- Email: [email protected]
Spectroscopic properties of Na(Sr,Ba)VO4:Eu3+ novel red-emitting phosphors for solid state lighting applications
Journal of Alloys and Compounds 561 (2013) 59–64
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Spectroscopic properties of Na(Sr,Ba)VO4:Eu3+ novel red-emitting phosphors for solid state lighting applications C.R. Kesavulu a, Dong Gi Lee b, Soung Soo Yi b,⇑, Kiwan Jang a, Jae Hyung Park c, Jae Seung Leem c, Yun Hyuk Choi c, Yeon Ju Jang c, Jung Gon Choi c a b c
Department of Physics, Changwon National University, Changwon 641-773, Republic of Korea Department of Electronic Materials Engineering, Silla University, Busan 617-736, Republic of Korea BUSANIL Science High School, Busan 604-828, Republic of Korea
a r t i c l e
i n f o
Article history: Received 3 December 2012 Received in revised form 22 January 2013 Accepted 23 January 2013 Available online 10 February 2013 Keywords: Phosphors Vanadate X-ray diffraction Optical properties
a b s t r a c t Eu3+-activated novel alkaline earth metal (Sr and Ba) vanadate phosphors, Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 (x = 0–0.97) have been successfully synthesized using solid state reaction method and characterized through structure, morphology, elemental analysis, luminescence (excitation, emission and CIE coordinate) and decay rate properties as a function of Ba ion concentration. Phosphors show a broad excitation band (monitored for 5D0 ? 7F2 transition of Eu3+) in the 230–430 nm wavelength regions which make them highly suitable for GaN-based LED chips. These phosphors can be efficiently excited by near UV light and exhibit a dominant red emission at 613 nm (5D0 ? 7F2). The decay lifetime and color coordinates were evaluated for Na(Sr,Ba)VO4:Eu3+ phosphors. Hence, these phosphors could be a potential candidate for light emitting diodes and display applications. Ó 2013 Published by Elsevier B.V.
1. Introduction Presently, phosphors-converted white light-emitting diodes (LEDs) have stepped into lighting industry due to their high efficiency, energy saving, lifetime and more importantly at a relatively low cost. However, most of these products are fabricated by combining a blue-emitting LED chip with a yellow-emitting phosphors (YAG:Ce3+), which suffers from low color rending index Ra (CRI < 75) and unsatisfactory high color temperature (>4500 K) because of the lack of red spectral component. These white LEDs cannot meet the requirement of general lighting applications [1–6]. The above drawbacks could be dissolved by using ultraviolet LED chip exciting red, green and blue tricolor phosphors. The luminescent effect of red phosphor is lower than that of the green and blue phosphor at UV excitation region. Thus, people are interested in the red phosphor that can be excited by blue or UV light. Rare earth ions have been playing an important role in modern lighting and display field due to the abundant emission colors based on their 4f–4f or 5d–4f transitions. Currently, much effort have been directed towards white light emitting diodes (WLEDs) owing to their strong red emission, mainly research is focused on novel Eu3+-doped nanocrystalline materials. Eu3+-doped molybdates, tungstates and vanadates phosphors usually show not only a broad and intense charger-transfer band (CTB) absorption in
⇑ Corresponding author. Tel.: +82 51 999 5704; fax: +82 51 999 5465. E-mail address: [email protected] (S.S. Yi). 0925-8388/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jallcom.2013.01.162
the UV region from oxygen to metal ions but also effective typical f–f transitions from Eu3+, which indicates that these phosphors have been used as potential candidates for UV or blue LEDs [7–10]. As an important family of phosphors host, Eu3+-doped vandate-based phosphors are widely used as red emitting phosphors for LEDs, fluorescent lamps and flat panel display (FPD) due to their extending absorptions from the UV region to the longer wavelength visible region excitation properties and higher chemical stabilities [10–18]. Luckily, Vanadate has strong absorption in UV region and ceramics based on alkaline earth metal (such as Sr and Ba) vanadates have been identified as an excellent material [13–17]. The crystal chemistry of many of these alkaline earth metal vandates is well explored. However, very rare studies regarding optical properties of rare earth ions doped in such vanadates, have been done. In the present work, we have synthesized a novel Eu3+-doped NaSrVO4 phosphor and further Ba ion have been introduced into the host matrix to change the crystal field symmetry. For this, a series of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 (x varies from 0 to 0.97) phosphor have been synthesized by solid state reaction method. All these samples were characterized by XRD, FT-IR, XPS, FE-SEM with EDS, photoluminescence excitation and emission spectra and decay time measurements. The effect of change in crystal symmetry on the intensity of hypersensitive transition of Eu3+ (5D0 ? 7F2) and thus overall change in CIE (international commission on illumination) chromaticity coordinate has been studied in detail to explore the best suitable phosphor for light emitting diodes (LEDs) application.
60
C.R. Kesavulu et al. / Journal of Alloys and Compounds 561 (2013) 59–64
Fig. 1. X-ray diffraction (XRD) patterns of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphor samples with various barium ion concentration.
Fig. 2. X-ray photoelectron spectroscopy (XPS) spectra of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphors synthesized at different Ba ion concentration. The peak at 132 eV corresponds to the binding energy of Sr while peaks at 780 eV and 796 eV are due to Ba.
2. Experimental details The Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphors were synthesized by solid-state reaction method. The starting materials were prepared from sodium carbonate (Na2CO3, JUNSEI, 99.8%), strontium carbonate (SrCO3, ALDRICH, 99.9%), barium carbonate (BaCO3, ALDRICH, 99.98%), Vanadium oxide (V2O5, ALDRICH, 99.6%) and europium oxide (Eu2O3, ALDRICH, 99.9%). The pure raw materials were taken in stoichiometric ratio and mixed with an agate mortar and then sintered at 1150 °C for 4 h (heating rate was 10 °C/min). The synthesis temperature above 1150 °C was not attempted because the evaporation of V ion could take place to a considerable amount above this temperature. X-ray diffraction (XRD, SHIMADZU XRD-6000) patterns of the powder phosphors were obtained using Cu Ka radiation (k = 1.54056 Å). X-ray photoelectron spectroscopic (XPS) measurements were carried out using ESCALAB 250 XPS spectrometer to investigate the effect of barium substitution. FT-IR spectrum of the powder phosphor was recorded on a FT-IR-200E spectrometer (JASCO) with KBr pellet technique from 4000 cm1 to 400 cm1. Morphology and particle size of
the powder samples were observed by field emission scanning electron microscopy (FE-SEM, Quanta 200, 15 kV). To identify the presence of constituents in the prepared phosphor, an energy dispersive X-ray spectroscopy (EDS) was employed equipped with an X-ray detector attached to FE-SEM instrument. Photoluminescence excitation (PLE) and emission (PL) characteristics were examined by spectrophotometer (FS-2, Scinco) equipped with a monochoromator and a xenon lamp (150 W continuous). The lifetime measurements were carried out using JOBIN YVON Fluorolog-3 Spectrofluorimeter using xenon arc lamp as an excitation source. All the measurements were carried out at room temperature.
3. Results and discussion The crystallinity and morphology of phosphors have been important factors to determine luminescent properties. Fig. 1 shows XRD patterns of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphor samples with various Ba ion concentration (x). For x = 0 and x = 0.97,
C.R. Kesavulu et al. / Journal of Alloys and Compounds 561 (2013) 59–64
two pure crystalline phases namely NaSr0.97VO4 (JCPDS No. 321160) and NaBa0.97VO4 (JCPDS No. 32-1043), respectively are obtained. NaSr0.97VO4:Eu3þ 0:03 phase crystallizes into a cubic structure. As the barium ion is introduced into the NaSrVO4, the Sr ions are replaced by Ba ions resulting into a different crystal structure of NaBa0.97VO4 phase (JCPDS No. 32-1043) finally for x = 0.97. However, compounds in intermediate steps (for 0 < x < 0.97) shows a poly-phase structure with the presence of peaks due to both NaSrVO4 and NaBaVO4 phases. Remarkably, intensity of the diffraction peaks corresponding to NaSrVO4 phase becomes week and also faces a change with increasing concentrations of Ba ion. This is possibly due to strain (shrinkage) in the crystal structure by replacing Sr ions through Ba ions, as the ionic radius of Ba ion (1.35 Å) is higher than that of Sr ion (1.13 Å) [19]. Beside, in the intermediate compounds, some of the diffraction peaks remain unindexed, especially if the concentration of Ba ion is high enough. As a result, incorporation of the Ba ions at this level (x = 0.25) is found to form a nearly single-phase compound just having minor impurity phase in its XRD pattern (Fig. 1). These impurity peaks are expected due to the solubility limit of Ba ion in the NaSrVO4 matrix. It is well known that as the amount of doping ion will increase beyond the solubility limit, impurity phase with unindexed peaks can be obviously observed [20]. The solubility limit of Ba ion in 4 matrix would be an interesting problem for crystallography and needs a detailed Rietveld refinement of XRD data, but beyond the scope of this study. Interestingly, a significant increase in emission intensity is observed by introducing the Ba ion at x = 0.25 in the matrix structure which is the focus of the present study. The effect of the introduction of Ba ion in the matrix could be more clearly realized through XPS measurement. Fig. 2 presents the XPS spectra of phosphors as a function of Ba ion concentration. The peak of binding energy for Sr ions is located at 132 eV and the peaks of binding energy for Ba ions are located at 780 eV and 796 eV [21–23]. The intensity of the peaks corresponding to Ba ion (at 780 and 796 eV) is absent in pure NaSrVO4 phase while it appears with a weak intensity for Ba ion doped phosphors. The intensity corresponding to the XPS peak of Ba ion uniformly increases, likewise, a uniform decrease in the intensity of Sr peak (at 132 eV) is observed as the concentration of Ba ions increases. Finally, in the pure NaBaVO4 phase no peak due to Sr ion is observed. This clearly indicates the replacement of Sr ions by Ba ion in the matrix as have also been observed through XRD analysis. Fig. 3 shows the FT-IR spectrum of Na(Sr0.47,Ba0.50)VO4:Eu3þ 0:03 phosphor in the spectral region of 400–4000 cm1. The FT-IR
Fig. 3. FT-IR spectrum of Na(Sr0.47,Ba0.50)VO4:Eu3þ 0:03 (x = 0.50) phosphor.
61
spectrum shows three absorption regions. In the first region, the broad band in the range of 2400–3700 cm1 with maximum at 3392 cm1 assigned to the O–H stretching vibration, the second region in the range of 1300–1800 cm1, which originated from the absorption of the H–O–H bending vibrations centered at 1447 cm1. These two bands are the characteristic vibrations of water from air, physically absorbed on the sample surface [24] and third region, Modes in the range of 450–600 cm1 and 750– 1250 cm1 are due to (VO4)3 absorption bands, which are assigned to the O–V–O bending vibrations and V–O stretching vibrations, respectively [17,25,26]. It is known that the presence of OH content increases the optical losses and then decreases the quantum efficiency of rare-earth doped materials. However, for the present phosphors, the intensity of the IR band associated to OH group is extremely low, which indicates that these prepared phosphors are suitable for practical applications. The peak assignments of the FT-IR spectrum of the present phosphors have been compared with the results reported in literature [17,25,27]. FE-SEM has been used to study the morphology and particle size of the phosphor with various concentrations of Ba ions, as shown in Fig. 4a–e. The particles show irregular and polygonal shape morphology. The particle size for almost all the phosphor powders lies in the range of sub-micrometer to a few micrometers. In addition, the synthesized samples showed some aggregation also which is due to high temperature involved in the synthesis of phosphor material. Detailed composition of the prepared Na(Sr0.47,Ba0.50)VO4:Eu3þ 0:03 (x = 0.50) phosphor is further analyzed by EDS. A representative EDS spectrum is shown in Fig. 5. EDS spectrum shows several specific line, the signal of Na, Sr, Ba, V, O and Eu elements in the prepared samples. This confirms the phase purity of the obtained phosphor and is in consistent with XRD analysis. Fig. 6 shows PLE spectra of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 powders. The PLE spectra (kem = 613 nm) show a broad band (lying in the wavelength range of 230–350 nm), due to the absorption edge of host matrix, along with other relatively weak and sharp bands (in the wavelength range of 350–450 nm), due to well known transitions due to Eu3+ ions. The replacement of Sr ion by Ba ion shows a significant change in the peak position corresponding to broad excitation band while the peak positions of the sharp bands remain almost unaltered. The intensity of PLE band in 230–350 nm region gain in intensity with an increase in Ba ion concentration and attains a maximum at for x = 0.25 (with its maxima at 313 nm wavelength). This broad band excitation of the material makes it highly suitable for LEDs application, particularly for GaN-based LED chips [28]. The UV/blue emission will be strongly absorbed by the host band edge which will activate the Eu3+ ion for efficient red emission. Fig. 7a shows the PL spectra of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphors. The PL spectra (kex = 313 nm) show the presence of most of the characteristic bands (5D0 ? 7Fj = 1,2,3,4) due to Eu3+ ion. The bands are observed and could be assigned to the transitions of 5 D0 ? 7F1 (592 nm), 5D0 ? 7F2 (613 nm and 623 nm), 5D0 ? 7F3 (641 nm and 656 nm) and 5D0 ? 7F4 (687 nm and 704 nm) Eu3+ ions. Among these transitions, the most significant emission peak located at 613 nm corresponds to forced electric dipole transition (5D0 ? 7F2) of Eu3+ ion. The intensity of the PL spectra also follows a similar trend, as PLE, and thus the PL intensity also shows an increase with increase in Ba ion concentration and attains the maximum emission intensity for Na(Sr0.72,Ba0.25)VO4:Eu3þ 0:03 phase (for x = 0.25), and then it decreases due to the effect of Ba ion concentration. The increase in the intensity of the emission with a variation in Ba ion concentration could be related to the variation in asymmetry around the rare earth ion which could be estimated by monochromaticity (R). The 5D0 ? 7F1 transition is used as a reference to judge the environmental asymmetry of the rare earth because it is allowed by the
62
C.R. Kesavulu et al. / Journal of Alloys and Compounds 561 (2013) 59–64
Fig. 4. The field scanning electron microscopy (FE-SEM) micrograph of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphor with varying concentration of Ba ion (a) x = 0.0, (b) x = 0.25, (c) x = 0.50, (d) x = 0.75 and (e) x = 0.97.
magnetic dipole transition and its intensity is independent of the environment where as the hypersensitive transition (5D0 ? F2) strongly depends on it. The intensity ratio (R) of 5D0 ? 7F2 to 5 D0 ? 7F1 transition (also called monochromaticity and denoted by R) is commonly used as a measure of the rare earth site symmetry [29]. Lower value of R signifies higher symmetry and vice versa. Fig. 7b shows intensity ratio of 5D0 ? 7F2 to 5D0 ? 7F1 transition (called monocromaticity, R), versus Ba ion concentration (in% value). The intensity ratio of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphors increases with the increase in Ba ion content up to a value of x = 0.25 is reached, and then it decreases due to the slight changes in local symmetry around the Eu3+ ion increases with increasing Ba ion concentration. The higher value of intensity ratio of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 (x = 0.25) phosphor is found to be 7.82 (seen in Table 1). This is favorable to improve the color purity of the red phosphor.
In order to obtain additional information on the luminescence properties of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 powders, the decay curves have been measured. Fig. 8 shows the decay curves for 5D0 ? 7F2 (613 nm) emission for these samples under excited at 313 nm. All the decay curves are well fitted by the single exponential equation [30], I = I0exp(t/s), where I and I0 are the luminescence intensity at time t and 0, respectively, and s is the PL lifetime. The estimated lifetime values from the fits are presented in Table 1. The single exponential behavior of fits for all samples evidences the presence of the Eu3+ ion environment is unique in according to the crystal structure. In addition, there is no wavelength shift or peak for new site has been observed for various Ba ion concentrations. This strongly suggests that the local environment of the Eu3+ ions in the lattice is the same in different positions. It is noticed from the Table 1, that the slight variation of lifetimes for different Ba ion concentrations may be due to the different particle sizes of the synthesized
63
C.R. Kesavulu et al. / Journal of Alloys and Compounds 561 (2013) 59–64
Fig. 5. EDS profile of Na(Sr0.47,Ba0.50)VO4:Eu3þ 0:03 (x = 0.50) phosphor.
corresponding emission spectra excited by 313 nm and have shown in Fig. 9, and also values are tabulated in Table 1. The obtained CIE color coordinates of all the samples are lies in the red region. The changes in the color coordinates may be due to the variation of the asymmetric ratios of various Ba ion concentrations of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphors. Further, the present color coordinates are comparable with the commercially available red phosphor of Y2O2S:Eu3+ (0.647, 0.343) [31]. The present results suggest that the Eu3+-doped Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 (x = 0– 0.97) phosphors could be a potential candidate as red emitting phosphor in display/lamp applications. 4. Conclusions
Fig. 6. Photoluminescence excitation (monitored at kem = 613 nm) spectra of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphor as a function of barium concentrations.
phosphors, which is in consistent with those results obtained from XRD analysis. This result shows that the lifetime is short enough for potential applications in displays and lights. In general, the emission color of two phosphors is compared by means of color coordinates and is good certification for photoluminescence applications. In 1931, the Commission International de I’Eclairage (CIE) established a universal quantitative model of color spaces. The chromaticity coordinates of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphors are calculated (using CIE calculated software) from their
In summary, Eu3+ ion-doped Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 (x = 0–0.97) phosphors have been synthesized successfully and characterized by XRD, XPS, FT-IR, FE-SEM with EDS and optical properties. Structural analysis depicts that phosphor crystallizes into pure 3þ NaSr0.97VO4:Eu3þ 0:03 and NaBa0.97VO4:Eu0:03 phases for x = 0 and x = 0.97 respectively, while intermediate compounds contains mixed phases. Doping of Ba ion into pure NaSrVO4 phase affects the crystal symmetry and hence a change in the asymmetry around the emitting species (Eu3+ ions) gives rise to an enhancement in the overall fluorescence intensity. The optimum emission is achieved for mixed Na(Sr0.72,Ba0.25)VO4:Eu3þ 0:03 phase phosphor. The excellent optical features, such as broad excitation band (monitored for 5 D0 ? 7F2 transition of Eu3+) lying in the range of 230–430 nm and excellent emission in red region (at 613 nm). The present results indicate that Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphors as red component for white LEDs in solid state lighting applications.
Table 1 Lifetimes, monochromaticity and CIE chromaticity coordinates of the Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 (where x = 0.0, 0.25, 0.50, 0.75 and 0.97) phosphors. S. no.
Value (x)
Lifetimes (ms)
Monochromaticity (R)
CIE coordinates (x, y)
1 2 3 4 5
x = 0.0 x = 0.25 x = 0.50 x = 0.75 x = 0.97
0.93 0.87 0.85 0.98 0.90
1.80 7.82 4.66 3.90 1.67
(0.656, (0.656, (0.651, (0.636, (0.635,
0.340) 0.340) 0.347) 0.361) 0.362)
64
C.R. Kesavulu et al. / Journal of Alloys and Compounds 561 (2013) 59–64
5 7 5 7 Fig. 7. (a) Emission (kex = 313 nm) spectra of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphor as a function of barium concentrations and (b) intensity ratio of D0 ? F2 to D0 ? F1 transition (called monocromaticity, R), versus Ba concentration (in% value) graph.
Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010– 0021456) and the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2010–0029634). References [1] [2] [3] [4] [5]
[6] Fig. 8. Photoluminescence decay curves of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 (x = 0.0, 0.25, 0.50, 0.75 and x = 0.97) phosphors (kex = 313 nm and kem = 613 nm).
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
Fig. 9. CIE chromaticity diagram of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphors under UV (313 nm) excitation, (a) x = 0.0, (b) x = 0.25, (c) x = 0.50, (d) x = 0.75 and (e) x = 0.97.
[31]
E.F. Schubert, J.K. Kim, Science 308 (2005) 1274–1278. S. Ye, F. Xiao, Y. Pan, Y. Ma, Q. Zhang, Mater. Sci. Eng. Rep. 71 (2010) 1–34. X. Li, L. Guan, X. Li, J. Wen, Z. Yang, Powder Technol. 200 (2010) 12–15. C. Cao, H.K. Yang, J.W. Chung, B.K. Moon, B.C. Choi, J.H. Jeong, K.H. Kim, J. Am. Ceram. Soc. 94 (2011) 3405–3411. S.K. Singh, Min Jae Kim, Chuer Hyun Moon, Dong Gi Lee, Soung Soo Yi, Kiwan Jang, Jung Hyun Jeong, Jong-Seong Bae, Dong-Soo Shin, Mater. Res. Bull. 47 (2012) 2676–2679. Chuer Hyun Moon, S.K. Singh, Dong Gi Lee, Soung Soo Yi, Kiwan Jang, Jung Hyun Jeong, Jong-Seong Bae, Dong-Soo Shin, Ceram. Int. (2012) 6789–6794. C. Guo, F. Gao, Y. Xu, L. Liang, F. Shi, B. Yan, J. Phys. D: Appl. Phys. 42 (2009) 095407 (7pp). A. Kumar, J. Kumar, J. Mater. Chem. 21 (2011) 3788–3795. Z. Ju, R. Wei, X. Gao, W. Liu, C. Pang, Opt. Mater. 33 (2011) 909–913. A.A. Setlur, H.A. Comanzo, A.M. Srivastava, W.W. Beers, J. Electrochem. Soc. 152 (2005) H205–H208. S. Choi, Y.M. Moon, K. Kim, H.K. Jung, S. Nahm, J. Lumin. 129 (2009) 988–990. R. Jagannathan, J. Lumin. 68 (1998) 211–216. T. Nakajima, M. Isobe, T. Tsuchiya, Y. Ueda, T. Kumagai, J. Lumin. 129 (2009) 1598–1601. Roshani Singh, S.J. Dhoble, Adv. Mater. Lett. 2 (5) (2011) 341–344. J. Zhao, C. Guo, J. Yu, R. Yu, Opt. Laser. Technol. 45 (2013) 62–68. T. Nakajima, M. Isobe, T. Tsuchiya, Y. Ueda, T. Manabe, Opt. Mater. 32 (2010) 1618–1621. Sheetal, V.B. Taxak, S.P. Khatkar, J. Fluoresc. 22 (2012) 891–897. J. Gu, B. Yan, J. Alloys Compd. 476 (2009) 619–623. R.D. Shannon, Acta Cryst. A32 (1976) 751–767. Y.Q. Li, G. de With, H.T. Hintzen, J. Solid State Chem. 181 (2008) 515–524. V. Young, T. Otagawa, Appl. Surf. Sci. 20 (1985) 228–248. B. Demri, D. Muster, J. Mater. Proc. Technol. 55 (1995) 311–314. S. Kumar, V.S. Raju, T.R.N. Kutty, Appl. Surf. Sci. 206 (2003) 250–261. F. Lei, B. Yan, Solid State Chem. 181 (2008) 855–862. I.O. Mazali, O.L. Alves, J. Braz, Chem. Soc. 15 (2004) 464–467. U. Rambabu, Sang-Do Han, Ceram. Int. 39 (2013) 701–708. V.B. Taxak, Sheetal, Dayawati, S.P. Khatkar, Curr. Appl. Phys. 13 (2013) 594–598. H. Hirayama, J. Appl. Phys. 97 (2005) 091101–0911019. A.K. Singh, S.K. Singh, H. Mishra, R. Prakash, S.B. Rai, J. Phys. Chem. B 114 (2010) 13042–13051. A.K. Parchur, R.S. Ningthoujam, S.B. Rai, G.S. Okram, R.A. Singh, M. Tyagi, S.C. Gadkari, R. Tewari, R.K. Vasta, Dalton Trans. 40 (2011) 7595–7601. C.H. Huang, T.W. Kuo, T.M. Chen, Opt. Exp. 19 (S1) (2011) A1–A6.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Spectroscopic properties of Na(Sr,Ba)VO4:Eu3+ novel red-emitting phosphors for solid state lighting applications C.R. Kesavulu a, Dong Gi Lee b, Soung Soo Yi b,⇑, Kiwan Jang a, Jae Hyung Park c, Jae Seung Leem c, Yun Hyuk Choi c, Yeon Ju Jang c, Jung Gon Choi c a b c
Department of Physics, Changwon National University, Changwon 641-773, Republic of Korea Department of Electronic Materials Engineering, Silla University, Busan 617-736, Republic of Korea BUSANIL Science High School, Busan 604-828, Republic of Korea
a r t i c l e
i n f o
Article history: Received 3 December 2012 Received in revised form 22 January 2013 Accepted 23 January 2013 Available online 10 February 2013 Keywords: Phosphors Vanadate X-ray diffraction Optical properties
a b s t r a c t Eu3+-activated novel alkaline earth metal (Sr and Ba) vanadate phosphors, Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 (x = 0–0.97) have been successfully synthesized using solid state reaction method and characterized through structure, morphology, elemental analysis, luminescence (excitation, emission and CIE coordinate) and decay rate properties as a function of Ba ion concentration. Phosphors show a broad excitation band (monitored for 5D0 ? 7F2 transition of Eu3+) in the 230–430 nm wavelength regions which make them highly suitable for GaN-based LED chips. These phosphors can be efficiently excited by near UV light and exhibit a dominant red emission at 613 nm (5D0 ? 7F2). The decay lifetime and color coordinates were evaluated for Na(Sr,Ba)VO4:Eu3+ phosphors. Hence, these phosphors could be a potential candidate for light emitting diodes and display applications. Ó 2013 Published by Elsevier B.V.
1. Introduction Presently, phosphors-converted white light-emitting diodes (LEDs) have stepped into lighting industry due to their high efficiency, energy saving, lifetime and more importantly at a relatively low cost. However, most of these products are fabricated by combining a blue-emitting LED chip with a yellow-emitting phosphors (YAG:Ce3+), which suffers from low color rending index Ra (CRI < 75) and unsatisfactory high color temperature (>4500 K) because of the lack of red spectral component. These white LEDs cannot meet the requirement of general lighting applications [1–6]. The above drawbacks could be dissolved by using ultraviolet LED chip exciting red, green and blue tricolor phosphors. The luminescent effect of red phosphor is lower than that of the green and blue phosphor at UV excitation region. Thus, people are interested in the red phosphor that can be excited by blue or UV light. Rare earth ions have been playing an important role in modern lighting and display field due to the abundant emission colors based on their 4f–4f or 5d–4f transitions. Currently, much effort have been directed towards white light emitting diodes (WLEDs) owing to their strong red emission, mainly research is focused on novel Eu3+-doped nanocrystalline materials. Eu3+-doped molybdates, tungstates and vanadates phosphors usually show not only a broad and intense charger-transfer band (CTB) absorption in
⇑ Corresponding author. Tel.: +82 51 999 5704; fax: +82 51 999 5465. E-mail address: [email protected] (S.S. Yi). 0925-8388/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jallcom.2013.01.162
the UV region from oxygen to metal ions but also effective typical f–f transitions from Eu3+, which indicates that these phosphors have been used as potential candidates for UV or blue LEDs [7–10]. As an important family of phosphors host, Eu3+-doped vandate-based phosphors are widely used as red emitting phosphors for LEDs, fluorescent lamps and flat panel display (FPD) due to their extending absorptions from the UV region to the longer wavelength visible region excitation properties and higher chemical stabilities [10–18]. Luckily, Vanadate has strong absorption in UV region and ceramics based on alkaline earth metal (such as Sr and Ba) vanadates have been identified as an excellent material [13–17]. The crystal chemistry of many of these alkaline earth metal vandates is well explored. However, very rare studies regarding optical properties of rare earth ions doped in such vanadates, have been done. In the present work, we have synthesized a novel Eu3+-doped NaSrVO4 phosphor and further Ba ion have been introduced into the host matrix to change the crystal field symmetry. For this, a series of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 (x varies from 0 to 0.97) phosphor have been synthesized by solid state reaction method. All these samples were characterized by XRD, FT-IR, XPS, FE-SEM with EDS, photoluminescence excitation and emission spectra and decay time measurements. The effect of change in crystal symmetry on the intensity of hypersensitive transition of Eu3+ (5D0 ? 7F2) and thus overall change in CIE (international commission on illumination) chromaticity coordinate has been studied in detail to explore the best suitable phosphor for light emitting diodes (LEDs) application.
60
C.R. Kesavulu et al. / Journal of Alloys and Compounds 561 (2013) 59–64
Fig. 1. X-ray diffraction (XRD) patterns of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphor samples with various barium ion concentration.
Fig. 2. X-ray photoelectron spectroscopy (XPS) spectra of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphors synthesized at different Ba ion concentration. The peak at 132 eV corresponds to the binding energy of Sr while peaks at 780 eV and 796 eV are due to Ba.
2. Experimental details The Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphors were synthesized by solid-state reaction method. The starting materials were prepared from sodium carbonate (Na2CO3, JUNSEI, 99.8%), strontium carbonate (SrCO3, ALDRICH, 99.9%), barium carbonate (BaCO3, ALDRICH, 99.98%), Vanadium oxide (V2O5, ALDRICH, 99.6%) and europium oxide (Eu2O3, ALDRICH, 99.9%). The pure raw materials were taken in stoichiometric ratio and mixed with an agate mortar and then sintered at 1150 °C for 4 h (heating rate was 10 °C/min). The synthesis temperature above 1150 °C was not attempted because the evaporation of V ion could take place to a considerable amount above this temperature. X-ray diffraction (XRD, SHIMADZU XRD-6000) patterns of the powder phosphors were obtained using Cu Ka radiation (k = 1.54056 Å). X-ray photoelectron spectroscopic (XPS) measurements were carried out using ESCALAB 250 XPS spectrometer to investigate the effect of barium substitution. FT-IR spectrum of the powder phosphor was recorded on a FT-IR-200E spectrometer (JASCO) with KBr pellet technique from 4000 cm1 to 400 cm1. Morphology and particle size of
the powder samples were observed by field emission scanning electron microscopy (FE-SEM, Quanta 200, 15 kV). To identify the presence of constituents in the prepared phosphor, an energy dispersive X-ray spectroscopy (EDS) was employed equipped with an X-ray detector attached to FE-SEM instrument. Photoluminescence excitation (PLE) and emission (PL) characteristics were examined by spectrophotometer (FS-2, Scinco) equipped with a monochoromator and a xenon lamp (150 W continuous). The lifetime measurements were carried out using JOBIN YVON Fluorolog-3 Spectrofluorimeter using xenon arc lamp as an excitation source. All the measurements were carried out at room temperature.
3. Results and discussion The crystallinity and morphology of phosphors have been important factors to determine luminescent properties. Fig. 1 shows XRD patterns of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphor samples with various Ba ion concentration (x). For x = 0 and x = 0.97,
C.R. Kesavulu et al. / Journal of Alloys and Compounds 561 (2013) 59–64
two pure crystalline phases namely NaSr0.97VO4 (JCPDS No. 321160) and NaBa0.97VO4 (JCPDS No. 32-1043), respectively are obtained. NaSr0.97VO4:Eu3þ 0:03 phase crystallizes into a cubic structure. As the barium ion is introduced into the NaSrVO4, the Sr ions are replaced by Ba ions resulting into a different crystal structure of NaBa0.97VO4 phase (JCPDS No. 32-1043) finally for x = 0.97. However, compounds in intermediate steps (for 0 < x < 0.97) shows a poly-phase structure with the presence of peaks due to both NaSrVO4 and NaBaVO4 phases. Remarkably, intensity of the diffraction peaks corresponding to NaSrVO4 phase becomes week and also faces a change with increasing concentrations of Ba ion. This is possibly due to strain (shrinkage) in the crystal structure by replacing Sr ions through Ba ions, as the ionic radius of Ba ion (1.35 Å) is higher than that of Sr ion (1.13 Å) [19]. Beside, in the intermediate compounds, some of the diffraction peaks remain unindexed, especially if the concentration of Ba ion is high enough. As a result, incorporation of the Ba ions at this level (x = 0.25) is found to form a nearly single-phase compound just having minor impurity phase in its XRD pattern (Fig. 1). These impurity peaks are expected due to the solubility limit of Ba ion in the NaSrVO4 matrix. It is well known that as the amount of doping ion will increase beyond the solubility limit, impurity phase with unindexed peaks can be obviously observed [20]. The solubility limit of Ba ion in 4 matrix would be an interesting problem for crystallography and needs a detailed Rietveld refinement of XRD data, but beyond the scope of this study. Interestingly, a significant increase in emission intensity is observed by introducing the Ba ion at x = 0.25 in the matrix structure which is the focus of the present study. The effect of the introduction of Ba ion in the matrix could be more clearly realized through XPS measurement. Fig. 2 presents the XPS spectra of phosphors as a function of Ba ion concentration. The peak of binding energy for Sr ions is located at 132 eV and the peaks of binding energy for Ba ions are located at 780 eV and 796 eV [21–23]. The intensity of the peaks corresponding to Ba ion (at 780 and 796 eV) is absent in pure NaSrVO4 phase while it appears with a weak intensity for Ba ion doped phosphors. The intensity corresponding to the XPS peak of Ba ion uniformly increases, likewise, a uniform decrease in the intensity of Sr peak (at 132 eV) is observed as the concentration of Ba ions increases. Finally, in the pure NaBaVO4 phase no peak due to Sr ion is observed. This clearly indicates the replacement of Sr ions by Ba ion in the matrix as have also been observed through XRD analysis. Fig. 3 shows the FT-IR spectrum of Na(Sr0.47,Ba0.50)VO4:Eu3þ 0:03 phosphor in the spectral region of 400–4000 cm1. The FT-IR
Fig. 3. FT-IR spectrum of Na(Sr0.47,Ba0.50)VO4:Eu3þ 0:03 (x = 0.50) phosphor.
61
spectrum shows three absorption regions. In the first region, the broad band in the range of 2400–3700 cm1 with maximum at 3392 cm1 assigned to the O–H stretching vibration, the second region in the range of 1300–1800 cm1, which originated from the absorption of the H–O–H bending vibrations centered at 1447 cm1. These two bands are the characteristic vibrations of water from air, physically absorbed on the sample surface [24] and third region, Modes in the range of 450–600 cm1 and 750– 1250 cm1 are due to (VO4)3 absorption bands, which are assigned to the O–V–O bending vibrations and V–O stretching vibrations, respectively [17,25,26]. It is known that the presence of OH content increases the optical losses and then decreases the quantum efficiency of rare-earth doped materials. However, for the present phosphors, the intensity of the IR band associated to OH group is extremely low, which indicates that these prepared phosphors are suitable for practical applications. The peak assignments of the FT-IR spectrum of the present phosphors have been compared with the results reported in literature [17,25,27]. FE-SEM has been used to study the morphology and particle size of the phosphor with various concentrations of Ba ions, as shown in Fig. 4a–e. The particles show irregular and polygonal shape morphology. The particle size for almost all the phosphor powders lies in the range of sub-micrometer to a few micrometers. In addition, the synthesized samples showed some aggregation also which is due to high temperature involved in the synthesis of phosphor material. Detailed composition of the prepared Na(Sr0.47,Ba0.50)VO4:Eu3þ 0:03 (x = 0.50) phosphor is further analyzed by EDS. A representative EDS spectrum is shown in Fig. 5. EDS spectrum shows several specific line, the signal of Na, Sr, Ba, V, O and Eu elements in the prepared samples. This confirms the phase purity of the obtained phosphor and is in consistent with XRD analysis. Fig. 6 shows PLE spectra of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 powders. The PLE spectra (kem = 613 nm) show a broad band (lying in the wavelength range of 230–350 nm), due to the absorption edge of host matrix, along with other relatively weak and sharp bands (in the wavelength range of 350–450 nm), due to well known transitions due to Eu3+ ions. The replacement of Sr ion by Ba ion shows a significant change in the peak position corresponding to broad excitation band while the peak positions of the sharp bands remain almost unaltered. The intensity of PLE band in 230–350 nm region gain in intensity with an increase in Ba ion concentration and attains a maximum at for x = 0.25 (with its maxima at 313 nm wavelength). This broad band excitation of the material makes it highly suitable for LEDs application, particularly for GaN-based LED chips [28]. The UV/blue emission will be strongly absorbed by the host band edge which will activate the Eu3+ ion for efficient red emission. Fig. 7a shows the PL spectra of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphors. The PL spectra (kex = 313 nm) show the presence of most of the characteristic bands (5D0 ? 7Fj = 1,2,3,4) due to Eu3+ ion. The bands are observed and could be assigned to the transitions of 5 D0 ? 7F1 (592 nm), 5D0 ? 7F2 (613 nm and 623 nm), 5D0 ? 7F3 (641 nm and 656 nm) and 5D0 ? 7F4 (687 nm and 704 nm) Eu3+ ions. Among these transitions, the most significant emission peak located at 613 nm corresponds to forced electric dipole transition (5D0 ? 7F2) of Eu3+ ion. The intensity of the PL spectra also follows a similar trend, as PLE, and thus the PL intensity also shows an increase with increase in Ba ion concentration and attains the maximum emission intensity for Na(Sr0.72,Ba0.25)VO4:Eu3þ 0:03 phase (for x = 0.25), and then it decreases due to the effect of Ba ion concentration. The increase in the intensity of the emission with a variation in Ba ion concentration could be related to the variation in asymmetry around the rare earth ion which could be estimated by monochromaticity (R). The 5D0 ? 7F1 transition is used as a reference to judge the environmental asymmetry of the rare earth because it is allowed by the
62
C.R. Kesavulu et al. / Journal of Alloys and Compounds 561 (2013) 59–64
Fig. 4. The field scanning electron microscopy (FE-SEM) micrograph of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphor with varying concentration of Ba ion (a) x = 0.0, (b) x = 0.25, (c) x = 0.50, (d) x = 0.75 and (e) x = 0.97.
magnetic dipole transition and its intensity is independent of the environment where as the hypersensitive transition (5D0 ? F2) strongly depends on it. The intensity ratio (R) of 5D0 ? 7F2 to 5 D0 ? 7F1 transition (also called monochromaticity and denoted by R) is commonly used as a measure of the rare earth site symmetry [29]. Lower value of R signifies higher symmetry and vice versa. Fig. 7b shows intensity ratio of 5D0 ? 7F2 to 5D0 ? 7F1 transition (called monocromaticity, R), versus Ba ion concentration (in% value). The intensity ratio of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphors increases with the increase in Ba ion content up to a value of x = 0.25 is reached, and then it decreases due to the slight changes in local symmetry around the Eu3+ ion increases with increasing Ba ion concentration. The higher value of intensity ratio of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 (x = 0.25) phosphor is found to be 7.82 (seen in Table 1). This is favorable to improve the color purity of the red phosphor.
In order to obtain additional information on the luminescence properties of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 powders, the decay curves have been measured. Fig. 8 shows the decay curves for 5D0 ? 7F2 (613 nm) emission for these samples under excited at 313 nm. All the decay curves are well fitted by the single exponential equation [30], I = I0exp(t/s), where I and I0 are the luminescence intensity at time t and 0, respectively, and s is the PL lifetime. The estimated lifetime values from the fits are presented in Table 1. The single exponential behavior of fits for all samples evidences the presence of the Eu3+ ion environment is unique in according to the crystal structure. In addition, there is no wavelength shift or peak for new site has been observed for various Ba ion concentrations. This strongly suggests that the local environment of the Eu3+ ions in the lattice is the same in different positions. It is noticed from the Table 1, that the slight variation of lifetimes for different Ba ion concentrations may be due to the different particle sizes of the synthesized
63
C.R. Kesavulu et al. / Journal of Alloys and Compounds 561 (2013) 59–64
Fig. 5. EDS profile of Na(Sr0.47,Ba0.50)VO4:Eu3þ 0:03 (x = 0.50) phosphor.
corresponding emission spectra excited by 313 nm and have shown in Fig. 9, and also values are tabulated in Table 1. The obtained CIE color coordinates of all the samples are lies in the red region. The changes in the color coordinates may be due to the variation of the asymmetric ratios of various Ba ion concentrations of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphors. Further, the present color coordinates are comparable with the commercially available red phosphor of Y2O2S:Eu3+ (0.647, 0.343) [31]. The present results suggest that the Eu3+-doped Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 (x = 0– 0.97) phosphors could be a potential candidate as red emitting phosphor in display/lamp applications. 4. Conclusions
Fig. 6. Photoluminescence excitation (monitored at kem = 613 nm) spectra of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphor as a function of barium concentrations.
phosphors, which is in consistent with those results obtained from XRD analysis. This result shows that the lifetime is short enough for potential applications in displays and lights. In general, the emission color of two phosphors is compared by means of color coordinates and is good certification for photoluminescence applications. In 1931, the Commission International de I’Eclairage (CIE) established a universal quantitative model of color spaces. The chromaticity coordinates of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphors are calculated (using CIE calculated software) from their
In summary, Eu3+ ion-doped Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 (x = 0–0.97) phosphors have been synthesized successfully and characterized by XRD, XPS, FT-IR, FE-SEM with EDS and optical properties. Structural analysis depicts that phosphor crystallizes into pure 3þ NaSr0.97VO4:Eu3þ 0:03 and NaBa0.97VO4:Eu0:03 phases for x = 0 and x = 0.97 respectively, while intermediate compounds contains mixed phases. Doping of Ba ion into pure NaSrVO4 phase affects the crystal symmetry and hence a change in the asymmetry around the emitting species (Eu3+ ions) gives rise to an enhancement in the overall fluorescence intensity. The optimum emission is achieved for mixed Na(Sr0.72,Ba0.25)VO4:Eu3þ 0:03 phase phosphor. The excellent optical features, such as broad excitation band (monitored for 5 D0 ? 7F2 transition of Eu3+) lying in the range of 230–430 nm and excellent emission in red region (at 613 nm). The present results indicate that Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphors as red component for white LEDs in solid state lighting applications.
Table 1 Lifetimes, monochromaticity and CIE chromaticity coordinates of the Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 (where x = 0.0, 0.25, 0.50, 0.75 and 0.97) phosphors. S. no.
Value (x)
Lifetimes (ms)
Monochromaticity (R)
CIE coordinates (x, y)
1 2 3 4 5
x = 0.0 x = 0.25 x = 0.50 x = 0.75 x = 0.97
0.93 0.87 0.85 0.98 0.90
1.80 7.82 4.66 3.90 1.67
(0.656, (0.656, (0.651, (0.636, (0.635,
0.340) 0.340) 0.347) 0.361) 0.362)
64
C.R. Kesavulu et al. / Journal of Alloys and Compounds 561 (2013) 59–64
5 7 5 7 Fig. 7. (a) Emission (kex = 313 nm) spectra of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphor as a function of barium concentrations and (b) intensity ratio of D0 ? F2 to D0 ? F1 transition (called monocromaticity, R), versus Ba concentration (in% value) graph.
Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010– 0021456) and the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2010–0029634). References [1] [2] [3] [4] [5]
[6] Fig. 8. Photoluminescence decay curves of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 (x = 0.0, 0.25, 0.50, 0.75 and x = 0.97) phosphors (kex = 313 nm and kem = 613 nm).
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
Fig. 9. CIE chromaticity diagram of Na(Sr0.97x,Bax)VO4:Eu3þ 0:03 phosphors under UV (313 nm) excitation, (a) x = 0.0, (b) x = 0.25, (c) x = 0.50, (d) x = 0.75 and (e) x = 0.97.
[31]
E.F. Schubert, J.K. Kim, Science 308 (2005) 1274–1278. S. Ye, F. Xiao, Y. Pan, Y. Ma, Q. Zhang, Mater. Sci. Eng. Rep. 71 (2010) 1–34. X. Li, L. Guan, X. Li, J. Wen, Z. Yang, Powder Technol. 200 (2010) 12–15. C. Cao, H.K. Yang, J.W. Chung, B.K. Moon, B.C. Choi, J.H. Jeong, K.H. Kim, J. Am. Ceram. Soc. 94 (2011) 3405–3411. S.K. Singh, Min Jae Kim, Chuer Hyun Moon, Dong Gi Lee, Soung Soo Yi, Kiwan Jang, Jung Hyun Jeong, Jong-Seong Bae, Dong-Soo Shin, Mater. Res. Bull. 47 (2012) 2676–2679. Chuer Hyun Moon, S.K. Singh, Dong Gi Lee, Soung Soo Yi, Kiwan Jang, Jung Hyun Jeong, Jong-Seong Bae, Dong-Soo Shin, Ceram. Int. (2012) 6789–6794. C. Guo, F. Gao, Y. Xu, L. Liang, F. Shi, B. Yan, J. Phys. D: Appl. Phys. 42 (2009) 095407 (7pp). A. Kumar, J. Kumar, J. Mater. Chem. 21 (2011) 3788–3795. Z. Ju, R. Wei, X. Gao, W. Liu, C. Pang, Opt. Mater. 33 (2011) 909–913. A.A. Setlur, H.A. Comanzo, A.M. Srivastava, W.W. Beers, J. Electrochem. Soc. 152 (2005) H205–H208. S. Choi, Y.M. Moon, K. Kim, H.K. Jung, S. Nahm, J. Lumin. 129 (2009) 988–990. R. Jagannathan, J. Lumin. 68 (1998) 211–216. T. Nakajima, M. Isobe, T. Tsuchiya, Y. Ueda, T. Kumagai, J. Lumin. 129 (2009) 1598–1601. Roshani Singh, S.J. Dhoble, Adv. Mater. Lett. 2 (5) (2011) 341–344. J. Zhao, C. Guo, J. Yu, R. Yu, Opt. Laser. Technol. 45 (2013) 62–68. T. Nakajima, M. Isobe, T. Tsuchiya, Y. Ueda, T. Manabe, Opt. Mater. 32 (2010) 1618–1621. Sheetal, V.B. Taxak, S.P. Khatkar, J. Fluoresc. 22 (2012) 891–897. J. Gu, B. Yan, J. Alloys Compd. 476 (2009) 619–623. R.D. Shannon, Acta Cryst. A32 (1976) 751–767. Y.Q. Li, G. de With, H.T. Hintzen, J. Solid State Chem. 181 (2008) 515–524. V. Young, T. Otagawa, Appl. Surf. Sci. 20 (1985) 228–248. B. Demri, D. Muster, J. Mater. Proc. Technol. 55 (1995) 311–314. S. Kumar, V.S. Raju, T.R.N. Kutty, Appl. Surf. Sci. 206 (2003) 250–261. F. Lei, B. Yan, Solid State Chem. 181 (2008) 855–862. I.O. Mazali, O.L. Alves, J. Braz, Chem. Soc. 15 (2004) 464–467. U. Rambabu, Sang-Do Han, Ceram. Int. 39 (2013) 701–708. V.B. Taxak, Sheetal, Dayawati, S.P. Khatkar, Curr. Appl. Phys. 13 (2013) 594–598. H. Hirayama, J. Appl. Phys. 97 (2005) 091101–0911019. A.K. Singh, S.K. Singh, H. Mishra, R. Prakash, S.B. Rai, J. Phys. Chem. B 114 (2010) 13042–13051. A.K. Parchur, R.S. Ningthoujam, S.B. Rai, G.S. Okram, R.A. Singh, M. Tyagi, S.C. Gadkari, R. Tewari, R.K. Vasta, Dalton Trans. 40 (2011) 7595–7601. C.H. Huang, T.W. Kuo, T.M. Chen, Opt. Exp. 19 (S1) (2011) A1–A6.