Tm3+ tri-doped Gd2Mo3O9 phosphors

Materials Science and Engineering B 178 (2013) 822–825

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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb

Short communication

White upconverted luminescence of Ho3+ /Yb3+ /Tm3+ tri-doped Gd2 Mo3 O9 phosphors Jiayue Sun ∗ , Bing Xue, Haiyan Du School of Science, Beijing Technology and Business University, Beijing 100048, China

a r t i c l e

i n f o

Article history: Received 17 December 2012 Received in revised form 29 March 2013 Accepted 16 April 2013 Available online 30 April 2013 Keywords: White-light emission Rare earths Gd2 Mo3 O9 phosphors

a b s t r a c t Yb3+ /Tm3+ /Ho3+ tri-doped Gd2 Mo3 O9 phosphors were synthesized by the high-temperature solid-state method. Under 980 nm near-infrared excitation, the white-light emission can be observed, which is consists of the blue, green, and red UC emissions. The green and red emission at 547 nm and 660 nm originated from the transition of Ho3+ (5 S2 , 5 F4 → 5 I8 and 5 F5 → 5 I8 ) and the blue emission at 475 nm attributed to the transition of Tm3+ (5 G4 → 5 H6 ). In this experiment, we selected the optimum concentration ratio of the three rare earths for the bright white emission. The Commission internationale de L’Eclairage (CIE) coordinates for the samples were calculated, and chromaticity coordinates were very close to white light regions. We find that the calculated CIE color coordinates of the Yb3+ / Tm3+ /Ho3+ tri-doped Gd2 Mo3 O9 phosphors changed with the incident pump power from 400 mW/cm2 to 1000 mW/cm2 . The upconversion luminescence mechanism of the samples was discussed on its spectral. The white light may be proved to be a candidate material for applications in various fields. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Recently, the generation of white light sources through upconversion (UC) process has received significant interest for a variety of applications, such as multicolor display, solid-state lighting, and light emitting diodes (LEDs) [1]. Especially phosphorconverted w-LEDs, because of their high luminous efficiency, long lifetimes, low environmental impact, and small structure type [2,3]. So far, the matrixes doped by several rare earths for white light emitting diodes have been developed, such as oxides [4–6], tungstate [7], molybdate [8,9] and glasses [10,11]. Moreover, the molybdate and tungstate families have been extensively studied due to their luminescent behavior and structural properties [12–14]. It is well-known that a high efficient luminescence system consists of three parts, the host, the activator and the sensitizer. Commonly, Yb3+ acts as a sensitizer to enhance UC luminescence efficiency, owing to its large absorption cross-section and efficient energy transfer (ET) to other lanthanide ions [15–18]. While, Ho3+ and Tm3+ ions are excellent dopant candidates as activator, because of their favorable intra-atomic 4f energy level structure. However, the host materials that are suitable for upconversion white luminescence are still limited and need to be further explored. Selecting a suitable host is a key factor for upconversion luminescent materials. Many experiments proved that the

rare earth tungstate and molybdate matrix are highly chemically, photo-thermally, and photo-chemically stable and have a broad optical transparence from the visible to the NIR regions [19,20]. As we know, Eu3+ doped Gd2 Mo3 O9 phosphor for redlight-emitting-diodes has been widely studied [21,22]. But the white light emission in this phosphor has not been researched. So, in this article, Yb3+ /Tm3+ /Ho3+ /Gd2 Mo3 O9 phosphors for whitelight emitting were successfully prepared by the high temperature solid-state method. The efficient energy transfer from Yb3+ to Ho3+ or Tm3+ ions contributes to this emission. Meanwhile, the UC properties and the optimal conditions for white light emission were investigated. Addition, we explored the changing tendency between the pump power and the white-light emitting. 2. Experimental 2.1. Materials The starting materials were Na2 CO3 (99.9%), MoO3 (A.R.), Gd2 O3 (99.99%), Yb2 O3 (99.99%), Tm2 O3 (99.99%) and Ho2 O3 (99.99%). Na2 CO3 with purity of 99.9% was used as flux to improve the chemical reaction. All the materials were used without further purification. 2.2. Preparation of Gd2 Mo3 O9 :Tm3+ /Ho3+ /Yb3+ phosphors

∗ Corresponding author. Tel.: +86 10 6898 5467; fax: +86 10 6898 5467. E-mail address: jiayue [email protected] (J. Sun). 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.04.006

According to certain stoichiometric ratio, we prepared three group experiments,

J. Sun et al. / Materials Science and Engineering B 178 (2013) 822–825

2.3. Characterization The structures were analyzed on a Shimadzu model XRD-6000 powder diffractometer (Cu-Ka radiation, 40 kV, 30 mA and a step size of 0.06◦ (2)). XRD patterns were recorded in the range of 20–60◦ . The room temperature UCL spectra among the wavelength range of 400–700 nm were measured using a FluoroLog-3 spectrophotometer (HORIBAJOBINYVON, USA) equipped with an external power-controllable 980 nm diode laser (Beijing Viasho Technology Company, China). 3. Results and discussion Fig. 1 shows the XRD patterns of the sample of Gd1.875 Mo3 O9 :0.02Tm3+ /0.005Ho3+ /0.1Yb3+ , Gd1.778 Mo3 O9 :0.02Tm3+ / 0.002Ho3+ /0.2Yb3+ , Gd1.845 Mo3 O9 :0.05Tm3+ /0.005Ho3+ /0.1Yb3+ , which was calcined in a muffle furnace at 800 ◦ C for 4 h, and the 7 mol% Na2 CO3 as the flux used to depress the sintering temperature, shorten the reaction time and improve the crystallization [21–23]. At here, we just show three XRD curves, which are all doped with the maximum rare earths in the three groups experiments. As shown in Fig. 1, the XRD patterns of Tm3+ /Ho3+ /Yb3+ tri-doped Gd2 Mo3 O9 match well with Gd2 Mo3 O9 structure (JCPDS card 33-0548). It indicates that the crystal lattice and basic structure of can be obtained successfully by the high temperature solid-state method and will not be affected by the little dopants of rare earths. The UC fluorescence emission spectra of the Gd1.978−x Mo3 O9 doped with 0.02Tm3+ /0.002Ho3+ /xYb3+ (x = 0.1, 0.15, 0.20) phosphors under the diode laser excitation of 980 nm are shown in Fig. 2. The strong blue, green and red emission are observed. The blue emission peaks can be attributed to 1 G4 → 3 H6 transition of Tm3+ . The green and red emission corresponds to 5 S2 , 5 F4 → 5 I8 and

3+

3+

3+

3+

3+

3+

3+

450

500

3+

Gd1.845Mo3O9,0.005Ho ,0.1Yb ,0.05Tm

30

35

40

45

50

600

700

5 F → 5 I transition, which are assigned to the intra-4f transitions of 5 8 Ho3+ ions. From Fig. 2, we can observe that the blue and green or red

emission have different varying tendency. The blue emission first increases slightly and then increases, but the green and red emission increases first and then decreases with increasing the Yb3+ ions doping concentration. The intensities of green and red emission are changing obviously compared to the blue emission. It may due to the more energy will be absorbed and transferred; besides, the photon count needed for blue emission is larger than red or green emission [24]. To measure the color of the visible emission that the human eye perceives, the Commission internationale de L’Eclairage (CIE) coordinates were calculated. The calculated CIE coordinates at the Yb3+ concentration of 0.1 mol, 0.15 mol and 0.2 mol are (0.35, 0.41), (0.29, 0.35) and (0.26, 0.38) respectively. Because of the coordinate (0.29, 0.35) is most close to (0.33, 0.33). So we choose 0.15 mol Yb3+ as the following synthesis concentration. Fig. 3 shows the emission of Gd1.83−x Mo3 O9 :0.02Tm3+ /xHo3+ /0.15Yb3+ (x = 0.001, 0.002, 0.005) phosphor, which exhibits that the red and green emissions are changed greatly and blue emission is changed slightly. That is because the probability of receiving energy for Ho3+

3+

3+

0.001Ho ,0.02Tm ,0.15Yb 3+ 3+ 3+ 0.002Ho ,0.02Tm ,0.15Yb 3+ 3+ 3+ 0.005Ho ,0.02Tm ,0.15Yb

450

55

2θ degree

650

Fig. 2. Photoluminescence spectra of Gd2 Mo3 O9 UC phosphors with various Yb3+ concentration with a fixed Ho3+ concentration at 0.2 mol% and Tm3+ concentration 2 mol%.

Intensity(a.u.)

Intensity(a.u.)

3+

550

3+

Gd1.778Mo3O9,0.002Ho ,0.2Yb ,0.02Tm

3+

Wavelength(nm)

Gd1.875Mo3O9,0.005Ho ,0.1Yb ,0.02Tm

3+

3+

0.10Yb ,0.002Ho ,0.02Tm 3+ 3+ 3+ 0.15Yb ,0.002Ho ,0.02Tm 3+ 3+ 3+ 0.20Yb ,0.002Ho ,0.02Tm

Intensity(a.u.)

(1) Gd1.978−x Mo3 O9 :0.02Tm3+ /0.002Ho3+ /xYb3+ (x = 0.1, 0.15, 0.2), (2) Gd1.88−x Mo3 O9 :0.02Tm3+ /xHo3+ /0.1Yb3+ (x = 0.001, 0.002, 0.005), (3) Gd1.895−x Mo3 O9 :xTm3+ /0.005Ho3+ /0.1Yb3+ (x = 0.01, 0.02, 0.03, 0.04, 0.05). The raw material was grinded thoroughly in an agate mortar. Then the mixtures were put into alumina crucibles and calcined in a muffle furnace at 800 ◦ C for 4 h. Finally, the corresponding phosphors were obtained when the furnace cooled naturally down to room temperature.

823

500

550

600

650

700

Wavelength(nm) 3+

3+

3+

Fig. 1. The XRD patterns of samples Gd1.845 Mo3 O9 :0.05Tm /0.1Yb /0.005Ho , Gd1.778 Mo3 O9 :0.02Tm3+ /0.2Yb3+/ 0.002Ho3+ and Gd1.875 Mo3 O9 :0.02Tm3+ /0.1Yb3+ / 0.005Ho3+ prepared at 800 ◦ C for 4 h.

Fig. 3. Photoluminescence spectra of Gd2 Mo3 O9 UC phosphors with various Ho3+ concentration with a fixed Yb3+ concentration at 15 mol% and Tm3+ concentration 2 mol%.

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J. Sun et al. / Materials Science and Engineering B 178 (2013) 822–825

3+

3+

3+

Intensity(a.u.)

0.01Tm ,0.005Ho ,0.15Yb 3+ 3+ 3+ 0.02Tm ,0.005Ho ,0.15Yb 3+ 3+ 3+ 0.03Tm ,0.005Ho ,0.15Yb 3+ 3+ 3+ 0.04Tm ,0.005Ho ,0.15Yb 3+ 3+ 3+ 0.05Tm ,0.005Ho ,0.15Yb

450

500

550

600

650

700

Wavelength(nm) Fig. 4. Photoluminescence spectra of Gd2 Mo3 O9 UC phosphors with various Tm3+ concentration with a fixed Yb3+ concentration at 15 mol% and Ho3+ concentration 0.5 mol%.

I ∝ Pm

(1)

where m is the number of infrared photons absorbed for emitting a visible photon. As shown in Fig. 6, the double-log coordinate has been displayed. The slopes of the curve Ln(I) versus Ln(P) for blue emission were measured to be 3.04, while for the green and red emissions are 2.06 and 2.08 respectively. These results indicate that

Fig. 5. The CIE chromaticity diagram with the calculated color coordinates under the excitation of 980 nm LD with various pump powers. (a) 400 mW/cm2 , (b) 600 mW/cm2 , (c) 800 mW/cm2 , (d) 1000 mW/cm2 , and the corresponding coordinates are (0.36, 0.40), (0.31, 0.35), (0.30, 0.35) and (0.28, 0.31), respectively.

three-photon processes contribute to the blue UC emissions and two-photon processes contribute to the red and green UC emissions. At here, we must point out that the CIE coordinates toward blue area with increasing pump power. This result may explain as follows: three-photon process are involved to produce the blue emission, which is higher than that of the green and red emission, resulting in that the blue emission excitation process is more sensitive to pump changing [1]. The schematic energy level diagrams of the Ho3+ , Yb3+ , Tm3+ tri-doped Gd2 Mo3 O9 ions are shown in Fig. 7. The mechanism of the UC emission for white light system from the Ho3+ , Yb3+ , Tm3+ tri-doped phosphors and Er3+ , Yb3+ , Tm3+ tri-doped phosphors has

8.0

Ln I(Intensity(a.u.))

ions will be increased with the increase in Ho3+ contents. The calculated coordinates are (0.38, 0.44), (0.29, 0.35) and (0.30, 0.35). At here 0.005 mol was selected as the optimum of Ho3+ concentration, because of its coordinate most close to (0.33, 0.33). Finally, Fig. 4 shows the Gd1.845−x Mo3 O9 :xTm3+ /0.005Ho3+ /0.15Yb3+ (x = 0.01, 0.02, 0.03, 0.04, 0.05) emission. From the figure, we can see that the blue emission intensity increases first and then decreases, and has a maximum concentration at 0.02 mol of Tm3+ , the green and red emission decreases monotonically with the Tm3+ increasing. It is speculated that decrease in green, red emissions are dominated by lower probability of receiving energy for Ho3+ ions with the increase of Tm3+ contents [24]. The color coordinates for the Gd1.845−x Mo3 O9 :xTm3+ /0.005Ho3+ /0.15Yb3+ (x = 0.01, 0.02, 0.03, 0.04, 0.05) phosphors had been analyzed. The corresponding coordinates are (0.39, 0.42), (0.30, 0.35), (0.31, 0.35), (0.34, 0.37), and (0.37, 0.42), respectively. From the above digits, we can get that the digit (0.31, 0.35) is most closely to (0.33, 0.33), which was obtained by the Gd2 Mo3 O9 :0.03Tm3+ /0.005Ho3+ /0.15Yb3+ phosphors and fall well within the white region of the CIE diagram. The UC emission spectrum of the sample is converted to the chromaticity coordinate of CIE 1931 standard colorimetric system. The calculated color coordinates when changing the pump for the Gd2 Mo3 O9 :Yb3+ /Tm3+ /Ho3+ samples are marked as sites (a–d) in CIE chromaticity diagram (Fig. 5), which are (0.36, 0.40), (0.31, 0.35), (0.30, 0.35) and (0.28, 0.31), respectively. The CIE chromaticity coordinates are very close to white light regions. So we can propose that the white luminescence upon 980 nm near-infrared excitation can be produced via an UC process by adjusting the concentration of dopants based on the above spectral analysis. But we also knew that further efforts should be put into effect to adjust the color components and enhance the upconversion efficiency for practical application. To study the mechanism of the UC process, the upconverted luminescence intensity (I) of the green, red and blue emission in Gd2 Mo3 O9 :0.03Tm3+ /0.005Ho3+ /0.1Yb3+ phosphors was measured as a function of the pump power (P) [25,26].

Blue Green Red

7.5 Slope=2.08

7.0 6.5 6.0

Slope=3.04 Slope=2.06

5.5 5.0 -0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

Ln P(mW) Fig. 6. Power dependence of the red, green and blue upconversion emission intensity of 0.005Ho3+ /0.03Tm3+ /0.15Yb3+ co-doped Gd2 Mo3 O9 phosphors.

J. Sun et al. / Materials Science and Engineering B 178 (2013) 822–825

1

5

G4

3

5

ET6

F3

H4

5 5

ET3

ET5

5

3

ESA

F5/2

H6

2

3+

Tm

F7/2

I5

I6

ET1

Green

5

Red

ET4

5

980nm

F4

Blue

3

H5

2

GSA

3

F5

I4

3

-1

Energy/10 (cm )

F2

5

S2 F4

ET2

3 3

F2

I7

5

3+

Yb

3+

Ho

I8

Fig. 7. Energy level diagram of Tm3+ , Ho3+ and Yb3+ tri-doped Gd2 Mo3 O9 and possible UC mechanisms under 980 nm excitation.

been widely researched. It is well known that there exists an efficient energy transfer process between Yb3+ and Tm3+ or Yb3+ and Ho3+ because the Yb3+ ions have a much larger absorption cross section around 980 nm [26]. In this work, the excitation mechanism for green and red emission at 547 nm and 660 nm associated to Ho3+ ions is accomplished through two successive energy transfer steps from Yb3+ ions [27] Yb3+ (2 F5/2 ) + Ho3+ (5 I8 ) → Yb3+ (2 F7/2 ) + Ho3+ (5 I6 )phononenergy

(ET1)

Yb3+ (2 F5/2 ) + Ho3+ (5 I6 ) → Yb3+ (2 F7/2 ) + Ho3+ (5 S2 /5 F4 )phononenergy

(ET2)

5 S /5 F 2 4

Then, the green emission produced by the radiatively to the 5 I8 level. While, the red emission caused by the 5 F level radiatively to the 5 I level. The 5 F level populated 5 5 8 by the following process: the 5 I6 level non-radiatively to the 5 I level and reached the 5 F level by another energy trans7 5 fer Yb3+ (2 F5/2 ) + Ho3+ (5 I7 ) → Yb3+ (2 F7/2 ) + Ho3+ (5 F5 ) phonon energy (ET3). The blue emission at 475 nm related to Tm3+ of three-photon process, indicates that a three successive energy transfer was needed [28]. We described as follows: Yb3+ (2 F5/2 ) + Tm3+ (3 H6 ) → Yb3+ (2 F7/2 ) + Tm3+ (3 H5 ) phonon energy(ET4), and then the Tm3+ ions in the 3 H5 excited level relax non-resonantly to the 3 F4 metastable level. From that level, a second ET process takes place: Yb3+ (2 F5/2 ) + Tm3+ (3 F4 ) → Yb3+ (2 F7/2 ) + Tm3+ (3 F2,3 ) phonon energy (ET5), and the 3 F2,3 states also relax by multi-phonon process to the 3 H4 state. Finally, the third ET process is occurred, Yb3+ (2 F5/2 ) + Tm3+ (3 H4 ) → Yb3+ (2 F7/2 ) + Tm3+ (1 G4 ) phonon energy (ET6). From the 1 G4 level, the Tm3+ ions radiatively relax to the 3 H6 states, generating the blue upconversion fluorescence emission around 475 nm. In summary, the energy matches between the two levels and the long lifetimes in the metastable states allow the energy transfer process occurring efficiently [29]. 4. Conclusions The phosphor samples of Gd2 Mo3 O9 doped with different lanthanide ions (Yb3+ /Tm3+ /Ho3+ ) were prepared by the hightemperature solid-state reaction. In this work, by retuning the proportion of Yb3+ , Tm3+ and Ho3+ ions concentration, the white light emission from Yb3+ /Tm3+ /Ho3+ tri-doped Gd2 Mo3 O9 materials had achieved in the present study. The optimum chemical composition is 10%Yb3+ , 3%Tm3+ , 0.5%Ho3+ in the Gd2 Mo3 O9 matrix. The white luminescence upon 980 nm semiconductor laser excitation comes from the combination of the blue (from Tm3+ ), green (from Ho3+ ) and red (from Ho3+ ), which corresponding to the

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transitions 1 G4 → 3 H6 of Tm3+ , 5 F4 (5 S2 ) → 5 I8 , and 5 F5 → 5 I8 of Ho3+ ions, respectively. The CIE coordinates for the Gd2 Mo3 O9 samples were calculated, and the color coordinates fall close to white light regions of the 1931 CIE diagram in a wide range of pump power. The spectra and pump power dependence analysis demonstrate that the three-photon for the blue emissions and two-photon processes are responsible for the green and red emissions. Finally, the upconversion luminescence mechanism for Yb3+ , Ho3+ and Tm3+ tri-doped Gd2 Mo3 O9 and the sensitizing ion Yb3+ was investigated. All the study indicates that the Gd2 Mo3 O9 phosphors can be a promising host to achieve white-light materials. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 20976002), the Beijing Natural Science Foundation (No. 2122012), Key Projects for Science and Technology of Beijing Education Commission (KZ201310011013), Project of Transformation and Industrialization of College Scientific & Technological Achievements, and Projects of the combination of Manufacture, Education & Research of Guangdong Province (No. 2011B090400100). References [1] L.N. Guo, Y.H. Wang, J. Zhang, P.Y. Dong, Journal of Electrochemical Society 158 (2011) 225–229. [2] S. Nakamura, M. Senoh, T. Mukai, Applied Physics Letters 62 (1993) 2390. [3] G. Sinha, A. Patra, Chemical Physics Letters 473 (2009) 151. [4] V. Mahalingam, R. Naccache, F. Vetrone, J.A. Capobianco, Optics Express 20 (2012) 111–119. [5] Y. Li, J. Zhang, Y. Luo, X. Zhang, Z. Hao, X. Wang, Journal of Materials Chemistry 21 (2011) 2895–2900. [6] Y.F. Bai, Y.X. Wang, G. Peng, W. Zhang, K. Yang, X.R. Zhang, Y.L. Song, Optics Communications 282 (2009) 1922–1924. [7] J.H. Chung, S.Y. Lee, K.B. Shim, J.H. Ryu, Applied Physics Express 5 (2012) 052602. [8] J.H. Chung, J.H. Ryu, S.W. Mhin, K.M. Kim, K.B. Shim, Journal of Materials Chemistry 22 (2012) 3997–4002. [9] D.Y. Li, Y.X. Wang, X.R. Zhang, G. Shi, G. Liu, Y.L. Song, Journal of Alloys and Compounds 550 (2013) 509–513. [10] G. Lakshminarayana, R. Yang, J.R. Qiu, M.G. Brik, G.A. Kumar, I.V. Kityk, Journal of Physics D: Applied Physics 42 (2009) 015414. [11] G. Lakshminarayana, J. Qiu, M.G. Brik, G.A. Kumar, Journal of Physics: Condensed Matter 20 (2008) 3745101. [12] P. Yang, C. Li, W. Wang, Z. Qu an, S. Gai, J. Li n, Journal of Solid State Chemistry 182 (2009) 2510–2520. [13] E.F. Paski, M. Blades, Analytical Chemistry 60 (1988) 1224–1230. [14] G. Ahmad, M.B. Dickerson, B.C. Church, Y. Cai, S.E. Jones, R.R. Naik, J.S. King, C.J. Summers, N. Kröger, K.H. Sandhage, Advanced Materials 18 (2006) 1759–1763. [15] X.Q. Chen, Y.L. Li, F. Kong, L.P. Li, Q. Sun, F.P. Wang, Journal of Alloys and Compounds 541 (2012) 505–509. [16] Z. Wang, L. Wu, H. Liang, W. Cai, Z. Zhang, Z. Jiang, Journal of Alloys and Compounds 509 (2011) 9144–9149. [17] G. Glaspell, J. Anderson, J.R. Wilkins, M.S. El-Shall, Journal of Physical Chemistry C 112 (2008) 11527–11531. [18] F. Vetrone, J.C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, Journal of Physical Chemistry B 107 (2003) 1107–1112. [19] Z.G. Xia, W. Zhou, H.Y. Du, J.Y. Sun, Materials Research Bulletin 45 (2010) 1199–1202. [20] J. Wang, X. Jing, C. Yan, J. Lin, F. Liao, Journal of Luminescence 121 (2006) 57. [21] X.X. Zhao, X.J. Wang, B.J. Chen, Q.Y. Meng, W.H. Di, G.Z. Ren, Y.M. Yang, Journal of Alloys and Compounds 433 (2007) 352–355. [22] X.X. Zhao, X.J. Wang, B.J. Chen, Q.Y. Meng, W.H. Di, G.Z. Ren, Y.M. Yang, Journal of Rare Earth 25 (2007) 15–18. [23] L.H. Zhang, H.Y. Zhong, X.P. Li, L.H. Cheng, L. Yao, J.S. Sun, J.S. Zhang, R.N. Hua, B.J. Chen, Physica B 407 (2012) 68–72. [24] L.L. Xing, R. Wang, W. Xu, Y.N. Qian, Y.L. Xu, C.H. Yang, X.R. Liu, Journal of Luminescence 132 (2012) 1568–1574. [25] T. Passuello, F. Piccinelli, M. Pedroni, M. Bettinelli, F. Mangiarini, R. Naccache, F. Vetrone, J.A. Capobianco, A. Speghini, Optical Materials 33 (2011) 643–646. [26] X.L. Pang, C.H. Jia, G.Q. Li, W.F. Zhang, Optical Materials 34 (2011) 234–238. [27] H.Y. Du, Y.J. Lan, Z.G. Xia, J.Y. Sun, Journal of Rare Earth 28 (2010) 697. [28] D.Y. Li, Y.X. Wang, X.R. Zhang, K. Yang, L. Liu, Y.L. Song, Optics Communications 285 (2012) 1925–1928. [29] H. Hu, Y. Bai, Journal of Alloys and Compounds 527 (2012) 25–29.