to yellowish green (0.3154, 0.4060) position by changing the Eu3+ concentration. With increasing

Fig. 7. The excitation and emission mechanism of the vanadate group (VO43−), Eu3+ and Sm3+, as well as the energy transfer (ET) process among the corresponding energy levels.

of the Sm3+ concentration, the CIE coordinates slightly shift from bluish green (0.2146, 0.3380) to yellow (0.2931, 0.3741) position. And the selected results for Ca2NaZn2V3O12:Sm3+ samples were shown in CIE diagram of Fig. 9. It is found that the emitting color is tunable in a large color gamut by adjusting the doping content of the rare earth

X. Chen et al. / Journal of Physics and Chemistry of Solids 74 (2013) 1439–1443

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Ca2NaZn2V3O12:0.08Eu3+ and Ca2NaZn2V3O12:Sm3+ are determined to be 11.4%, 17.4% and 15.1%, respectively.

4. Conclusions

Fig. 8. The room temperature decay curves and lifetimes of VO43− in Ca2NaZn2V3O12 host (1), Ca1.94Na1.06Zn2V3O12:0.03Eu3+ (2) and Ca1.94Na1.06Zn2V3O12:0.03Sm3+ (3).

Table 2 CIE chromaticity coordinates of Ca2−xNa1+xZn2V3O12:xEu3+ 3+ phosphors excited at 365 nm. +yZn2V3O12:ySm

and

Ca2−yNa1

x(Eu3+)

(x,y)

y(Sm3+)

(x,y)

0 0.01 0.03 0.08 0.10

(0.2146,0.3380) (0.2759,0.3837) (0.2688,0.3711) (0.2914,0.3591) (0.3154,0.4060)

0 0.01 0.03 0.08 0.10

(0.2146,0.3380) (0.2548,0.3840) (0.2673,0.3794) (0.2794,0.3740) (0.2931,0.3741)

In summary, we have successfully synthesized the selfactivated and Eu3+ or Sm3+ doped vanadate Ca2NaZn2V3O12 phosphors via the solid state reaction. XRD results indicate the formation of single phase compound with garnet structure. Through the study of luminescence properties and decay curves, we can find that energy transfer takes place from the VO43− to the dopant RE ions. Ca2NaZn2V3O12 consists of a broad emission band covering 400–700 nm with emission peak near 497 nm. For the Eu3+ or Sm3+ doped Ca2NaZn2V3O12 phosphors, the emission spectra are composed of the broad-band emission located near 497 nm and the emission lines ascribed to the f–f transitions of Eu3 + /Sm3+. Because of the energy transfer between VO43− and the dopant RE ions, the CIE coordinates of Ca2NaZn2V3O12:Eu3+ and Ca2NaZn2V3O12:Sm3+ phosphors can be adjusted by controlling the rare earth ions concentration. The results in this work indicate that the obtained phosphor has great potential to develop a suitable phosphor for the application on near-UV excited white LEDs.

Acknowledgments This present work was supported by the National Natural Science Foundations of China (Grant Nos. 51002146 and 51272242), the Natural Science Foundations of Beijing (2132050), the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-12-0950), the Fundamental Research Funds for the Central Universities (2011YYL131) and the College Student Research Innovation Program of China University of Geosciences, Beijing (2012AG0025).

References

Fig. 9. CIE chromaticity diagram for Ca2−xNa1+xZn2V3O12:Sm3+(x ¼ 0, 0.03, 0.08 and 0.10) phosphors (points a–d) excited by 365 nm UV lamp and the corresponding images.

ions from green emission to yellow, even the white emission. The absolute QE values of the selected phosphors of Ca2NaZn2V3O12 host,

[1] Z.G. Xia, J.Q. Zhuang, H.K. Liu, L.B. Liao, J. Phys. D: Appl. Phys. 45 (2012) 015302. [2] L.L. Han, Y.H. Wang, J. Zhang, Y.Z. Wang, Mater. Chem. Phys. 139 (2013) 87. [3] G. Gundiah, Y. Shimomura, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 455 (2008) 279. [4] K.J. Park, M.A. Lim, H.D. Park, Appl. Phys. Lett. 82 (2003) 683. [5] T. Nakajima, M. Isobe, T. Tsuchiya, Y. Ueda, T. Kumagai, J. Lumin. 129 (2009) 1598. [6] A. Nakatsukaa, Y. Ikutab, A. Yoshiasac, K. Iishi, Mater. Res. Bull. 39 (2004) 949. [7] Y.L. Huang, Y.M. Yu, T. Tsuboi, H.J. Seo, Opt. Express 20 (2012) 4360. [8] X.Q. Su, B. Yan, J. Alloys Compd. 421 (2006) 273. [9] G. Bayer, J. Am. Ceram. Soc. 48 (1965) 600. [10] A.R. Dhobale, M. Mohapatra, V. Natarajan, S.V. Godbole, J. Lumin. 132 (2012) 293. [11] J.F. Li, K.H. Qiu, J.F. Li, W. Li, Q. Yang, J.H. Li, Mater. Res. Bull. 45 (2010) 598. [12] X.H. Dou, W.R. Zhao, E.H. Song, G.X. Zhou, C.Y. Yi, M.K. Zhou, Spectrochim. Acta A 78 (2011) 821. [13] S.X. Yan, J.H. Zhang, X. Zhang, S.Z. Lu, X.G. Ren, Z.G. Nie, X.J. Wang, J. Phys. Chem. C 111 (2007) 13256. [14] Z.G. Xia, D.M. Chen, J. Am. Ceram. Soc. 464 (2010) 565. [15] Z.G. Xia, L.B. Liao, Z.P. Zhang, Y.F. Wang, Mater. Res. Bull. 47 (2012) 405. [16] Z.G. Xia, X.M. Wang, Y.X. Wang, L.B. Liao, X.P. Jing, Inorg. Chem. 50 (2011) 10134.