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Structure and luminescence properties of Ba3WO6:Eu3+ nanowire phosphors obtained by conventional solid-state reaction method
Optical Materials xxx (2014) xxx–xxx
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Structure and luminescence properties of Ba3WO6:Eu3+ nanowire phosphors obtained by conventional solid-state reaction method Yuntong Li, Xiaohua Liu ⇑ School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, People’s Republic of China
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Article history: Received 9 July 2014 Received in revised form 6 October 2014 Accepted 13 October 2014 Available online xxxx Keywords: Photoluminescence Charge compensation Phosphor
a b s t r a c t For the first time, novel Ba3xWO6:xEu3+ (x = 0.01, 0.03, 0.05, 0.08, 0.1) nanowire phosphors were synthesized by the conventional solid state method. The X-ray pattern indicates that Ba3WO6 belongs to the cubic system with space group Fm-3m. The photoluminescence (PL) spectra demonstrate that the phosphors emit strong red light centered at 595 nm corresponding to 5D0 ? 7F1 transition of Eu3+ ion under CT band excitation. The position of charge transfer (CT) band of Ba2.95WO6:0.05Eu3+ shifts to a lower energy region (red shift) with the increase of annealing temperature. The co-doped effect of alkali-metal ions (Li+, Na+, and K+) on the luminescence behavior of Ba3WO6:Eu3+ has been discussed in this paper. The luminescence properties suggest that the Ba3WO6:Eu3+ phosphor may be a promising candidate in solid-state lighting applications. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction White light-emitting diodes (w-LEDs) have been expected to be the next generation light source due to their high luminous efficiency, long lifespan, energy conservation, high reliability, environmental friendliness and easy assembly [1–3]. Typically, w-LEDs can be generated by several different methods. One approach to obtain w-LEDs with high luminous efficiency is to combine InGaN-based blue chips with a yellow phosphor (Y3Al5O12:Ce3+) [4,5]. Unfortunately, the commercial w-LEDs exhibit a low color rendering index due to the scarcity of red light component in the emission spectra. Another approach, based on the combination of near ultraviolet LED chip and tricolor phosphors or coupling a blue LED to green and red phosphors, offers better color rendering performance. However, the efficiency of the red phosphor Y2O2S:Eu3+ is far lower than that of blue phosphor BaMgAl10O17:Eu2+ and green phosphor ZnS:Cu+, Al3+ phosphors. Besides, sulfide-based phosphors easily generate poisonous gas as it contains toxic elements and its lifetime is inadequate under the excitation of UV light [6–8]. In view of these reasons, much more efforts should be put into the research of novel red phosphors with high efficiency and excellent chemical stability and can be excited in the near UV range. Recently, a mass of Eu3+ activated phosphors with high quantum efficiency in red or reddish-orange region have been investigated due to their potential application for w-LEDs. Among these ⇑ Corresponding author. Fax: +86 20 39322265. E-mail address: [email protected] (X. Liu).
researches, much attention has been paid on Eu3+ ions doped molybdate and tungstate compounds owing to their high efficiency 2 energy transfer between WO2 complexes and the rare 4 , MoO4 2 earth ions [6,9]. However, materials containing WO2 4 and MoO4 groups with scheelite structure are not good enough to give high emission intensity for near-UV excitation due to their CT bands are not well matched with the near UV LED chip [10,11]. It is known that the ordered double perovskite compounds A2BB0 O6, in which A (12-coordination) is an alkaline earth ion, B (6-coordination), a divalent metal ion such as Mg, Ca, Zn and B0 (6-coordination) a hexavalent W or Mo ion, are particularly suitable as model compounds for fundamental studies of certain physical properties such as huge magnetoresistance, superconducting and solid oxide fuel cells [12,13]. In addition, these compounds are excellent matrix materials of phosphors because of their structure diversity, excellent optical transparency, good thermal and chemical stability. There have been some reports about Eu3+-activated double perovskite materials containing WO6 and MoO6 groups, such as Sr2ZnWO6:Eu3+ [14], Ba2MgW(Mo)O6:Eu3+ [15] and Ba2CaMoO6: Eu3+ [16]. But to our knowledge, little work has been performed on the photoluminescence properties and the structure of Eu3+-doped Ba3WO6 phosphors. In this paper, Ba3WO6:Eu3+ nanowire phosphors with different calcining temperatures were synthesized by solid-state reaction method. The morphologies and luminescence properties of the phosphors and the effect of calcining temperature and the codoped effects of alkali-metal ions on the luminescence properties were investigated.
http://dx.doi.org/10.1016/j.optmat.2014.10.032 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
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2. Experimental 2.1. Synthesis The powder samples of Ba2.95WO6:0.05Eu3+ phosphors with different calcining temperatures were synthesized via solid-state reaction technique. The stoichiometric starting materials BaCO3 (A.R.), WO3 (A.R.) and Eu2O3 (4 N) were grinded and mixed homogeneously in an agate mortar. The homogeneous mixture was divided into four parts and calcined at different temperatures, viz. 1000–1200 °C with an interval of 50 °C, respectively, in air for 5 h. Finally, the samples were grinded slightly to obtain the phosphor powders. 2.2. Characterization of samples The phase composition was determined using X-ray diffractometer (Philips XD-2) with Cu Ka radiation (k = 0.15406 nm) at 36 kV tube voltage and 20 mA tube current. The data were collected from 10° to 70° in 2h range with a scanning step of 0.02° and a scanning rate 4.0°/min. The morphology of the sample was characterized by a S3400N scanning electronic microscope (SEM) (S3400N, Hitachi, Japan) with accelerating voltage of 15 kV. Emission and excitation spectra were measured on a Fluorescence Spectrophotometer (Hitachi F-7000) equipped with a 150 W xenon lamp as excitation source. All measurements were carried out at room temperature. Raman spectra were recorded using an Invia-Inflex high-resolution Raman spectrometer with a 633 nm laser beam as light source. 3. Results and discussions 3.1. Morphology characterization SEM photographs of Ba3WO6:Eu3+ samples calcined at different temperatures (1050, 1100, 1150, and 1200 °C) are shown in Fig. 1. As can be seen, the products with the good morphology calcined at 1050 °C, 1100 °C, 1150 °C are mainly composed of nanowires with orderly diameter of tens nanometers and their lengths are several micrometers. It is easy to note that with the increase of calcining
temperature the diameters of the nanowires increase slightly. However, when the synthesized temperature rises to 1200 °C, the nanowires shorten and become disorderly and unsystematic in comparison with the samples calcined at 1050, 1100 and 1150 °C. As well known, in general, metal oxide nanowires grow by attaching of atoms to their tips either from a solution or a vapor phase. For the solution phase growth, Ostwald ripening and oriented attachment are the main growth mechanisms [17]. While the vapor phase growth is based on the catalyst-assisted vapor–liquid–solid (VLS) or the vapor–solid (VS) mechanisms [18]. For the current study, the formation of the nanowire structures may be through VS mechanisms. In this VS-typed process, the decomposition of BaCO3 produces BaO by the reaction: BaCO3 ? BaO + CO2. Solid BaO and WO3 evaporate to form BaO and WO3 vapors. Then, the vapors will react to form Ba3WO6 crystal nuclei on the surface of solid reactant powders by the reaction: BaO + WO3 ? Ba3WO6. Finally, Ba3WO6 crystal nuclei grow up and form the nanowires with the continuous decomposition and vaporization of BaCO3 and WO3. In this growth process, atoms may reach at the tip of nanowire by the solid state diffusion process along the sidewalls of the nanowire [19]. Further investigations are underway to understand the mechanism in more details. In the best of our knowledge, the present study for the first time provides evidence that a metal oxide nanowire can be grown from the corresponding metal oxide mixtures by conventional solid-state reaction method.
3.2. XRD analysis and structure The crystallinity and phase purity of the Ba2.95WO6:0.05Eu3+ phosphors sintered at different temperatures were determined by XRD, as shown in Fig. 2. It can be seen clearly that all peaks of the as-prepared samples obtained at 1150 and 1200 °C match well with the standard pattern of pure Ba3WO6 cubic structure (JCPDS No. 33-0182) and no extra peaks of impurity are observed in the patterns. This fact indicates that Eu3+ ions have been successfully incorporated into the Ba3WO6 host lattice and do not destroy the lattice structure of host. From the XRD patterns, it is easy to note that with the increase of temperature up to 1150 °C, the peaks of
Fig. 1. SEM images of Ba2.95WO6:0.05Eu3+ phosphors synthesized at different calcination temperatures: (a) 1050 °C, (b) 1100 °C, (c) 1150 °C and (d) 1200 °C.
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(A-site) ions are coordinated by twelve oxygen atoms and Ba2+ (B-site)/W6+ (B0 -site) ions are coordinated by six O atoms forming BaO6/WO6 octahedra. These octahedra appear alternately in the cell and are linked together by sharing vertex oxygen atoms [21]. The ionic radii of Ba2+ (A-site, twelve-coordination), Ba2+ (B-site, six-coordination) ions and W6+ (six-coordination), are 1.61 Å, 1.35 Å and 0.6 Å, respectively [22]. Considering the ionic radius of Eu3+ (0.95 Å, six-coordination) is relatively near that of Ba2+ (B-site, six-coordination), Eu3+ ions are expected to substitute for Ba2+ (B-site, six-coordination) ions in Ba3WO6 host. This substitution might induce the Ba2+ vacancy or interstitial oxygen 3+ 00 00 O2 between two Eu3+ ions, i.e. [2(Eu3+ i Ba) –VBa ] and [2(EuBa) –Oi ], to compensate for the excess positive charge. 3.3. Excitation and emission of Ba2.95WO6:0.05Eu3+
Ba3WO6, such as (2 2 2), (4 4 0) and (5 3 3) get stronger. At the meantime, the impurity peak of WO3 becomes weaker. Besides, the XRD patterns of the samples synthesized at 1150 and 1200 °C show the similar peak intensities. The evolution of the XRD pattern indicates that the crystallinity of the powders improves with the increase of calcining temperature and the effect of temperature on the crystallinity of Ba2.95WO6:0.05Eu3+ will become delicate beyond 1150 °C. Therefore, the calcining temperature of 1150 °C is selected to synthesize Ba2.95WO6:0.05Eu3+ and Ba2.9WO6:0.05Eu3+, 0.05R+ (R = Li, Na, K) phosphors. Alkaline earth tungstates of the type A2BB0 O6 crystallize in an ordered perovskite structure. For Ba3WO6, A-site and B-site are exclusively occupied by Ba2+ ions, B0 -site is occupied by W6+ ions and its crystal structure is shown in Fig. 3 [20]. Ba3WO6 belongs to cubic system (space group Fm-3m (2 2 5)) with lattice constants of a = b = c = 17.7 Å, a = b = c = 90°, V = 5067.6 Å3 and Z = 32. In Ba3WO6 host, the cations locate at three different sites, viz. Ba2+ (A-site) at the 8c sites with Td symmetry, Ba2+ (B-site) and Mo6+ at the 4a and 4b sites with Oh symmetry, respectively [21]. Ba2+
Fig. 4 shows the excitation spectra of Eu3+-doped Ba3WO6 synthesized at different temperatures. From the excitation spectra by monitoring the 5D0 ? 7F1 emission (595 nm) of Eu3+, it is clearly seen that these excitation spectra are similar in shape, which consist of a broad band and some lines. The intense broad band centered at around 318 nm is assigned to the charge transfer transitions from oxygen to europium atom and tungstate atom (O2 ? Eu3+, O2 ? W6+), the sharp bands between 355 and 550 nm are assigned to the transitions of Eu3+: 7F0 ? 5L7, 7 F0 ? 5L6, 7F0 ? 5D3, 7F0 ? 5D2 and 7F0 ? 5D1, respectively. Besides, it is noted that the CT position of Ba2.95WO6:0.05Eu3+ shifts to a lower energy region (red shift) with the increase of annealing temperature. A reasonable explanation is that accompanied with the increase of annealing temperature, lattice expansions occurs, resulting in the widening of bond length and decrease of interaction between the atoms. Therefore, the crystal field posed on the W and Eu by ligand O atoms decreases, which reduces the band gap width of W–O and the charge transfer energy [23]. The emission spectra of Ba2.95WO6:0.05Eu3+ phosphors synthesized at different temperatures were obtained under CT band excitation of 318 nm and are shown in Fig. 5. As can be seen, the emission spectra are composed of three major emission peaks ranging from 500 to 750 nm, which are originated from the 5 D0 ? 7FJ transitions of Eu3+ (J = 0, 1 and 2). It is noted that the emission peak corresponding to the parity-allowed magnetic dipole transition of 5D0 ? 7F1 is predominant. Generally, the intensities of different 5D0 ? 7FJ transitions rely on the local symmetry of the crystal field of Eu3+ ions. On the basis of the Judd–Ofeld
Fig. 3. The crystal structure of Ba3WO6. White polyhedra, WO6 octahedra; green polyhedra, BaO6 octahedra. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. The excitation spectra (kem = 595 nm) of Ba2.95WO6:0.05Eu3+ phosphors synthesized at different calcination temperatures: 1000, 1050, 1100, 1150, and 1200 °C.
Fig. 2. The XRD patterns of Ba2.95WO6:0.05Eu3+ samples synthesized at different calcination temperatures. (‘‘’’ stands for the WO3 phase.)
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substitutions of Eu3+. Comparing the two spectra in Fig. 6, it can be clearly seen that the peak (145 cm1) of mode T2g (1) keeps in the same position, while the peak (800 cm1) of mode A1g has a slight blue shift due to the substitutions of Eu3+. This is the evidence in support of the B site substitutions of Eu3+, i.e. Eu3+ ions are located at 4a Ba2+ sites (six-coordination, Oh point symmetry) with inversion symmetry in the matrix. 3.5. Dependence of emission intensities on Eu3+ concentrations
Fig. 5. Emission spectra of Ba2.95WO6:0.05Eu3+ samples synthesized at different temperatures: 1000, 1050, 1100, 1150, and 1200 °C.
theory [24], when Eu3+ ion occupies the lattice site with inversion center, the magnetic dipole transition 5D0 ? 7F1 would be predominant. Therefore, Eu3+ ions are mainly located at 4a site Ba2+ (sixcoordination, Oh point symmetry) with inversion symmetry in the matrix. This result is different from that of Ca3WO6:Eu3+ phosphor [25,26], where the electric dipole transition 5D0 ? 7F2 is predominant since Eu3+ ions substitute for the noncentrosymmetric Ca2+ cations (Td symmetry). Besides, it is worth to note that these emission spectra are similar in shape and with the increase of calcining temperature, their intensities increase firstly and reach a maximum, and then decrease. As it seems, this appears due to the higher crystallinity, good morphology and larger size of the sample synthesized at 1150 °C. 3.4. Raman spectra of Ba3WO6 and Ba2.95WO6:0.05Eu3+ To identify the substitution properties of Eu3+ ions in the Ba3WO6 matrix, Raman spectra of Ba3WO6 and Ba2.95WO6:0.05Eu3+ samples were measured and are shown in Fig. 6. On the basis of previous reports [27,28], mode T2g (1) is an A-site cation related vibration contributed by oxygen atoms and an A-site cation. While mode A1g is a fully symmetric breathing vibration of oxygen octahedra and is as a consequence of the B-site ordering. The vibration manner evolution of A1g and T2g (1) can be used to identify the site
Fig. 6. Raman spectra of Ba3WO6 and Ba2.95WO6:0.05Eu3+ samples.
Generally speaking, the doping concentration of luminescent centers has a significant effect on the phosphor performance. Confirming the optimum doping concentration is of great significant, therefore, the concentration dependent emission intensities of Ba3xWO6:xEu3+ (x = 0.01, 0.03, 0.05, 0.08 and 0.10) is studied and the effect of Eu3+ doping concentration on the relative intensity of the magnetic dipole transition is shown as an inset in Fig. 7. As can be seen, the emission intensity initially increases with Eu3+ concentration, then reaches a maximum at 5 mol% Eu3+, and finally a decrease in the emission intensity is observed as the Eu3+ doping ratio is higher above 0.05. Thus, the optimal doping concentration of Eu3+ in Ba3xWO6:xEu3+ phosphor is just about 5 mol%. Apparently, a low concentration quenching occurs in Ba3xWO6:xEu3+ and quenching interactions between neighboring Eu3+ ion can not take place due to such a low doping Eu3+ concentration. As well known, quenching correlates with the probability of pairing Eu3+ ions is related to intervening oxygens [29]. And it depends upon the overlap of the wave function of Eu3+ ion with those of a dumbbell-shaped p-orbital of an intervening oxygen ion. When the Eu–O–Eu angle is 180°, the distance between Eu–O is closest, so is the strongest interaction. In Ba3WO6:Eu3+, the Ba–O–Ba angle is equal to 136.4° and Ba2+ is substituted for Eu3+. Hence, interactions between neighboring Eu3+ ions through intervening oxygens are involved in quenching. Dexter also argued that quenching may occur as a result of energy transfer by means of identical ions to one which acts as a sink due to its presence next to a defect [30]. For Ba3WO6: Eu3+, the lattice defects appear due to the nonequivalent substitution. Therefore, the Eu3+ ion next to the defect might lose the energy to the host through interaction with the phonon spectrum. So a low concentration quenching occurs in Ba3WO6:Eu3+. 3.6. Photoluminescence characteristics of Ba3WO6:Eu3+, R+ (R = Li, Na, K) Generally, the charge imbalance is unfavorable to the emission intensity, because the charge imbalance could induce point defects
Fig. 7. Emission spectra (kex = 318 nm) of Ba3xWO6:xEu3+ (x = 0.01, 0.03, 0.05, 0.08, 0.10) samples. The inset is the concentration dependence of the integrated intensity of Eu3+ ion.
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Fig. 9 exhibits the emission spectra of Ba2.95WO6:0.05Eu3+ and Ba2.9WO6:0.05Eu3+, 0.05R+ (R = Li, Na, K) phosphors. It can be seen that the co-doped Li+, Na+ and K+ can all lead to the increasing luminescent intensity of Eu3+ ion, indicating that alkali metal ions R+ could enhance the emission intensity of the Ba2.95WO6:0.05Eu3+. Besides, it can be found that the luminescent intensity increase in the order: K+ > Na+ > Li+. A reasonable explanation is that the radius of K+ ion (1.38 Å, CN = 6) is closely similar to that of Ba2+ ion (1.35 Å, CN = 6). When 2Ba2+ are replaced by Eu3+ (0.95 Å, CN = 6) and K+ ions, the change of crystal structure is very small. While the gap of the radius between Ba2+ ion and Li+ ion (0.76 Å, CN = 6) or Na+ ion (1.02 Å, CN = 6) is bigger in comparison with that of K+ ion, therefore, a larger lattice distortion might be induced when Li+ or Na+ ions is introduced to the host lattice [35]. Thus, K+ ions may be the optimal compensator charge for the Ba2.95WO6:0.05Eu3+ phosphor. Fig. 8. The XRD patterns of Ba2.9WO6:0.05Eu3+, 0.05R+ (R = Li, Na, K) samples.
in the structure, which would increase the nonradiative process and reduce emission intensity [31]. Interestingly, alkali mental ions like Li+, Na+ and K+ have the chemical nature of low oxidation states and distinct ionic radii, and thus can be used to modify charge imbalance to enhance the luminescence efficiency [32–34]. For Ba3WO6:Eu3+, When Eu3+ ion is incorporated into Ba3WO6 host, Eu3+ ion should substitute for the Ba2+ site, which may induce the vacancy defect. In order to offset the charge imbalance generated by Eu3+ substitution for Ba2+, reduce the lattice distort and enhance the luminescent intensity, alkali metal ions R+ (R = Li, Na, K) are added to Ba3WO6:Eu3+. The X-ray diffraction patterns of Ba2.9WO6:0.05Eu3+, 0.05R+ (R = Li, Na, K) phosphors which were sintered at 1150 °C for 5 h are shown in Fig. 8. It can be seen that all patterns of samples with different charge compensation (Li+, Na+ and K+) are in good agreement with the standard data of orthorhombic structure Ba3WO6 (JCPDS 33-0182), and no extra peaks belonging to other impurities are observed in the patterns. This fact suggests that alkali metal ions R+ (R = Li, Na, K) and Eu3+ ions have been successfully incorporated into the Ba3WO6 host lattice and do not change the lattice structure of Ba3WO6 host. For Ba2.9WO6:0.05Eu3+, 0.05R+ phosphors, two Ba2+ ions were substituted by one Eu3+ ion and one R+ ion, expressing as 2BaBa ? EuBa + RBa0 .
Fig. 9. Emission spectra (kex = 318 nm) of Ba2.95WO6:0.05Eu3+ and Ba2.9WO6:0.05Eu3+, 0.05R+ (R = Li, Na, K) synthesized at 1150 °C.
4. Conclusions In summary, novel Ba3WO6:Eu3+ nanowire phosphors were synthesized by solid-state reaction method. Ba3WO6:Eu3+ shows an intense reddish orange emission at 595 nm under the CT band excitation. The optimal doping concentration of Eu3+ in Ba3WO6 is about 0.05. The CT position of Ba2.95WO6:0.05Eu3+ shifts to a lower energy region (red shift) with the increase of annealing temperature. K+ ions have the best charge compensation effect for Ba3WO6:Eu3+ phosphor among alkaline mental ions. Ba3WO6:Eu3+ phosphors might have a potential application for w-LEDs. Acknowledgements This work is supported by the Natural Science Foundation of Guangdong Province of China (Grant Nos. S2013010013567 and 7008121), the Ministry of Education of China, and the Ministry of Personnel of China. References [1] P.F. Smet, A.B. Parmentier, D. Poelman, J. Electrochem. Soc. 158 (2011) R37. [2] S. Pimputkar, J.S. Speck, S.P. DenBaars, S. Nakamura, Nat. Photonics 3 (2009) 180. [3] M.Y. Wang, J.H. Zhang, X. Zhang, Y.S. Luo, X.G. Ren, S.Z. Lu, X.R. Liu, X.J. Wang, J. Phys. D Appl. Phys. 41 (2008) 205103. [4] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1997. [5] R. Muller-Mach, G.O. Mueller, IEEE J. Sel. Top. Quantum Electron. 8 (2002) 339. [6] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2. [7] S.X. Yan, J.H. Zhang, X. Zhang, S.Z. Lu, X.G. Ren, Z.G. Nie, X.J. Wang, J. Phys. Chem. C111 (2007) 13256. [8] Z.L. Wang, H.B. Liang, M.L. Gong, Q. Su, Electrochem. Solid-State Lett. 8 (2005) H33. [9] Y.H. Zheng, H.P. You, K. Liu, Y.H. Song, G. Jia, Y.J. Huang, M. Yang, L.H. Zhang, G. Ning, Cryst. Eng. Commun. 13 (2011) 3001. [10] V. Sivakumar, U.V. Varadaraju, J. Electrochem. Soc. 153 (2006) H54. [11] V. Sivakumar, U.V. Varadaraju, J. Electrochem. Soc. 152 (2005) H168. [12] E.G. Steward, H.P. Rooksby, Acta Crystallogr. 4 (1951) 503. [13] G. Blasse, J. Phys. Chem. Solids 26 (1965) 1969. [14] Z.X. Liang, Z.S. Li, H.T. Zhang, S.X. Ouyang, Z.G. Zou, J. Lumin. 469 (2009) L6. [15] H.Y. Li, H.K. Yang, B.K. Moon, B.C. Choi, J.H. Jeong, K. Jang, H.S. Lee, S.S. Yi, J. Alloys Compd. 509 (2011) 8788. [16] X.Y. Sun, Z.D. Hao, C.J. Li, X.G. He, H.Y. Qi, L.J. Yu, Y.S. Luo, J.H. Zhang, J. Lumin. 134 (2013) 191. [17] G. Xi, J. Ye, Inorg. Chem. 49 (2010) 2302. [18] C. Cheng, G. Xu, H. Zhang, Y. Li, Y. Luo, P. Zhang, Mater. Sci. Eng., B 147 (2008) 79. [19] E.I. Givargizov, Highly Anisotropic Crystals, Reidel, Dordrecht, 1987. [20] V. Sivakumar, U.V. Varadaraju, Electrochem. Solid State Lett. 9 (2006) H35. [21] V.S. Filip’ev, G.E. Shatalova, E.G. Fesenko, Kristallografiya 19 (1974) 386. [22] R.D. Shannon, Acta Cryst. A32 (1976) 751. [23] H.Y. Li, S.Y. Zhang, S.H. Zhou, X.Q. Cao, Y.H. Zheng, J. Phys. Chem. C 113 (2009) 13115. [24] S. Shigeo, M. William, Phosphor Handbook, CRC Press, Washington, DC, 1998. [25] S.A. Zhang, Y.H. Hu, L. Chen, J.X. Wang, G.F. Ju, Y. Fan, J. Lumin. 142 (2013) 116. [26] X. Zhao, J.J. Wang, L. Fan, Y.F. Ding, Z.S. Li, T. Yu, Z.G. Zou, Dalton Trans. 42 (2013) 13502.
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Structure and luminescence properties of Ba3WO6:Eu3+ nanowire phosphors obtained by conventional solid-state reaction method Yuntong Li, Xiaohua Liu ⇑ School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 9 July 2014 Received in revised form 6 October 2014 Accepted 13 October 2014 Available online xxxx Keywords: Photoluminescence Charge compensation Phosphor
a b s t r a c t For the first time, novel Ba3xWO6:xEu3+ (x = 0.01, 0.03, 0.05, 0.08, 0.1) nanowire phosphors were synthesized by the conventional solid state method. The X-ray pattern indicates that Ba3WO6 belongs to the cubic system with space group Fm-3m. The photoluminescence (PL) spectra demonstrate that the phosphors emit strong red light centered at 595 nm corresponding to 5D0 ? 7F1 transition of Eu3+ ion under CT band excitation. The position of charge transfer (CT) band of Ba2.95WO6:0.05Eu3+ shifts to a lower energy region (red shift) with the increase of annealing temperature. The co-doped effect of alkali-metal ions (Li+, Na+, and K+) on the luminescence behavior of Ba3WO6:Eu3+ has been discussed in this paper. The luminescence properties suggest that the Ba3WO6:Eu3+ phosphor may be a promising candidate in solid-state lighting applications. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction White light-emitting diodes (w-LEDs) have been expected to be the next generation light source due to their high luminous efficiency, long lifespan, energy conservation, high reliability, environmental friendliness and easy assembly [1–3]. Typically, w-LEDs can be generated by several different methods. One approach to obtain w-LEDs with high luminous efficiency is to combine InGaN-based blue chips with a yellow phosphor (Y3Al5O12:Ce3+) [4,5]. Unfortunately, the commercial w-LEDs exhibit a low color rendering index due to the scarcity of red light component in the emission spectra. Another approach, based on the combination of near ultraviolet LED chip and tricolor phosphors or coupling a blue LED to green and red phosphors, offers better color rendering performance. However, the efficiency of the red phosphor Y2O2S:Eu3+ is far lower than that of blue phosphor BaMgAl10O17:Eu2+ and green phosphor ZnS:Cu+, Al3+ phosphors. Besides, sulfide-based phosphors easily generate poisonous gas as it contains toxic elements and its lifetime is inadequate under the excitation of UV light [6–8]. In view of these reasons, much more efforts should be put into the research of novel red phosphors with high efficiency and excellent chemical stability and can be excited in the near UV range. Recently, a mass of Eu3+ activated phosphors with high quantum efficiency in red or reddish-orange region have been investigated due to their potential application for w-LEDs. Among these ⇑ Corresponding author. Fax: +86 20 39322265. E-mail address: [email protected] (X. Liu).
researches, much attention has been paid on Eu3+ ions doped molybdate and tungstate compounds owing to their high efficiency 2 energy transfer between WO2 complexes and the rare 4 , MoO4 2 earth ions [6,9]. However, materials containing WO2 4 and MoO4 groups with scheelite structure are not good enough to give high emission intensity for near-UV excitation due to their CT bands are not well matched with the near UV LED chip [10,11]. It is known that the ordered double perovskite compounds A2BB0 O6, in which A (12-coordination) is an alkaline earth ion, B (6-coordination), a divalent metal ion such as Mg, Ca, Zn and B0 (6-coordination) a hexavalent W or Mo ion, are particularly suitable as model compounds for fundamental studies of certain physical properties such as huge magnetoresistance, superconducting and solid oxide fuel cells [12,13]. In addition, these compounds are excellent matrix materials of phosphors because of their structure diversity, excellent optical transparency, good thermal and chemical stability. There have been some reports about Eu3+-activated double perovskite materials containing WO6 and MoO6 groups, such as Sr2ZnWO6:Eu3+ [14], Ba2MgW(Mo)O6:Eu3+ [15] and Ba2CaMoO6: Eu3+ [16]. But to our knowledge, little work has been performed on the photoluminescence properties and the structure of Eu3+-doped Ba3WO6 phosphors. In this paper, Ba3WO6:Eu3+ nanowire phosphors with different calcining temperatures were synthesized by solid-state reaction method. The morphologies and luminescence properties of the phosphors and the effect of calcining temperature and the codoped effects of alkali-metal ions on the luminescence properties were investigated.
http://dx.doi.org/10.1016/j.optmat.2014.10.032 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
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2. Experimental 2.1. Synthesis The powder samples of Ba2.95WO6:0.05Eu3+ phosphors with different calcining temperatures were synthesized via solid-state reaction technique. The stoichiometric starting materials BaCO3 (A.R.), WO3 (A.R.) and Eu2O3 (4 N) were grinded and mixed homogeneously in an agate mortar. The homogeneous mixture was divided into four parts and calcined at different temperatures, viz. 1000–1200 °C with an interval of 50 °C, respectively, in air for 5 h. Finally, the samples were grinded slightly to obtain the phosphor powders. 2.2. Characterization of samples The phase composition was determined using X-ray diffractometer (Philips XD-2) with Cu Ka radiation (k = 0.15406 nm) at 36 kV tube voltage and 20 mA tube current. The data were collected from 10° to 70° in 2h range with a scanning step of 0.02° and a scanning rate 4.0°/min. The morphology of the sample was characterized by a S3400N scanning electronic microscope (SEM) (S3400N, Hitachi, Japan) with accelerating voltage of 15 kV. Emission and excitation spectra were measured on a Fluorescence Spectrophotometer (Hitachi F-7000) equipped with a 150 W xenon lamp as excitation source. All measurements were carried out at room temperature. Raman spectra were recorded using an Invia-Inflex high-resolution Raman spectrometer with a 633 nm laser beam as light source. 3. Results and discussions 3.1. Morphology characterization SEM photographs of Ba3WO6:Eu3+ samples calcined at different temperatures (1050, 1100, 1150, and 1200 °C) are shown in Fig. 1. As can be seen, the products with the good morphology calcined at 1050 °C, 1100 °C, 1150 °C are mainly composed of nanowires with orderly diameter of tens nanometers and their lengths are several micrometers. It is easy to note that with the increase of calcining
temperature the diameters of the nanowires increase slightly. However, when the synthesized temperature rises to 1200 °C, the nanowires shorten and become disorderly and unsystematic in comparison with the samples calcined at 1050, 1100 and 1150 °C. As well known, in general, metal oxide nanowires grow by attaching of atoms to their tips either from a solution or a vapor phase. For the solution phase growth, Ostwald ripening and oriented attachment are the main growth mechanisms [17]. While the vapor phase growth is based on the catalyst-assisted vapor–liquid–solid (VLS) or the vapor–solid (VS) mechanisms [18]. For the current study, the formation of the nanowire structures may be through VS mechanisms. In this VS-typed process, the decomposition of BaCO3 produces BaO by the reaction: BaCO3 ? BaO + CO2. Solid BaO and WO3 evaporate to form BaO and WO3 vapors. Then, the vapors will react to form Ba3WO6 crystal nuclei on the surface of solid reactant powders by the reaction: BaO + WO3 ? Ba3WO6. Finally, Ba3WO6 crystal nuclei grow up and form the nanowires with the continuous decomposition and vaporization of BaCO3 and WO3. In this growth process, atoms may reach at the tip of nanowire by the solid state diffusion process along the sidewalls of the nanowire [19]. Further investigations are underway to understand the mechanism in more details. In the best of our knowledge, the present study for the first time provides evidence that a metal oxide nanowire can be grown from the corresponding metal oxide mixtures by conventional solid-state reaction method.
3.2. XRD analysis and structure The crystallinity and phase purity of the Ba2.95WO6:0.05Eu3+ phosphors sintered at different temperatures were determined by XRD, as shown in Fig. 2. It can be seen clearly that all peaks of the as-prepared samples obtained at 1150 and 1200 °C match well with the standard pattern of pure Ba3WO6 cubic structure (JCPDS No. 33-0182) and no extra peaks of impurity are observed in the patterns. This fact indicates that Eu3+ ions have been successfully incorporated into the Ba3WO6 host lattice and do not destroy the lattice structure of host. From the XRD patterns, it is easy to note that with the increase of temperature up to 1150 °C, the peaks of
Fig. 1. SEM images of Ba2.95WO6:0.05Eu3+ phosphors synthesized at different calcination temperatures: (a) 1050 °C, (b) 1100 °C, (c) 1150 °C and (d) 1200 °C.
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(A-site) ions are coordinated by twelve oxygen atoms and Ba2+ (B-site)/W6+ (B0 -site) ions are coordinated by six O atoms forming BaO6/WO6 octahedra. These octahedra appear alternately in the cell and are linked together by sharing vertex oxygen atoms [21]. The ionic radii of Ba2+ (A-site, twelve-coordination), Ba2+ (B-site, six-coordination) ions and W6+ (six-coordination), are 1.61 Å, 1.35 Å and 0.6 Å, respectively [22]. Considering the ionic radius of Eu3+ (0.95 Å, six-coordination) is relatively near that of Ba2+ (B-site, six-coordination), Eu3+ ions are expected to substitute for Ba2+ (B-site, six-coordination) ions in Ba3WO6 host. This substitution might induce the Ba2+ vacancy or interstitial oxygen 3+ 00 00 O2 between two Eu3+ ions, i.e. [2(Eu3+ i Ba) –VBa ] and [2(EuBa) –Oi ], to compensate for the excess positive charge. 3.3. Excitation and emission of Ba2.95WO6:0.05Eu3+
Ba3WO6, such as (2 2 2), (4 4 0) and (5 3 3) get stronger. At the meantime, the impurity peak of WO3 becomes weaker. Besides, the XRD patterns of the samples synthesized at 1150 and 1200 °C show the similar peak intensities. The evolution of the XRD pattern indicates that the crystallinity of the powders improves with the increase of calcining temperature and the effect of temperature on the crystallinity of Ba2.95WO6:0.05Eu3+ will become delicate beyond 1150 °C. Therefore, the calcining temperature of 1150 °C is selected to synthesize Ba2.95WO6:0.05Eu3+ and Ba2.9WO6:0.05Eu3+, 0.05R+ (R = Li, Na, K) phosphors. Alkaline earth tungstates of the type A2BB0 O6 crystallize in an ordered perovskite structure. For Ba3WO6, A-site and B-site are exclusively occupied by Ba2+ ions, B0 -site is occupied by W6+ ions and its crystal structure is shown in Fig. 3 [20]. Ba3WO6 belongs to cubic system (space group Fm-3m (2 2 5)) with lattice constants of a = b = c = 17.7 Å, a = b = c = 90°, V = 5067.6 Å3 and Z = 32. In Ba3WO6 host, the cations locate at three different sites, viz. Ba2+ (A-site) at the 8c sites with Td symmetry, Ba2+ (B-site) and Mo6+ at the 4a and 4b sites with Oh symmetry, respectively [21]. Ba2+
Fig. 4 shows the excitation spectra of Eu3+-doped Ba3WO6 synthesized at different temperatures. From the excitation spectra by monitoring the 5D0 ? 7F1 emission (595 nm) of Eu3+, it is clearly seen that these excitation spectra are similar in shape, which consist of a broad band and some lines. The intense broad band centered at around 318 nm is assigned to the charge transfer transitions from oxygen to europium atom and tungstate atom (O2 ? Eu3+, O2 ? W6+), the sharp bands between 355 and 550 nm are assigned to the transitions of Eu3+: 7F0 ? 5L7, 7 F0 ? 5L6, 7F0 ? 5D3, 7F0 ? 5D2 and 7F0 ? 5D1, respectively. Besides, it is noted that the CT position of Ba2.95WO6:0.05Eu3+ shifts to a lower energy region (red shift) with the increase of annealing temperature. A reasonable explanation is that accompanied with the increase of annealing temperature, lattice expansions occurs, resulting in the widening of bond length and decrease of interaction between the atoms. Therefore, the crystal field posed on the W and Eu by ligand O atoms decreases, which reduces the band gap width of W–O and the charge transfer energy [23]. The emission spectra of Ba2.95WO6:0.05Eu3+ phosphors synthesized at different temperatures were obtained under CT band excitation of 318 nm and are shown in Fig. 5. As can be seen, the emission spectra are composed of three major emission peaks ranging from 500 to 750 nm, which are originated from the 5 D0 ? 7FJ transitions of Eu3+ (J = 0, 1 and 2). It is noted that the emission peak corresponding to the parity-allowed magnetic dipole transition of 5D0 ? 7F1 is predominant. Generally, the intensities of different 5D0 ? 7FJ transitions rely on the local symmetry of the crystal field of Eu3+ ions. On the basis of the Judd–Ofeld
Fig. 3. The crystal structure of Ba3WO6. White polyhedra, WO6 octahedra; green polyhedra, BaO6 octahedra. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. The excitation spectra (kem = 595 nm) of Ba2.95WO6:0.05Eu3+ phosphors synthesized at different calcination temperatures: 1000, 1050, 1100, 1150, and 1200 °C.
Fig. 2. The XRD patterns of Ba2.95WO6:0.05Eu3+ samples synthesized at different calcination temperatures. (‘‘’’ stands for the WO3 phase.)
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substitutions of Eu3+. Comparing the two spectra in Fig. 6, it can be clearly seen that the peak (145 cm1) of mode T2g (1) keeps in the same position, while the peak (800 cm1) of mode A1g has a slight blue shift due to the substitutions of Eu3+. This is the evidence in support of the B site substitutions of Eu3+, i.e. Eu3+ ions are located at 4a Ba2+ sites (six-coordination, Oh point symmetry) with inversion symmetry in the matrix. 3.5. Dependence of emission intensities on Eu3+ concentrations
Fig. 5. Emission spectra of Ba2.95WO6:0.05Eu3+ samples synthesized at different temperatures: 1000, 1050, 1100, 1150, and 1200 °C.
theory [24], when Eu3+ ion occupies the lattice site with inversion center, the magnetic dipole transition 5D0 ? 7F1 would be predominant. Therefore, Eu3+ ions are mainly located at 4a site Ba2+ (sixcoordination, Oh point symmetry) with inversion symmetry in the matrix. This result is different from that of Ca3WO6:Eu3+ phosphor [25,26], where the electric dipole transition 5D0 ? 7F2 is predominant since Eu3+ ions substitute for the noncentrosymmetric Ca2+ cations (Td symmetry). Besides, it is worth to note that these emission spectra are similar in shape and with the increase of calcining temperature, their intensities increase firstly and reach a maximum, and then decrease. As it seems, this appears due to the higher crystallinity, good morphology and larger size of the sample synthesized at 1150 °C. 3.4. Raman spectra of Ba3WO6 and Ba2.95WO6:0.05Eu3+ To identify the substitution properties of Eu3+ ions in the Ba3WO6 matrix, Raman spectra of Ba3WO6 and Ba2.95WO6:0.05Eu3+ samples were measured and are shown in Fig. 6. On the basis of previous reports [27,28], mode T2g (1) is an A-site cation related vibration contributed by oxygen atoms and an A-site cation. While mode A1g is a fully symmetric breathing vibration of oxygen octahedra and is as a consequence of the B-site ordering. The vibration manner evolution of A1g and T2g (1) can be used to identify the site
Fig. 6. Raman spectra of Ba3WO6 and Ba2.95WO6:0.05Eu3+ samples.
Generally speaking, the doping concentration of luminescent centers has a significant effect on the phosphor performance. Confirming the optimum doping concentration is of great significant, therefore, the concentration dependent emission intensities of Ba3xWO6:xEu3+ (x = 0.01, 0.03, 0.05, 0.08 and 0.10) is studied and the effect of Eu3+ doping concentration on the relative intensity of the magnetic dipole transition is shown as an inset in Fig. 7. As can be seen, the emission intensity initially increases with Eu3+ concentration, then reaches a maximum at 5 mol% Eu3+, and finally a decrease in the emission intensity is observed as the Eu3+ doping ratio is higher above 0.05. Thus, the optimal doping concentration of Eu3+ in Ba3xWO6:xEu3+ phosphor is just about 5 mol%. Apparently, a low concentration quenching occurs in Ba3xWO6:xEu3+ and quenching interactions between neighboring Eu3+ ion can not take place due to such a low doping Eu3+ concentration. As well known, quenching correlates with the probability of pairing Eu3+ ions is related to intervening oxygens [29]. And it depends upon the overlap of the wave function of Eu3+ ion with those of a dumbbell-shaped p-orbital of an intervening oxygen ion. When the Eu–O–Eu angle is 180°, the distance between Eu–O is closest, so is the strongest interaction. In Ba3WO6:Eu3+, the Ba–O–Ba angle is equal to 136.4° and Ba2+ is substituted for Eu3+. Hence, interactions between neighboring Eu3+ ions through intervening oxygens are involved in quenching. Dexter also argued that quenching may occur as a result of energy transfer by means of identical ions to one which acts as a sink due to its presence next to a defect [30]. For Ba3WO6: Eu3+, the lattice defects appear due to the nonequivalent substitution. Therefore, the Eu3+ ion next to the defect might lose the energy to the host through interaction with the phonon spectrum. So a low concentration quenching occurs in Ba3WO6:Eu3+. 3.6. Photoluminescence characteristics of Ba3WO6:Eu3+, R+ (R = Li, Na, K) Generally, the charge imbalance is unfavorable to the emission intensity, because the charge imbalance could induce point defects
Fig. 7. Emission spectra (kex = 318 nm) of Ba3xWO6:xEu3+ (x = 0.01, 0.03, 0.05, 0.08, 0.10) samples. The inset is the concentration dependence of the integrated intensity of Eu3+ ion.
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Fig. 9 exhibits the emission spectra of Ba2.95WO6:0.05Eu3+ and Ba2.9WO6:0.05Eu3+, 0.05R+ (R = Li, Na, K) phosphors. It can be seen that the co-doped Li+, Na+ and K+ can all lead to the increasing luminescent intensity of Eu3+ ion, indicating that alkali metal ions R+ could enhance the emission intensity of the Ba2.95WO6:0.05Eu3+. Besides, it can be found that the luminescent intensity increase in the order: K+ > Na+ > Li+. A reasonable explanation is that the radius of K+ ion (1.38 Å, CN = 6) is closely similar to that of Ba2+ ion (1.35 Å, CN = 6). When 2Ba2+ are replaced by Eu3+ (0.95 Å, CN = 6) and K+ ions, the change of crystal structure is very small. While the gap of the radius between Ba2+ ion and Li+ ion (0.76 Å, CN = 6) or Na+ ion (1.02 Å, CN = 6) is bigger in comparison with that of K+ ion, therefore, a larger lattice distortion might be induced when Li+ or Na+ ions is introduced to the host lattice [35]. Thus, K+ ions may be the optimal compensator charge for the Ba2.95WO6:0.05Eu3+ phosphor. Fig. 8. The XRD patterns of Ba2.9WO6:0.05Eu3+, 0.05R+ (R = Li, Na, K) samples.
in the structure, which would increase the nonradiative process and reduce emission intensity [31]. Interestingly, alkali mental ions like Li+, Na+ and K+ have the chemical nature of low oxidation states and distinct ionic radii, and thus can be used to modify charge imbalance to enhance the luminescence efficiency [32–34]. For Ba3WO6:Eu3+, When Eu3+ ion is incorporated into Ba3WO6 host, Eu3+ ion should substitute for the Ba2+ site, which may induce the vacancy defect. In order to offset the charge imbalance generated by Eu3+ substitution for Ba2+, reduce the lattice distort and enhance the luminescent intensity, alkali metal ions R+ (R = Li, Na, K) are added to Ba3WO6:Eu3+. The X-ray diffraction patterns of Ba2.9WO6:0.05Eu3+, 0.05R+ (R = Li, Na, K) phosphors which were sintered at 1150 °C for 5 h are shown in Fig. 8. It can be seen that all patterns of samples with different charge compensation (Li+, Na+ and K+) are in good agreement with the standard data of orthorhombic structure Ba3WO6 (JCPDS 33-0182), and no extra peaks belonging to other impurities are observed in the patterns. This fact suggests that alkali metal ions R+ (R = Li, Na, K) and Eu3+ ions have been successfully incorporated into the Ba3WO6 host lattice and do not change the lattice structure of Ba3WO6 host. For Ba2.9WO6:0.05Eu3+, 0.05R+ phosphors, two Ba2+ ions were substituted by one Eu3+ ion and one R+ ion, expressing as 2BaBa ? EuBa + RBa0 .
Fig. 9. Emission spectra (kex = 318 nm) of Ba2.95WO6:0.05Eu3+ and Ba2.9WO6:0.05Eu3+, 0.05R+ (R = Li, Na, K) synthesized at 1150 °C.
4. Conclusions In summary, novel Ba3WO6:Eu3+ nanowire phosphors were synthesized by solid-state reaction method. Ba3WO6:Eu3+ shows an intense reddish orange emission at 595 nm under the CT band excitation. The optimal doping concentration of Eu3+ in Ba3WO6 is about 0.05. The CT position of Ba2.95WO6:0.05Eu3+ shifts to a lower energy region (red shift) with the increase of annealing temperature. K+ ions have the best charge compensation effect for Ba3WO6:Eu3+ phosphor among alkaline mental ions. Ba3WO6:Eu3+ phosphors might have a potential application for w-LEDs. Acknowledgements This work is supported by the Natural Science Foundation of Guangdong Province of China (Grant Nos. S2013010013567 and 7008121), the Ministry of Education of China, and the Ministry of Personnel of China. References [1] P.F. Smet, A.B. Parmentier, D. Poelman, J. Electrochem. Soc. 158 (2011) R37. [2] S. Pimputkar, J.S. Speck, S.P. DenBaars, S. Nakamura, Nat. Photonics 3 (2009) 180. [3] M.Y. Wang, J.H. Zhang, X. Zhang, Y.S. Luo, X.G. Ren, S.Z. Lu, X.R. Liu, X.J. Wang, J. Phys. D Appl. Phys. 41 (2008) 205103. [4] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1997. [5] R. Muller-Mach, G.O. Mueller, IEEE J. Sel. Top. Quantum Electron. 8 (2002) 339. [6] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2. [7] S.X. Yan, J.H. Zhang, X. Zhang, S.Z. Lu, X.G. Ren, Z.G. Nie, X.J. Wang, J. Phys. Chem. C111 (2007) 13256. [8] Z.L. Wang, H.B. Liang, M.L. Gong, Q. Su, Electrochem. Solid-State Lett. 8 (2005) H33. [9] Y.H. Zheng, H.P. You, K. Liu, Y.H. Song, G. Jia, Y.J. Huang, M. Yang, L.H. Zhang, G. Ning, Cryst. Eng. Commun. 13 (2011) 3001. [10] V. Sivakumar, U.V. Varadaraju, J. Electrochem. Soc. 153 (2006) H54. [11] V. Sivakumar, U.V. Varadaraju, J. Electrochem. Soc. 152 (2005) H168. [12] E.G. Steward, H.P. Rooksby, Acta Crystallogr. 4 (1951) 503. [13] G. Blasse, J. Phys. Chem. Solids 26 (1965) 1969. [14] Z.X. Liang, Z.S. Li, H.T. Zhang, S.X. Ouyang, Z.G. Zou, J. Lumin. 469 (2009) L6. [15] H.Y. Li, H.K. Yang, B.K. Moon, B.C. Choi, J.H. Jeong, K. Jang, H.S. Lee, S.S. Yi, J. Alloys Compd. 509 (2011) 8788. [16] X.Y. Sun, Z.D. Hao, C.J. Li, X.G. He, H.Y. Qi, L.J. Yu, Y.S. Luo, J.H. Zhang, J. Lumin. 134 (2013) 191. [17] G. Xi, J. Ye, Inorg. Chem. 49 (2010) 2302. [18] C. Cheng, G. Xu, H. Zhang, Y. Li, Y. Luo, P. Zhang, Mater. Sci. Eng., B 147 (2008) 79. [19] E.I. Givargizov, Highly Anisotropic Crystals, Reidel, Dordrecht, 1987. [20] V. Sivakumar, U.V. Varadaraju, Electrochem. Solid State Lett. 9 (2006) H35. [21] V.S. Filip’ev, G.E. Shatalova, E.G. Fesenko, Kristallografiya 19 (1974) 386. [22] R.D. Shannon, Acta Cryst. A32 (1976) 751. [23] H.Y. Li, S.Y. Zhang, S.H. Zhou, X.Q. Cao, Y.H. Zheng, J. Phys. Chem. C 113 (2009) 13115. [24] S. Shigeo, M. William, Phosphor Handbook, CRC Press, Washington, DC, 1998. [25] S.A. Zhang, Y.H. Hu, L. Chen, J.X. Wang, G.F. Ju, Y. Fan, J. Lumin. 142 (2013) 116. [26] X. Zhao, J.J. Wang, L. Fan, Y.F. Ding, Z.S. Li, T. Yu, Z.G. Zou, Dalton Trans. 42 (2013) 13502.
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