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Synthesis and calcination temperature dependent photoluminescence properties of novel bromosilicate phosphors
Materials Letters 63 (2009) 2600–2602
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis and calcination temperature dependent photoluminescence properties of novel bromosilicate phosphors Zhiguo Xia ⁎, Guowu Li, Daimei Chen, Haiyi Xiao School of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, PR China
a r t i c l e
i n f o
Article history: Received 18 June 2009 Accepted 4 September 2009 Available online 12 September 2009 Keywords: A. Optical materials D. Luminescence D. Optical properties
a b s t r a c t Novel Eu2+-doped bromosilicate phosphors, (CaO–CaBr2–SiO2):0.03Eu2+, were prepared by the traditional solid-state method under a different calcination temperature. The as-prepared (CaO–CaBr2–SiO2):0.03Eu2+ phosphors obtained under various reaction temperatures all indicated a broad excitation in the near ultraviolet region (350–450 nm). The phosphor system exhibits bluish-green light with a peak wavelength at 498 nm when the calcination temperature is below 800 °C, while it shows a strong blue emission light with a peak wavelength at 474 nm as the calcination temperature is above 800 °C, and it also gives an interesting greenish-yellow long-lasting phosphorescence with a peak wavelength at 540 nm in the dark. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the past decade, significant research efforts have been devoted to achieve novel material systems with excellent photoluminescence properties for illumination and display applications [1,2]. Silicates are one of the largest classes of compounds in inorganic chemistry, which are of special interest to us as host lattices for rare-earth-doped phosphors [3,4]. Among them, the studies on halosilicate materials, such as flurosilicate, chlorsilicate and bromosilicate matrix, have attracted more and more attention [5–9]. In recent years, the studies on Eu2+doped halosilicate-based phosphors, such as Ca3SiO4Cl2:Eu2+ [7], and Ca10(Si2O7)3Cl2:Eu2+ [8], Ba5SiO4(F,Cl)6:Eu2+ [9] have become a hot issue in exploring new phosphor materials, which have also been proven to be efficient in the application of the white LEDs' lightconversion phosphors. In our previous work, the peculiar long-lasting phosphorescent luminescent properties in the (CaO–CaBr2–SiO2) phosphor system have been studied [10]. In this paper, the synthesis and optical properties of (CaO–CaBr2–SiO2):0.03Eu2+ phosphor were studied as a function of calcination temperature, which was also investigated to develop new materials with potential application for white LEDs.
2. Experimental The chemical composition of the Eu2+-doped (CaO–CaBr2–SiO2) phosphor system is given by means of the imaginary formula ‘Ca3SiO4Br2:Eu2+’. The phosphor samples were synthesized by the
⁎ Corresponding author. Tel: + 86 1 0 8232 2759. E-mail address: [email protected] (Z. Xia). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.09.017
high-temperature solid-state synthesis method. The starting materials, Ca(OH)2 (A.R.), CaBr2·2H2O (A.R.) and SiO2 (A.R.) were weighted by a molar ratio of Ca(OH)2, CaBr2∙2H2O and SiO2 = 2:1∙1:1. According to our former reported data [10], an optimum mole amount of Eu2O3 (0.03 mol) was added in the mixture as the activator. The mixed samples were fired for 3 h at the selected temperature, 700 °C, 800 °C, 900 °C and 1000 °C in CO reducing atmosphere, and highly pure carbon grains were used as a reducing agent, by which the samples were covered during firing. X-ray diffraction (XRD) patterns were recorded by using an X-ray powder diffractometer (SHIMADZU, XRD-6000) operating at 40 kV, 30 mA and a scanning speed of 2.0° (2θ)/min, using monochromatized Cu Kα radiation. Diffuse reflection spectra of assynthesized phosphor powder samples were measured on a UV–vis– NIR spectrophotometer (UV-3600, SHIMADZU) attached to an integral sphere. BaSO4 was used as a reference standard. The photoluminescence (PL) spectra were recorded by using a Perkin-Elmer LS-55 fluorescence spectrophotometer with a photomultiplier tube operating at 400 V, and a 150-W Xe lamp was used as the excitation lamp. 3. Results and discussions Fig. 1 gives the XRD patterns of (CaO–CaBr2–SiO2):0.03Eu2+ phosphors for various calcination temperatures. As shown in Fig. 1, the obtained diffraction peaks of Eu2+-doped CaO–CaBr2–SiO2 phosphor systems synthesized at four different temperature conditions do not match any data in the JCPDS base after careful comparison. However, we also find that there are two kinds of diffraction peaks, which correspond to the two possible structural phases. The products obtained at 700 °C and 800 °C belong to the same series, while the products obtained at 900 °C and 1000 °C are the other types from the given diffraction data in Fig. 1. Hereafter, we denote them as L-phase for the (CaO–CaBr 2 –SiO 2 ):0.03Eu 2+
Z. Xia et al. / Materials Letters 63 (2009) 2600–2602
Fig. 1. XRD patterns of (CaO–CaBr2–SiO2):0.03Eu2+ phosphor for various calcination temperatures: (a) 700 °C, (b) 800 °C, (c) 900 °C, and (d) 1000 °C.
phosphors synthesized at 700 °C and 800 °C, and H-phase for those synthesized at 900 °C and 1000 °C. The UV–vis diffuse reflection spectra of (CaO–CaBr2–SiO2):0.03Eu2+ phosphors for various calcination temperatures are given in Fig. 2. There are two obvious absorption bands, 230–310 nm and 320–480 nm, attributed to transition from 4f 7 to 4f 65d1 of Eu2+ ions [11]. As the calcination temperature increases, the absorption intensity of an absorption band ranging from 320 nm to 480 nm increases. Another important difference lies in that it can be divided into two series absorption profiles. It testifies that there are two kinds of crystal phases in the (CaO–CaBr2–SiO2):0.03Eu2+ phosphor systems, and it also reflects that a different host system will bring about a different UV–vis diffuse reflection spectra characteristic. The inset in Fig. 2 gives the comparison of UV–vis diffuse spectra of (CaO–CaBr2–SiO2) host and 0.03Eu2+-doped (CaO–CaBr2–SiO2) system synthesized at 900 ºC. It is observed that the (CaO–CaBr2–SiO2) host shows a platform of high reflection in the wavelength range of 400–800 nm and then starts to decrease slightly from 400 to 200 nm, due to the host absorption. It is found that the absorption band of the Eu2+ doped (CaO–CaBr2–SiO2) system is different to the host absorption with two obvious absorption bands in the near-UV region, thereby (CaO–CaBr2–SiO2) host can't be efficiently excited by the near-UV light owing to the absence of Eu2+ ions.
Fig. 2. UV–vis diffuse reflection spectra of (CaO–CaBr2–SiO2):0.03Eu2+ phosphor for various calcination temperatures: (a) 700 °C, (b) 800 °C, (c) 900 °C, and (d) 1000 °C; and the inset shows the comparison of the UV–vis diffuse spectra of (CaO–CaBr2–SiO2) host and the 0.03Eu2+-doped (CaO–CaBr2–SiO2) system synthesized at 900 °C.
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Fig. 3. Emission spectra (1) and excitation spectra (2) of (CaO–CaBr2–SiO2):0.03Eu2+ phosphor for various calcination temperature: (a) 700 °C, (b) 800 °C, (c) 900 °C, and (d) 1000 °C.
Fig. 3 shows the excitation spectra and emission spectra of (CaO– CaBr2–SiO2):0.03Eu2+ phosphors for a different calcination temperature. As seen in Fig. 3 (1)a–b, L-phase (CaO–CaBr2–SiO2):0.03Eu2+ phosphor shows an intense bluish-green broad emission around 497 nm upon 420 nm excitation. Monitoring the bluish-green emission at 497 nm, there are several broad excitation bands around 311, 360, 393, 421 and 445 nm, as seen in Fig. 3 (2). In contrast, as shown in Fig. 3 (1)c–d, H-phase (CaO–CaBr2–SiO2):0.03Eu2+ phosphor shows a strong blue broad emission around 474 nm upon 420 nm excitation. It is also observed in Fig. 3 (2) that the excitation spectra of H-phase consist of four broad bands around 313, 362, 394 and 420 nm. As is known, the broad excitation profiles are mainly due to transition of Eu2+ from 4f ground state to 5d excited state, which also agree with the UV–vis diffuse reflection spectra [12]. However, the complicated structure of the excitation spectra of Eu2+ ion indicates that the site occupied by the Eu2+ ion has a lower symmetry [13]. It is found that both L-phase and H-phase have a similar excitation character, except for the 445 nm excitation band belonging to the L-phase. However, two different structure phases in the (CaO–CaBr2–SiO2) system can also be found, and
Fig. 4. Comparison of the normalized emission spectra of L-phase (a) and H-phase (b) upon 365 nm UV excitation, and afterglow spectrum of H-phase (c) of (CaO–CaBr2–SiO2):0.03Eu2+ phosphor. Inset: digital photographs of the photoluminescence and phosphorescence for the corresponding phosphors.
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Z. Xia et al. / Materials Letters 63 (2009) 2600–2602
H-phase should be a stable structure and has higher symmetry compared to L-phase. It is also found that, if the reaction temperature is above 850 °C, (CaO–CaBr2–SiO2):0.03Eu2+ phosphor system can show greenishyellow light-emitting, long-lasting phosphorescence, except for the blue emission under near-UV excitation [10]. Fig. 4 shows the comparison of the normalized emission spectra of L-phase (a) and Hphase (b) upon 365 nm UV excitation, and afterglow spectrum of Hphase (c) for the (CaO–CaBr2–SiO2):0.03Eu2+ phosphor, and their respective digital photographs are also given in the inset of Fig. 4. It is obvious that the three emission spectra are all asymmetric in the spectra profiles. As marked by a solid arrow at 498 nm, 474 nm and 540 nm in Fig. 4, it indicates that, the PL spectrum of L-phase exhibits a clear bluishgreen color with peak wavelength at 498 nm, the PL spectrum of Hphase gives a clear blue color with a peak wavelength at 474 nm, while the LLP spectrum of H-phase shows a greenish-yellow color with the main peak wavelength at 540 nm, as also shown by their respective digital photographs. 4. Conclusions The interesting calcination temperature dependence bluish-green and blue emission phosphors, which is based on the same Eu2+-doped bromosilicate phosphor systems, (CaO–CaBr2–SiO2):0.03Eu2+, were prepared by the traditional solid-state method. Under 420-nm near-UV light, L-phase phosphor exhibits a bluish-green light with a peak
wavelength at 498 nm, while H-phase phosphor shows strong blue emission light with a peak wavelength at 474 nm. The peculiar longlasting phosphorescence property belonging to the H-phase was also found. The present photoluminescent property results indicate that the (CaO–CaBr2–SiO2):0.03Eu2+ phosphors prepared by a different reaction temperature are promising to meet the application requirements for near-UV GaN-based light-emitting diodes (LEDs) as blue or greenemitting phosphors owing to their broad excitation in the near ultraviolet region (350–450 nm).
References [1] Hao ZD, Zhang JH, Zhang X, Ren XG, Shi Y, Luo S, Lu Z, Wang XJ. J Phys D: Appl Phys 2008;41:182001–182001-04. [2] Bin Im W, Kin II Y, Fellows NN, Masui H, Hirata GA, Denbaars SP, Seshadri R. Appl Phys Lett 2008;93:091905–091905-3. [3] Poort SHM, Blokoel WP, Blasse G. Chem Mater 1995;7:1547–51. [4] Suzuki Y, Kakihana M, Shimomura Y, Kijima N. J Mater Sci 2008;43:2213–6. [5] Zeng QH, Hiroaki T, Kiichirou E, Nobutsugu T, Zhang SX. Appl Phys Lett 2006;88:051906–051906-3. [6] Xia ZG, Sun JY, Du HY, Zhou W. Opt Mater 2006;28:524–9. [7] Ding WJ, Wang J, Liu ZM, Zhang M, Su Q, Tang JK. J Electrochem Soc 2008;155: J122–7. [8] Ding WJ, Wang J, Zhang M, Zhang QH, Su Q. Chem Phys Lett 2007;435:301–5. [9] Xia ZG, Li Q, Sun JY. Mater Lett 2007;61:1885–8. [10] Xia ZG, Liu J, Li Q, Sun JY. Electrochem Solid State Lett 2007;10:J4–8. [11] Meijerink A, Blasse G. J Lumin 1990;47:1–5. [12] Blasse G, Grabmaier BC. Lumin. Mater. Berlin: Springer Verlag; 1994. [13] Yang M, Zhang S. J Phys Chem Solids 2004;64:213–21.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis and calcination temperature dependent photoluminescence properties of novel bromosilicate phosphors Zhiguo Xia ⁎, Guowu Li, Daimei Chen, Haiyi Xiao School of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, PR China
a r t i c l e
i n f o
Article history: Received 18 June 2009 Accepted 4 September 2009 Available online 12 September 2009 Keywords: A. Optical materials D. Luminescence D. Optical properties
a b s t r a c t Novel Eu2+-doped bromosilicate phosphors, (CaO–CaBr2–SiO2):0.03Eu2+, were prepared by the traditional solid-state method under a different calcination temperature. The as-prepared (CaO–CaBr2–SiO2):0.03Eu2+ phosphors obtained under various reaction temperatures all indicated a broad excitation in the near ultraviolet region (350–450 nm). The phosphor system exhibits bluish-green light with a peak wavelength at 498 nm when the calcination temperature is below 800 °C, while it shows a strong blue emission light with a peak wavelength at 474 nm as the calcination temperature is above 800 °C, and it also gives an interesting greenish-yellow long-lasting phosphorescence with a peak wavelength at 540 nm in the dark. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the past decade, significant research efforts have been devoted to achieve novel material systems with excellent photoluminescence properties for illumination and display applications [1,2]. Silicates are one of the largest classes of compounds in inorganic chemistry, which are of special interest to us as host lattices for rare-earth-doped phosphors [3,4]. Among them, the studies on halosilicate materials, such as flurosilicate, chlorsilicate and bromosilicate matrix, have attracted more and more attention [5–9]. In recent years, the studies on Eu2+doped halosilicate-based phosphors, such as Ca3SiO4Cl2:Eu2+ [7], and Ca10(Si2O7)3Cl2:Eu2+ [8], Ba5SiO4(F,Cl)6:Eu2+ [9] have become a hot issue in exploring new phosphor materials, which have also been proven to be efficient in the application of the white LEDs' lightconversion phosphors. In our previous work, the peculiar long-lasting phosphorescent luminescent properties in the (CaO–CaBr2–SiO2) phosphor system have been studied [10]. In this paper, the synthesis and optical properties of (CaO–CaBr2–SiO2):0.03Eu2+ phosphor were studied as a function of calcination temperature, which was also investigated to develop new materials with potential application for white LEDs.
2. Experimental The chemical composition of the Eu2+-doped (CaO–CaBr2–SiO2) phosphor system is given by means of the imaginary formula ‘Ca3SiO4Br2:Eu2+’. The phosphor samples were synthesized by the
⁎ Corresponding author. Tel: + 86 1 0 8232 2759. E-mail address: [email protected] (Z. Xia). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.09.017
high-temperature solid-state synthesis method. The starting materials, Ca(OH)2 (A.R.), CaBr2·2H2O (A.R.) and SiO2 (A.R.) were weighted by a molar ratio of Ca(OH)2, CaBr2∙2H2O and SiO2 = 2:1∙1:1. According to our former reported data [10], an optimum mole amount of Eu2O3 (0.03 mol) was added in the mixture as the activator. The mixed samples were fired for 3 h at the selected temperature, 700 °C, 800 °C, 900 °C and 1000 °C in CO reducing atmosphere, and highly pure carbon grains were used as a reducing agent, by which the samples were covered during firing. X-ray diffraction (XRD) patterns were recorded by using an X-ray powder diffractometer (SHIMADZU, XRD-6000) operating at 40 kV, 30 mA and a scanning speed of 2.0° (2θ)/min, using monochromatized Cu Kα radiation. Diffuse reflection spectra of assynthesized phosphor powder samples were measured on a UV–vis– NIR spectrophotometer (UV-3600, SHIMADZU) attached to an integral sphere. BaSO4 was used as a reference standard. The photoluminescence (PL) spectra were recorded by using a Perkin-Elmer LS-55 fluorescence spectrophotometer with a photomultiplier tube operating at 400 V, and a 150-W Xe lamp was used as the excitation lamp. 3. Results and discussions Fig. 1 gives the XRD patterns of (CaO–CaBr2–SiO2):0.03Eu2+ phosphors for various calcination temperatures. As shown in Fig. 1, the obtained diffraction peaks of Eu2+-doped CaO–CaBr2–SiO2 phosphor systems synthesized at four different temperature conditions do not match any data in the JCPDS base after careful comparison. However, we also find that there are two kinds of diffraction peaks, which correspond to the two possible structural phases. The products obtained at 700 °C and 800 °C belong to the same series, while the products obtained at 900 °C and 1000 °C are the other types from the given diffraction data in Fig. 1. Hereafter, we denote them as L-phase for the (CaO–CaBr 2 –SiO 2 ):0.03Eu 2+
Z. Xia et al. / Materials Letters 63 (2009) 2600–2602
Fig. 1. XRD patterns of (CaO–CaBr2–SiO2):0.03Eu2+ phosphor for various calcination temperatures: (a) 700 °C, (b) 800 °C, (c) 900 °C, and (d) 1000 °C.
phosphors synthesized at 700 °C and 800 °C, and H-phase for those synthesized at 900 °C and 1000 °C. The UV–vis diffuse reflection spectra of (CaO–CaBr2–SiO2):0.03Eu2+ phosphors for various calcination temperatures are given in Fig. 2. There are two obvious absorption bands, 230–310 nm and 320–480 nm, attributed to transition from 4f 7 to 4f 65d1 of Eu2+ ions [11]. As the calcination temperature increases, the absorption intensity of an absorption band ranging from 320 nm to 480 nm increases. Another important difference lies in that it can be divided into two series absorption profiles. It testifies that there are two kinds of crystal phases in the (CaO–CaBr2–SiO2):0.03Eu2+ phosphor systems, and it also reflects that a different host system will bring about a different UV–vis diffuse reflection spectra characteristic. The inset in Fig. 2 gives the comparison of UV–vis diffuse spectra of (CaO–CaBr2–SiO2) host and 0.03Eu2+-doped (CaO–CaBr2–SiO2) system synthesized at 900 ºC. It is observed that the (CaO–CaBr2–SiO2) host shows a platform of high reflection in the wavelength range of 400–800 nm and then starts to decrease slightly from 400 to 200 nm, due to the host absorption. It is found that the absorption band of the Eu2+ doped (CaO–CaBr2–SiO2) system is different to the host absorption with two obvious absorption bands in the near-UV region, thereby (CaO–CaBr2–SiO2) host can't be efficiently excited by the near-UV light owing to the absence of Eu2+ ions.
Fig. 2. UV–vis diffuse reflection spectra of (CaO–CaBr2–SiO2):0.03Eu2+ phosphor for various calcination temperatures: (a) 700 °C, (b) 800 °C, (c) 900 °C, and (d) 1000 °C; and the inset shows the comparison of the UV–vis diffuse spectra of (CaO–CaBr2–SiO2) host and the 0.03Eu2+-doped (CaO–CaBr2–SiO2) system synthesized at 900 °C.
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Fig. 3. Emission spectra (1) and excitation spectra (2) of (CaO–CaBr2–SiO2):0.03Eu2+ phosphor for various calcination temperature: (a) 700 °C, (b) 800 °C, (c) 900 °C, and (d) 1000 °C.
Fig. 3 shows the excitation spectra and emission spectra of (CaO– CaBr2–SiO2):0.03Eu2+ phosphors for a different calcination temperature. As seen in Fig. 3 (1)a–b, L-phase (CaO–CaBr2–SiO2):0.03Eu2+ phosphor shows an intense bluish-green broad emission around 497 nm upon 420 nm excitation. Monitoring the bluish-green emission at 497 nm, there are several broad excitation bands around 311, 360, 393, 421 and 445 nm, as seen in Fig. 3 (2). In contrast, as shown in Fig. 3 (1)c–d, H-phase (CaO–CaBr2–SiO2):0.03Eu2+ phosphor shows a strong blue broad emission around 474 nm upon 420 nm excitation. It is also observed in Fig. 3 (2) that the excitation spectra of H-phase consist of four broad bands around 313, 362, 394 and 420 nm. As is known, the broad excitation profiles are mainly due to transition of Eu2+ from 4f ground state to 5d excited state, which also agree with the UV–vis diffuse reflection spectra [12]. However, the complicated structure of the excitation spectra of Eu2+ ion indicates that the site occupied by the Eu2+ ion has a lower symmetry [13]. It is found that both L-phase and H-phase have a similar excitation character, except for the 445 nm excitation band belonging to the L-phase. However, two different structure phases in the (CaO–CaBr2–SiO2) system can also be found, and
Fig. 4. Comparison of the normalized emission spectra of L-phase (a) and H-phase (b) upon 365 nm UV excitation, and afterglow spectrum of H-phase (c) of (CaO–CaBr2–SiO2):0.03Eu2+ phosphor. Inset: digital photographs of the photoluminescence and phosphorescence for the corresponding phosphors.
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Z. Xia et al. / Materials Letters 63 (2009) 2600–2602
H-phase should be a stable structure and has higher symmetry compared to L-phase. It is also found that, if the reaction temperature is above 850 °C, (CaO–CaBr2–SiO2):0.03Eu2+ phosphor system can show greenishyellow light-emitting, long-lasting phosphorescence, except for the blue emission under near-UV excitation [10]. Fig. 4 shows the comparison of the normalized emission spectra of L-phase (a) and Hphase (b) upon 365 nm UV excitation, and afterglow spectrum of Hphase (c) for the (CaO–CaBr2–SiO2):0.03Eu2+ phosphor, and their respective digital photographs are also given in the inset of Fig. 4. It is obvious that the three emission spectra are all asymmetric in the spectra profiles. As marked by a solid arrow at 498 nm, 474 nm and 540 nm in Fig. 4, it indicates that, the PL spectrum of L-phase exhibits a clear bluishgreen color with peak wavelength at 498 nm, the PL spectrum of Hphase gives a clear blue color with a peak wavelength at 474 nm, while the LLP spectrum of H-phase shows a greenish-yellow color with the main peak wavelength at 540 nm, as also shown by their respective digital photographs. 4. Conclusions The interesting calcination temperature dependence bluish-green and blue emission phosphors, which is based on the same Eu2+-doped bromosilicate phosphor systems, (CaO–CaBr2–SiO2):0.03Eu2+, were prepared by the traditional solid-state method. Under 420-nm near-UV light, L-phase phosphor exhibits a bluish-green light with a peak
wavelength at 498 nm, while H-phase phosphor shows strong blue emission light with a peak wavelength at 474 nm. The peculiar longlasting phosphorescence property belonging to the H-phase was also found. The present photoluminescent property results indicate that the (CaO–CaBr2–SiO2):0.03Eu2+ phosphors prepared by a different reaction temperature are promising to meet the application requirements for near-UV GaN-based light-emitting diodes (LEDs) as blue or greenemitting phosphors owing to their broad excitation in the near ultraviolet region (350–450 nm).
References [1] Hao ZD, Zhang JH, Zhang X, Ren XG, Shi Y, Luo S, Lu Z, Wang XJ. J Phys D: Appl Phys 2008;41:182001–182001-04. [2] Bin Im W, Kin II Y, Fellows NN, Masui H, Hirata GA, Denbaars SP, Seshadri R. Appl Phys Lett 2008;93:091905–091905-3. [3] Poort SHM, Blokoel WP, Blasse G. Chem Mater 1995;7:1547–51. [4] Suzuki Y, Kakihana M, Shimomura Y, Kijima N. J Mater Sci 2008;43:2213–6. [5] Zeng QH, Hiroaki T, Kiichirou E, Nobutsugu T, Zhang SX. Appl Phys Lett 2006;88:051906–051906-3. [6] Xia ZG, Sun JY, Du HY, Zhou W. Opt Mater 2006;28:524–9. [7] Ding WJ, Wang J, Liu ZM, Zhang M, Su Q, Tang JK. J Electrochem Soc 2008;155: J122–7. [8] Ding WJ, Wang J, Zhang M, Zhang QH, Su Q. Chem Phys Lett 2007;435:301–5. [9] Xia ZG, Li Q, Sun JY. Mater Lett 2007;61:1885–8. [10] Xia ZG, Liu J, Li Q, Sun JY. Electrochem Solid State Lett 2007;10:J4–8. [11] Meijerink A, Blasse G. J Lumin 1990;47:1–5. [12] Blasse G, Grabmaier BC. Lumin. Mater. Berlin: Springer Verlag; 1994. [13] Yang M, Zhang S. J Phys Chem Solids 2004;64:213–21.