- Email: [email protected]
Eu3+-doped Na5Al(PO4)2F2 single-phase phosphor
Materials Letters 160 (2015) 294–297
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Structure and tunable blue-white-red luminescence of Eu2 þ /Eu3 þ -doped Na5Al(PO4)2F2 single-phase phosphor Ruijin Yu a,b, Jing Wang a, Ze Zhao a, Mengxue Li a, Shuaidong Huo b, Junbo Li b, Jinyi Wang a,n a b
College of Science, Northwest A&F University, Yangling, Shaanxi 712100, PR China Department of Chemistry, University of Massachusetts Amherst 710 North Pleasant Street, Amherst, MA 01003, United States
art ic l e i nf o
a b s t r a c t
Article history: Received 6 June 2015 Received in revised form 27 July 2015 Accepted 29 July 2015 Available online 30 July 2015
Eu2 þ /Eu3 þ -doped fluorophosphate Na5Al(PO4)2F2 was synthesized by the high-temperature solid-state reaction. The structural XRD refinement was used to investigate the phase formation and structure. The photoluminescence (PL) excitation and emission spectra were measured. The host accommodates two kinds of Eu-centers in the lattices, i.e., Eu3 þ and Eu2 þ , which were confirmed by the emission spectra and decay curves. Under the excitation of UV and near-UV light, the broad emission peaked at 420 nm and 530 nm from the 4f65d–4f7 transition of Eu2 þ and narrow peaks at 592 nm, 615 nm from the 4f– 4f transitions of Eu3 þ ions were detected. The luminescence color could shift from blue to white and red by mixing the luminescence from the coexistence of Eu2 þ /Eu3 þ ions in the single-phase phosphor. & 2015 Elsevier B.V. All rights reserved.
Keywords: Eu2 þ Eu3 þ Luminescence Optical materials and properties Phosphors Microstructure
1. Introduction It is well-known that europium ion has two possible valences, i.e., Eu2 þ and Eu3 þ ions in a host, which can be identified because of the distinguished spectra and decay lifetime from each emission in solids [1,2]. Eu3 þ shows red-luminescence (570 750 nm) from the intra-configurational 4f6-4f6 parity forbidden transitions with narrow peaks of 5D0-7FJ (J ¼0–4) with lifetime in ms scale [3]. However, Eu2 þ has broad emission from the parity allowed transitions 4f65d1-4f7 with fast decay (ns scale) [4–6]. Eu2 þ presents wide emission range changing in blue-green-yellow or red band. As a result phosphors containing both Eu2 þ and Eu3 þ ions could provide tunable colors by making full use of emissions from Eu2 þ and Eu3 þ [7,8]. This is possible to develop a single-phased white-light-emitting phosphor by mixing from Eu2 þ and Eu3 þ centers in a host. In this work, Na5Al(PO4)2F2 was selected to develop a phosphor activated by Eu2 þ /Eu3 þ ions. The first motivation is involved in F ions in the lattices, which could draw electronic cloud more intensively than O2 . The incorporation of F in RE-activated oxides can enhance the luminescence efficiency and the optical damage [9,10]. Secondly, Na5Al(PO4)2F2 has the rigid three dimensional framework formed by PO4 tetrahedral and AlO4F2 octahedral corner-connecting with each other. This rigid characteristic can n
Corresponding author. Fax: þ86 29 87082520. E-mail address: [email protected] (J. Wang).
http://dx.doi.org/10.1016/j.matlet.2015.07.148 0167-577X/& 2015 Elsevier B.V. All rights reserved.
enhance its thermal stability of the luminescence. Na5Al(PO4)2F2:Eu was synthesized by traditional solid-state reaction method. The structures were characterized structural refinements. The luminescence was characterized by the photoluminescence spectra and decay. The Eu ions can be incorporated in Eu3 þ and Eu2 þ states, which induce tunable colors under different excitation wavelength.
2. Experimental Na5Al(PO4)2F2:Eu were prepared by solid state reaction. The doping concentration is 7.0 mol% of Eu ions. The starting materials were AlPO4 (99.9%), NaF (99.9%) and Eu2O3 (99.99%). The mixture was ground together in an agate mortar for enough time. Then the mixture was heated at 850 °C for 6 h in a crucible with a cover with the reducing agent (active carbon). Then the powders were quenched to room temperature. The phase purity was checked by powder X-ray diffraction (XRD) analysis collected on a Rigaku D/Max diffractometer operating at 40 kV, 30 mA with Bragg–Brentano geometry using Cu-Kα radiation (λ ¼1.5405 Å). The morphology of the powder was observed using a JEOL-5600 scanning electron microscope. The photoluminescence excitation and emission spectra were recorded on a Perkin-Elmer LS-50B luminescence spectrometer. The luminescence decay was measured using the fourth harmonic (266 nm) of a pulsed Nd:YAG laser.
R. Yu et al. / Materials Letters 160 (2015) 294–297
295
3. Results and discussion Fig. 1(a) shows the XRD pattern of Na5Al(PO4)2F2:Eu and the Rietveld structural refinement using the GSAS program. The observed pattern can be well indexed according to the standard card PDF#39-0325 (Na5Al(PO4)2F2) selected in the International Centre for Diffraction Data (ICDD) database. This reveals that the phosphor crystallized in a pure single-phase, and no impurity lines were observed. Fig. 1(b and c) shows the crystal structure of Na5Al (PO4)2F2:Eu modeled using the atomic coordinate’s data from the refinements. Na5AlF2(PO4)2 crystallizes in a trigonal cell with space −
group P3. The framework is formed by PO4-tetrahedra and AlO4F2octahedra connected via common vertices forming a two dimensionally extended hetero-polyanion. Na þ is located in interconnected spacing of [AlF2(PO4)2]-part with six kinds of similar coordination (Fig. 1c). Fig. 1(d) shows the representative SEM of the phosphor prepared at 850 °C for 6 h. The powders are composted by the closely-packed and well-grown particles. The small grains present irregular elliptical or ball shapes with a size in 1– 2 μm. Fig. 2 shows the excitation (λem ¼420, 530, 615 nm) in Na5Al (PO4)2F2:Eu. The signals contain two parts from Eu2 þ and Eu3 þ coexisting in the same lattices. By monitoring 420 and 530 nm, the broad band from 200 to 400 nm can be measured from the overlap of the 4f–5d transition of Eu2 þ and the O2 þ Eu3 þ charge–transfer (CT) transition [11]. Some peaks present the dominated intensity on the spectrum (λem ¼615 nm), which are from the transitions in the 4f6 configuration of Eu3 þ . The excitation spectra present the strong intensity in the range of near-UV matching with the emission of near UV chips [12]. This indicates Na5Al(PO4)2F2:Eu is a good candidate for white-LEDs driven by near UV light. The excitation indicates that Na5Al(PO4)2F2:Eu can show the luminescence of Eu3 þ and/or Eu2 þ under the appropriate excitation wavelengths. Fig. 3 is the emission under the excitation of
Fig. 2. Excitation spectra of Na5Al(PO4)2F2:Eu (λem ¼420 nm, 530 nm, and λem ¼ 615 nm).
250, 265, 325, 380, 395 and 465 nm. The spectra present the broad band (350–600 nm) and sharp peaks. Each spectrum contents the luminescence characteristics of Eu2 þ /Eu3 þ ions. The broad emission bands are from 4f65d-4f7 transition of Eu2 þ ions, and the emission lines are induced by 4f-4f transitions 5D0-7FJ (J ¼1, 2, 3 and 4) transitions of Eu3 þ [4]. It has been established that 4f65d-4f7 transitions from Eu2 þ ions in a compound are allowed; consequently the luminescence spectra are broad and the luminescence lifetimes are very short in nanosecond to microsecond regions. While, Eu3 þ ions show distinct spectra presenting emission lines in the region of 560– 720 nm from 5D0-7FJ (J ¼0–4) transitions with a lifetime longer than several milliseconds. So Eu2 þ and Eu3 þ can be well separated by emission spectra and luminescence decay curves [4]. To identify the Eu2 þ ions, the decay curves of 420 nm and 530 nm are shown
Fig. 1. (A): the XRD structural refinement using the GSAS program; (b): schematic structure views along [001] direction; (c): the coordination surrounding of Na þ ions; and (d): the SEM picture of Na5Al(PO4)2F2:Eu nanoparticles.
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R. Yu et al. / Materials Letters 160 (2015) 294–297
Fig. 3. Emission spectra of Na5Al(PO4)2F2:Eu (λex ¼ 250, 265, 325, 380, 395 and 465 nm).
Fig. 4. the decay curves of 420 nm and 530 nm for Eu2 þ centers and 615 nm for Eu3 þ ions in Na5Al(PO4)2F2:Eu.
in Fig. 4 (λex ¼ 355 nm). The two emission bands of 420 nm and 530 nm have lifetimes of 0.69 and 0.98 μs, respectively. The luminescence of 615 nm from 5D0 states of Eu3 þ (inset Fig. 4) shows the long lifetimes of 0.68 ms. It is reasonable that there several emission sites available for Eu2 þ and Eu3 þ centers as shown in Fig. 1(c), which closely overlap together because of the similar surrounding in the lattices. The result indicates the emission band is the emission of Eu2 þ ions, not from the defects such as cation vacancies in the sample. In Na5Al(PO4)2F2:Eu the charge compensation is necessary because the different charge states between the doping ions Eu2 þ /3 þ and Na þ ions in the same lattices. To keep the electroneutrality for the substitution of Eu2 þ /3 þ on Na þ , cation vacancy defect VNa´ with negative charge will compensate the induced positive defects of Eu3Naþ ̇ or Eu3Naþ ̇ , forming a dipole complexes of [(Eu3Naþ ) ̇ 2VNa´] and [(Eu2Naþ ) ̇ VNa´]. As shown in Fig. 2 each spectrum under different excitations presents the luminescence from both Eu2 þ and Eu3 þ . Therefore,
Fig. 5. CIE chromaticity coordinates of Na5Al(PO4)2F2:Eu under various excitation.
the final emitting color is obviously various with excitation. Fig. 5 shows the CIE chromaticity diagram calculated from the emission in Fig. 3. It can be seen that the phosphors present different colors under different excitation. This is resulted by the mixed emission wavelength from the Eu2 þ and Eu3 þ centers in the same lattices [4,13]. The sample shows blue under excitation of 250 nm, while it displays bright red luminescence under the excitation of 395 and 465 nm in Eu3 þ . It is noted that the white color from a singlephase in Na5Al(PO4)2F2:Eu can be realized under the excitation of 265, 325 and 380 nm as shown in Fig. 5.
4. Conclusions Eu2 þ /Eu3 þ -doped Na5Al(PO4)2F2 was prepared by solid state reaction. The crystal was investigated by the XRD structure refinement. Eu ions present two distinct valance states, þ2, and þ3 in the lattices. Eu2 þ ions present the broad bands peaked at
R. Yu et al. / Materials Letters 160 (2015) 294–297
420 nm and 530 nm, while Eu3 þ shows the red-emission with the dominated 5D0-7F2 at 615 nm. The tunable colors of blue-whitepink-red can be observed under different excitation due to the mixed emission components from Eu2 þ and Eu3 þ . It is valuable that the white color from a single-phase in Na5Al(PO4)2F2:Eu can be realized with the excitation of UV and near UV light. Na5Al (PO4)2F2:Eu could be a potential phosphor for white-LEDs.
Acknowledgments A Na5Al(PO4)2F2:Eu phosphor was supplied by the Display and Lighting Phosphor Bank at Pukyong National University. This work was supported by The National Natural Science Foundation of China, China (Grant no. 21201141), The Chinese Universities Scientific Fund (Grant no. QN2011119), and the Young Faculty Study Abroad Program of Northwest A&F University.
297
References [1] C. Jiang, Y. Qiuhong, S. Lu, Q. Lu, Y. Yuan, Mater. Lett. 130 (2014) 296–298. [2] N. Kumar, V. Kumar, H.C. Swart, A. Mishra, J. Ngila, V. Parashar, Mater. Lett. 146 (2015) 51–54. [3] R. Yu, C. Wang, J. Chen, Y. Wu, H. Li, H. Ma, ECS J. Solid State Sci. 3 (2014) R33–R37. [4] P. Dorenbos, J. Lumin. 104 (2003) 239–260. [5] T. Li, Y. Kotuska, T. Fukuda, T. Kurushima, N. Kamata, Mater. Lett. 145 (2015) 158–161. [6] H.K. Juwhari, W.B. White, Mater. Lett. 88 (2012) 16–18. [7] G.J. Gao, S. Reibstein, M.Y. Peng, L.J. Wondraczek, Mater. Chem. 21 (2011) 3156–3161. [8] A. Baran, J. Barzowsk, M. Grinberg, S. Mahlik, K. Szczodrowski, Y. Zorenko, Opt. Mater. 35 (2013) 2107–2114. [9] Z.F. Tian, H.B. Liang, H.H. Lin, Q. Su, B. Guo, G.B. Zhang, Y.B. Fu, J. Solid State Chem. 179 (2006) 1356–1362. [10] Z. Tian, H. Liang, W. Chen, Q. Su, G. Zhang, G. Yang, Opt. Express 17 (2009) 956–962. [11] X.Y. Chen, G.K. Liu, J. Solid State Chem. 178 (2005) 419–428. [12] C.C. Lin, R.S.J. Liu, Phys. Chem. Lett. 2 (2011) 1268–1277. [13] G. Blasse, B.C. Grabmarier, Luminescence Materials, Springer-Verlin, Heidelberg (1994), p. 41.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Structure and tunable blue-white-red luminescence of Eu2 þ /Eu3 þ -doped Na5Al(PO4)2F2 single-phase phosphor Ruijin Yu a,b, Jing Wang a, Ze Zhao a, Mengxue Li a, Shuaidong Huo b, Junbo Li b, Jinyi Wang a,n a b
College of Science, Northwest A&F University, Yangling, Shaanxi 712100, PR China Department of Chemistry, University of Massachusetts Amherst 710 North Pleasant Street, Amherst, MA 01003, United States
art ic l e i nf o
a b s t r a c t
Article history: Received 6 June 2015 Received in revised form 27 July 2015 Accepted 29 July 2015 Available online 30 July 2015
Eu2 þ /Eu3 þ -doped fluorophosphate Na5Al(PO4)2F2 was synthesized by the high-temperature solid-state reaction. The structural XRD refinement was used to investigate the phase formation and structure. The photoluminescence (PL) excitation and emission spectra were measured. The host accommodates two kinds of Eu-centers in the lattices, i.e., Eu3 þ and Eu2 þ , which were confirmed by the emission spectra and decay curves. Under the excitation of UV and near-UV light, the broad emission peaked at 420 nm and 530 nm from the 4f65d–4f7 transition of Eu2 þ and narrow peaks at 592 nm, 615 nm from the 4f– 4f transitions of Eu3 þ ions were detected. The luminescence color could shift from blue to white and red by mixing the luminescence from the coexistence of Eu2 þ /Eu3 þ ions in the single-phase phosphor. & 2015 Elsevier B.V. All rights reserved.
Keywords: Eu2 þ Eu3 þ Luminescence Optical materials and properties Phosphors Microstructure
1. Introduction It is well-known that europium ion has two possible valences, i.e., Eu2 þ and Eu3 þ ions in a host, which can be identified because of the distinguished spectra and decay lifetime from each emission in solids [1,2]. Eu3 þ shows red-luminescence (570 750 nm) from the intra-configurational 4f6-4f6 parity forbidden transitions with narrow peaks of 5D0-7FJ (J ¼0–4) with lifetime in ms scale [3]. However, Eu2 þ has broad emission from the parity allowed transitions 4f65d1-4f7 with fast decay (ns scale) [4–6]. Eu2 þ presents wide emission range changing in blue-green-yellow or red band. As a result phosphors containing both Eu2 þ and Eu3 þ ions could provide tunable colors by making full use of emissions from Eu2 þ and Eu3 þ [7,8]. This is possible to develop a single-phased white-light-emitting phosphor by mixing from Eu2 þ and Eu3 þ centers in a host. In this work, Na5Al(PO4)2F2 was selected to develop a phosphor activated by Eu2 þ /Eu3 þ ions. The first motivation is involved in F ions in the lattices, which could draw electronic cloud more intensively than O2 . The incorporation of F in RE-activated oxides can enhance the luminescence efficiency and the optical damage [9,10]. Secondly, Na5Al(PO4)2F2 has the rigid three dimensional framework formed by PO4 tetrahedral and AlO4F2 octahedral corner-connecting with each other. This rigid characteristic can n
Corresponding author. Fax: þ86 29 87082520. E-mail address: [email protected] (J. Wang).
http://dx.doi.org/10.1016/j.matlet.2015.07.148 0167-577X/& 2015 Elsevier B.V. All rights reserved.
enhance its thermal stability of the luminescence. Na5Al(PO4)2F2:Eu was synthesized by traditional solid-state reaction method. The structures were characterized structural refinements. The luminescence was characterized by the photoluminescence spectra and decay. The Eu ions can be incorporated in Eu3 þ and Eu2 þ states, which induce tunable colors under different excitation wavelength.
2. Experimental Na5Al(PO4)2F2:Eu were prepared by solid state reaction. The doping concentration is 7.0 mol% of Eu ions. The starting materials were AlPO4 (99.9%), NaF (99.9%) and Eu2O3 (99.99%). The mixture was ground together in an agate mortar for enough time. Then the mixture was heated at 850 °C for 6 h in a crucible with a cover with the reducing agent (active carbon). Then the powders were quenched to room temperature. The phase purity was checked by powder X-ray diffraction (XRD) analysis collected on a Rigaku D/Max diffractometer operating at 40 kV, 30 mA with Bragg–Brentano geometry using Cu-Kα radiation (λ ¼1.5405 Å). The morphology of the powder was observed using a JEOL-5600 scanning electron microscope. The photoluminescence excitation and emission spectra were recorded on a Perkin-Elmer LS-50B luminescence spectrometer. The luminescence decay was measured using the fourth harmonic (266 nm) of a pulsed Nd:YAG laser.
R. Yu et al. / Materials Letters 160 (2015) 294–297
295
3. Results and discussion Fig. 1(a) shows the XRD pattern of Na5Al(PO4)2F2:Eu and the Rietveld structural refinement using the GSAS program. The observed pattern can be well indexed according to the standard card PDF#39-0325 (Na5Al(PO4)2F2) selected in the International Centre for Diffraction Data (ICDD) database. This reveals that the phosphor crystallized in a pure single-phase, and no impurity lines were observed. Fig. 1(b and c) shows the crystal structure of Na5Al (PO4)2F2:Eu modeled using the atomic coordinate’s data from the refinements. Na5AlF2(PO4)2 crystallizes in a trigonal cell with space −
group P3. The framework is formed by PO4-tetrahedra and AlO4F2octahedra connected via common vertices forming a two dimensionally extended hetero-polyanion. Na þ is located in interconnected spacing of [AlF2(PO4)2]-part with six kinds of similar coordination (Fig. 1c). Fig. 1(d) shows the representative SEM of the phosphor prepared at 850 °C for 6 h. The powders are composted by the closely-packed and well-grown particles. The small grains present irregular elliptical or ball shapes with a size in 1– 2 μm. Fig. 2 shows the excitation (λem ¼420, 530, 615 nm) in Na5Al (PO4)2F2:Eu. The signals contain two parts from Eu2 þ and Eu3 þ coexisting in the same lattices. By monitoring 420 and 530 nm, the broad band from 200 to 400 nm can be measured from the overlap of the 4f–5d transition of Eu2 þ and the O2 þ Eu3 þ charge–transfer (CT) transition [11]. Some peaks present the dominated intensity on the spectrum (λem ¼615 nm), which are from the transitions in the 4f6 configuration of Eu3 þ . The excitation spectra present the strong intensity in the range of near-UV matching with the emission of near UV chips [12]. This indicates Na5Al(PO4)2F2:Eu is a good candidate for white-LEDs driven by near UV light. The excitation indicates that Na5Al(PO4)2F2:Eu can show the luminescence of Eu3 þ and/or Eu2 þ under the appropriate excitation wavelengths. Fig. 3 is the emission under the excitation of
Fig. 2. Excitation spectra of Na5Al(PO4)2F2:Eu (λem ¼420 nm, 530 nm, and λem ¼ 615 nm).
250, 265, 325, 380, 395 and 465 nm. The spectra present the broad band (350–600 nm) and sharp peaks. Each spectrum contents the luminescence characteristics of Eu2 þ /Eu3 þ ions. The broad emission bands are from 4f65d-4f7 transition of Eu2 þ ions, and the emission lines are induced by 4f-4f transitions 5D0-7FJ (J ¼1, 2, 3 and 4) transitions of Eu3 þ [4]. It has been established that 4f65d-4f7 transitions from Eu2 þ ions in a compound are allowed; consequently the luminescence spectra are broad and the luminescence lifetimes are very short in nanosecond to microsecond regions. While, Eu3 þ ions show distinct spectra presenting emission lines in the region of 560– 720 nm from 5D0-7FJ (J ¼0–4) transitions with a lifetime longer than several milliseconds. So Eu2 þ and Eu3 þ can be well separated by emission spectra and luminescence decay curves [4]. To identify the Eu2 þ ions, the decay curves of 420 nm and 530 nm are shown
Fig. 1. (A): the XRD structural refinement using the GSAS program; (b): schematic structure views along [001] direction; (c): the coordination surrounding of Na þ ions; and (d): the SEM picture of Na5Al(PO4)2F2:Eu nanoparticles.
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R. Yu et al. / Materials Letters 160 (2015) 294–297
Fig. 3. Emission spectra of Na5Al(PO4)2F2:Eu (λex ¼ 250, 265, 325, 380, 395 and 465 nm).
Fig. 4. the decay curves of 420 nm and 530 nm for Eu2 þ centers and 615 nm for Eu3 þ ions in Na5Al(PO4)2F2:Eu.
in Fig. 4 (λex ¼ 355 nm). The two emission bands of 420 nm and 530 nm have lifetimes of 0.69 and 0.98 μs, respectively. The luminescence of 615 nm from 5D0 states of Eu3 þ (inset Fig. 4) shows the long lifetimes of 0.68 ms. It is reasonable that there several emission sites available for Eu2 þ and Eu3 þ centers as shown in Fig. 1(c), which closely overlap together because of the similar surrounding in the lattices. The result indicates the emission band is the emission of Eu2 þ ions, not from the defects such as cation vacancies in the sample. In Na5Al(PO4)2F2:Eu the charge compensation is necessary because the different charge states between the doping ions Eu2 þ /3 þ and Na þ ions in the same lattices. To keep the electroneutrality for the substitution of Eu2 þ /3 þ on Na þ , cation vacancy defect VNa´ with negative charge will compensate the induced positive defects of Eu3Naþ ̇ or Eu3Naþ ̇ , forming a dipole complexes of [(Eu3Naþ ) ̇ 2VNa´] and [(Eu2Naþ ) ̇ VNa´]. As shown in Fig. 2 each spectrum under different excitations presents the luminescence from both Eu2 þ and Eu3 þ . Therefore,
Fig. 5. CIE chromaticity coordinates of Na5Al(PO4)2F2:Eu under various excitation.
the final emitting color is obviously various with excitation. Fig. 5 shows the CIE chromaticity diagram calculated from the emission in Fig. 3. It can be seen that the phosphors present different colors under different excitation. This is resulted by the mixed emission wavelength from the Eu2 þ and Eu3 þ centers in the same lattices [4,13]. The sample shows blue under excitation of 250 nm, while it displays bright red luminescence under the excitation of 395 and 465 nm in Eu3 þ . It is noted that the white color from a singlephase in Na5Al(PO4)2F2:Eu can be realized under the excitation of 265, 325 and 380 nm as shown in Fig. 5.
4. Conclusions Eu2 þ /Eu3 þ -doped Na5Al(PO4)2F2 was prepared by solid state reaction. The crystal was investigated by the XRD structure refinement. Eu ions present two distinct valance states, þ2, and þ3 in the lattices. Eu2 þ ions present the broad bands peaked at
R. Yu et al. / Materials Letters 160 (2015) 294–297
420 nm and 530 nm, while Eu3 þ shows the red-emission with the dominated 5D0-7F2 at 615 nm. The tunable colors of blue-whitepink-red can be observed under different excitation due to the mixed emission components from Eu2 þ and Eu3 þ . It is valuable that the white color from a single-phase in Na5Al(PO4)2F2:Eu can be realized with the excitation of UV and near UV light. Na5Al (PO4)2F2:Eu could be a potential phosphor for white-LEDs.
Acknowledgments A Na5Al(PO4)2F2:Eu phosphor was supplied by the Display and Lighting Phosphor Bank at Pukyong National University. This work was supported by The National Natural Science Foundation of China, China (Grant no. 21201141), The Chinese Universities Scientific Fund (Grant no. QN2011119), and the Young Faculty Study Abroad Program of Northwest A&F University.
297
References [1] C. Jiang, Y. Qiuhong, S. Lu, Q. Lu, Y. Yuan, Mater. Lett. 130 (2014) 296–298. [2] N. Kumar, V. Kumar, H.C. Swart, A. Mishra, J. Ngila, V. Parashar, Mater. Lett. 146 (2015) 51–54. [3] R. Yu, C. Wang, J. Chen, Y. Wu, H. Li, H. Ma, ECS J. Solid State Sci. 3 (2014) R33–R37. [4] P. Dorenbos, J. Lumin. 104 (2003) 239–260. [5] T. Li, Y. Kotuska, T. Fukuda, T. Kurushima, N. Kamata, Mater. Lett. 145 (2015) 158–161. [6] H.K. Juwhari, W.B. White, Mater. Lett. 88 (2012) 16–18. [7] G.J. Gao, S. Reibstein, M.Y. Peng, L.J. Wondraczek, Mater. Chem. 21 (2011) 3156–3161. [8] A. Baran, J. Barzowsk, M. Grinberg, S. Mahlik, K. Szczodrowski, Y. Zorenko, Opt. Mater. 35 (2013) 2107–2114. [9] Z.F. Tian, H.B. Liang, H.H. Lin, Q. Su, B. Guo, G.B. Zhang, Y.B. Fu, J. Solid State Chem. 179 (2006) 1356–1362. [10] Z. Tian, H. Liang, W. Chen, Q. Su, G. Zhang, G. Yang, Opt. Express 17 (2009) 956–962. [11] X.Y. Chen, G.K. Liu, J. Solid State Chem. 178 (2005) 419–428. [12] C.C. Lin, R.S.J. Liu, Phys. Chem. Lett. 2 (2011) 1268–1277. [13] G. Blasse, B.C. Grabmarier, Luminescence Materials, Springer-Verlin, Heidelberg (1994), p. 41.