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Synthesis of a new NASICON-type blue luminescent material
Journal of Alloys and Compounds 418 (2006) 73–76
Synthesis of a new NASICON-type blue luminescent material Toshiyuki Masui, Kazuhiko Koyabu, Shinji Tamura, Nobuhito Imanaka ∗ Department of Applied Chemistry, Faculty of Engineering, Osaka University and Handai Frontier Research Center, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Received 1 June 2005; received in revised form 31 August 2005; accepted 31 August 2005 Available online 18 January 2006
Abstract A new blue emitting phosphor, Eu0.5 Zr2 (PO4 )3 , was synthesized for the first time by the solid-state reaction method. The sample developed in the present study has a stable NASICON-type rhombohedral structure, which consists of a three-dimensional network formed by PO4 tetrahedra sharing corners with ZrO6 octahedra in a single phase with high crystallinity. The photoluminescent emission spectrum shows typical emission from 4f6 5d1 to 4f7 of Eu2+ , although the improvement of the luminescent property will be necessary by optimization of the preparation process and the modification of the particle morphology. © 2005 Elsevier B.V. All rights reserved. Keywords: NASICON-type structure; Phosphor; Blue emission; Divalent europium ion
1. Introduction There has been considerable interest in the development of advanced luminescent materials for applications such as flat panel displays, mercury-free lamps, and X-ray imaging systems. The properties of the luminescent materials arise from complex interactions among the host lattice, activators, defects, and interfaces, which are strongly dependent on the composition [1]. Among the activators in the luminescent materials, the optical properties of divalent europium ions present many interests because they are useful for visible and UV radiation sources and some of them being important for laser materials and optical communication devices [2,3]. In order to produce new luminescent materials, it is significant to select the crystal structure carefully. The structure of the NASICON (Na+ ion super ionic conductors)-type phosphates whose typical composition is expressed as NaZr2 (PO4 )3 consists of a three-dimensional network formed by PO4 tetrahedra sharing corners with ZrO6 octahedra [4,5]. This structure allows sodium cations to substitute with other cations leading to the creation of new luminescent materials with modulation along the a and c axes of the hexagonal cell. The crystal structure of the ∗
Corresponding author. Tel.: +81 6 6879 7352; fax: +81 6 6879 7354. E-mail address: [email protected] (N. Imanaka).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.08.097
MII 0.5 Zr2 (PO4 )3 phosphates is determined by the size of the MII ion. The phosphates with larger ions such as Eu2+ , Sr2+ , and Ba2+ crystallize in the NASICON-type (Fig. 1), whereas those with smaller ions exhibit the -Fe2 (SO4 )3 -type structure. In this study, a new luminescent material containing divalent europium ions in the NASICON-type structure, Eu0.5 Zr2 (PO4 )3 , was synthesized and the luminescent properties were characterized. 2. Experimental Eu0.5 Zr2 (PO4 )3 was prepared by a conventional solid-state reaction method. A stoichiometric amount of Eu2 O3 (99.9%), ZrO(NO3 )2 ·2H2 O (99.95%), and (NH4 )2 HPO4 (99.99%) was mixed in a mortar. A small amount (5 wt.%) of H2 BO3 was added as a flux and then the powder was mixed using a ball-milling apparatus (Fritsch P-7) for 6 h. The mixture was calcined at 1200 ◦ C for 5 h in a flow of 2%H2 –Ar gas. The phosphor was characterized with a scanning electron microscope (SEM, Hitachi S-4300SD) and with an X-ray powder diffractometer (XRD, Rigaku Multiflex) using Cu K␣ radiation. The lattice parameters of Eu0.5 Zr2 (PO4 )3 were obtained by the least squares method from observed d-value and h k l data corrected with those of ␣-Al2 O3 using the internal standard method. The mean particle size and the size distribution were estimated by measuring the diameters of 100 particles on the SEM photographs. The sample composition was confirmed by X-ray fluorescence analysis (Rigaku ZSX100e). The photoluminescent (PL) excitation and emission spectra of the Eu0.5 Zr2 (PO4 )3 phosphor were obtained at room temperature with a fluorescence spectrophotometer (Shimadzu RF-5300PC).
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T. Masui et al. / Journal of Alloys and Compounds 418 (2006) 73–76 Table 1 Comparison of the lattice parameters of the NASICON-type phosphates
Fig. 1. Structure of MII 0.5 Zr2 (PO4 )3 phosphates (M = Sr, Eu, and Ba).
3. Results and discussion X-ray powder diffraction (XRD) pattern for Eu0.5 Zr2 (PO4 )3 is shown in Fig. 2. The XRD pattern of the sample is in good agreement with a single phase of the NASICON-type structure [6–9], and there is no diffraction peak corresponding to any impurities in the XRD patterns. The cell parameters of the Eu0.5 Zr2 (PO4 )3 phosphor data are a = 0.8693 nm and c = 2.3388 nm, and the indexation of all reflections is consistent with the R3 space group. The comparison of these parameters with those of Ca0.5 Zr2 (PO4 )3 [8], Sr0.5 Zr2 (PO4 )3 [7], and Ba0.5 Zr2 (PO4 )3 [9] are summarized in Table 1. The ionic radii of Ca2+ , Eu2+ , Sr2+ , and Ba2+ are 0.100 nm, 0.117 nm, 0.118 nm, and 0.135 nm for six coordination, respectively [10]. It can be seen from Table 1 that in the series of the M0.5 Zr2 (PO4 )3 compounds with M = Ca, Eu, Sr, and Ba, the a-parameter decreases and the c-parameter increases with an increase in the ionic radius of the M cation. These tendencies in the behavior of the unit-cell parameters have been interpreted
Fig. 2. X-ray powder diffraction pattern of the Eu0.5 Zr2 (PO4 )3 sample.
Sample
a (nm)
c (nm)
Ca0.5 Zr2 (PO4 )3 [8] Eu0.5 Zr2 (PO4 )3 (present study) Sr0.5 Zr2 (PO4 )3 [7] Ba0.5 Zr2 (PO4 )3 [9]
0.8781 0.8693 0.8693 0.8646
2.2675 2.3388 2.3389 2.3984
from the viewpoint of the structural modification [11]. In the NASICON-type M0.5 Zr2 (PO4 )3 , M2+ cations occupy the positions inside the columns between two faces of the neighboring ZrO6 -octahedra located along the c-axis, which is elongated by the introduction of a larger M2+ cation. In addition, the correlated rotation of zirconium octahedra and phosphorous tetrahedra connecting the parallel columns is also caused by the introduction of the M2+ cation, which in turn, decreases the a-parameter. The photoluminescent (PL) excitation (left) and emission (right) spectra of the Eu0.5 Zr2 (PO4 )3 phosphor are depicted in Fig. 3. This phosphor exhibits a broad band emission which is well-known characteristic of the Eu2+ blue emission, corresponding to the transition from 4f6 5d1 to 4f7 . In the structure of Eu0.5 Zr2 (PO4 )3 , there are two different divalent cation sites: M(1) and M(2) [3–5]. The former site is elongated antiprism formed by triangular faces of two ZrO6 octahedra, while the latter is a large 10-fold coordinated site created by the phosphozirconate framework. Since it is suggested that all Eu2+ ions occupy the one-half of the M(1) sites [4,5,12–15], only one Eu2+ emission center should be expected in the Eu0.5 Zr2 (PO4 )3 phosphor. However, the broad emission bands in the range between 420 nm and 640 nm can be separated into two Gaussians with maxima at about 457 nm and 490 nm, respectively. This result suggests that there are two M(1) sites [3a (0 0 0) and 3b (0 0 1/2)] in Eu0.5 Zr2 (PO4 )3 as reported for Mn0.5 Zr2 (PO4 )3 [15]. Although the crystal environments of the Eu2+ ions on the (3a) and the (3b) sites are very similar, the splitting of 5dexcitation level of the Eu2+ ion in the solid-state compounds strongly depends on the strength of the crystal field around the
Fig. 3. PL excitation (λem = 450 nm) and emission (λex = 254 nm) spectra of the Eu0.5 Zr2 (PO4 )3 phosphor; broken lines represent the Gaussian fit of the 460 nm emission band.
T. Masui et al. / Journal of Alloys and Compounds 418 (2006) 73–76
75
Fig. 5. Particle size distribution histograms of the Eu0.5 Zr2 (PO4 )3 (open bars) and the commercial BaMgAl10 O17 (closed bars) phosphors.
the optimization of the synthesis procedure such as the utilization of a flux and the modification of the particle morphologies. Furthermore, it is advantageous that the phosphor particle is as small as possible because this potentially results in higher packing density and better paste rheology leading to high resolution in displays and lower loading in lamps. In addition, the granular shape of the phosphor particles is available to minimize the quantity of binder as well as vehicles required in the paste used in the process of making back plates of several displays by screen-printing or ink-jet printing. Fig. 4. Scanning electron microscopic images of the Eu0.5 Zr2 (PO4 )3 (a) and the commercial BaMgAl10 O17 (b) phosphors.
Eu2+ ion. When the crystal environments are analogous, the Eu2+ center with a shorter Eu2+ –O2− distance will give a longer wavelength emission. Since the distance of M2+ –O2− on the (3a) site is longer than that on the (3b) site [15], we ascribed the 457 nm emission to Eu2+ ions on the (3a) sites, and the 490 nm emission to those on the (3b) sites, respectively. Similar phenomena have been reported in the BaMgSiO4 :Eu2+ phosphor [16]. SEM photographs and the particle size distribution histograms of the Eu0.5 Zr2 (PO4 )3 and the commercial BaMgAl10 O17 (BAM) phosphors are shown in Figs. 4 and 5, respectively. It is obvious that the particle size of the present Eu0.5 Zr2 (PO4 )3 is considerably smaller than that of the commercial one. The present Eu0.5 Zr2 (PO4 )3 particles distribute in the range of 0.3–1.7 m and the average size was 0.76 m, while the BaMgAl10 O17 particles distribute in the range of 1.0–5.7 m and the average size was 2.6 m. In addition, the particle morphology of the Eu0.5 Zr2 (PO4 )3 is granular and that of the BaMgAl10 O17 is plate-like. Although the emission intensity of the Eu0.5 Zr2 (PO4 )3 phosphor was 15.1% of the commercial BaMgAl10 O17 one, this material is expected to be applicable for some optical devices by
4. Conclusions A new blue emitting phosphor, Eu0.5 Zr2 (PO4 )3 , was synthesized for the first time by the solid-state reaction method. The sample developed in the present study has the stable NASICONtype structure in a single phase with a high crystallinity, and the photoluminescent emission spectrum shows a typical emission from 4f6 5d1 to 4f7 of Eu2+ . Although the emission intensity of this phosphor under UV excitation is not sufficient in the present stage, improvement of the luminescent property is expected by the optimization of the preparation process and the modification of the morphology. Acknowledgements The authors sincerely thank Dr. Ken-ichi Nakayama and Prof. Dr. Masaaki Yokoyama (Osaka University) for their assistance with the SEM data collection. This work was supported by the Industrial Technology Research Grant Program in 02 (Project No. 02A27004c) from the New Energy and Industrial Technology Development Organization (NEDO) based on funds provided by the Ministry of Economy, Trade and Industry, Japan (METI).
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T. Masui et al. / Journal of Alloys and Compounds 418 (2006) 73–76
References [1] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer Verlag, Berlin, 1994. [2] K. Hirao, S. Todoroki, D.H. Cho, N. Soga, Opt. Lett. 18 (1993) 1586–1587. [3] V.P. Gapontsev, S.M. Matitsin, A.A. Isineev, V.B. Kravchenko, Opt. Laser Technol. 14 (1982) 189–196. [4] L. Hagman, P. Kierkegaard, Acta Chem. Scand. 22 (1968) 1822– 1832. [5] J.B. Goodenough, H.Y.-P. Hong, J.A. Kafalas, Mater. Res. Bull. 11 (1976) 203–220. [6] S. Subramaniam, D.P. Stinton, O.B. Cavin, C.R. Hubbard, P.F. Becher, S.Y. Limaye, Powder Diffr. 9 (1994) 111–114.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Nat. Bur. Stand. (U.S.) Monogr. 25 (1984) 20–94. J. Alamo, J.L. Rodrigo, Solid State Ionics 63–65 (1993) 678–683. Nat. Bur. Stand. (U.S.) Monogr. 25 (1983) 20–17. R.D. Shannon, Acta Crystallogr. A32 (1976) 751–767. V.I. Pet’kov, V.S. Kurazhkovskaya, A.I. Orlova, M.L. Spiridonova, Crystallogr. Rep. 47 (2002) 736–743. A. Mouline, M. Alami, R. Brochu, R. Olazcuaga, C. Parent, G. Le Flem, J. Solid State Chem. 152 (2000) 453–459. M. Alami Talbi, R. Brochu, J. Solid State Chem. 110 (1994) 350–355. M. Alami, R. Brochu, C. Parent, L. Rabardel, C. Delmas, G. Le Flem, J. Alloys Compd. 118 (1992) 117–119. A. Mouline, M. Alami, R. Brochu, R. Olazcuaga, C. Parent, G. Le Flem, Mater. Res. Bull. 35 (2000) 899–908. M. Peng, Z. Pei, G. Hong, Q. Su, J. Mater. Chem. 13 (2003) 1202–1205.
Synthesis of a new NASICON-type blue luminescent material Toshiyuki Masui, Kazuhiko Koyabu, Shinji Tamura, Nobuhito Imanaka ∗ Department of Applied Chemistry, Faculty of Engineering, Osaka University and Handai Frontier Research Center, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Received 1 June 2005; received in revised form 31 August 2005; accepted 31 August 2005 Available online 18 January 2006
Abstract A new blue emitting phosphor, Eu0.5 Zr2 (PO4 )3 , was synthesized for the first time by the solid-state reaction method. The sample developed in the present study has a stable NASICON-type rhombohedral structure, which consists of a three-dimensional network formed by PO4 tetrahedra sharing corners with ZrO6 octahedra in a single phase with high crystallinity. The photoluminescent emission spectrum shows typical emission from 4f6 5d1 to 4f7 of Eu2+ , although the improvement of the luminescent property will be necessary by optimization of the preparation process and the modification of the particle morphology. © 2005 Elsevier B.V. All rights reserved. Keywords: NASICON-type structure; Phosphor; Blue emission; Divalent europium ion
1. Introduction There has been considerable interest in the development of advanced luminescent materials for applications such as flat panel displays, mercury-free lamps, and X-ray imaging systems. The properties of the luminescent materials arise from complex interactions among the host lattice, activators, defects, and interfaces, which are strongly dependent on the composition [1]. Among the activators in the luminescent materials, the optical properties of divalent europium ions present many interests because they are useful for visible and UV radiation sources and some of them being important for laser materials and optical communication devices [2,3]. In order to produce new luminescent materials, it is significant to select the crystal structure carefully. The structure of the NASICON (Na+ ion super ionic conductors)-type phosphates whose typical composition is expressed as NaZr2 (PO4 )3 consists of a three-dimensional network formed by PO4 tetrahedra sharing corners with ZrO6 octahedra [4,5]. This structure allows sodium cations to substitute with other cations leading to the creation of new luminescent materials with modulation along the a and c axes of the hexagonal cell. The crystal structure of the ∗
Corresponding author. Tel.: +81 6 6879 7352; fax: +81 6 6879 7354. E-mail address: [email protected] (N. Imanaka).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.08.097
MII 0.5 Zr2 (PO4 )3 phosphates is determined by the size of the MII ion. The phosphates with larger ions such as Eu2+ , Sr2+ , and Ba2+ crystallize in the NASICON-type (Fig. 1), whereas those with smaller ions exhibit the -Fe2 (SO4 )3 -type structure. In this study, a new luminescent material containing divalent europium ions in the NASICON-type structure, Eu0.5 Zr2 (PO4 )3 , was synthesized and the luminescent properties were characterized. 2. Experimental Eu0.5 Zr2 (PO4 )3 was prepared by a conventional solid-state reaction method. A stoichiometric amount of Eu2 O3 (99.9%), ZrO(NO3 )2 ·2H2 O (99.95%), and (NH4 )2 HPO4 (99.99%) was mixed in a mortar. A small amount (5 wt.%) of H2 BO3 was added as a flux and then the powder was mixed using a ball-milling apparatus (Fritsch P-7) for 6 h. The mixture was calcined at 1200 ◦ C for 5 h in a flow of 2%H2 –Ar gas. The phosphor was characterized with a scanning electron microscope (SEM, Hitachi S-4300SD) and with an X-ray powder diffractometer (XRD, Rigaku Multiflex) using Cu K␣ radiation. The lattice parameters of Eu0.5 Zr2 (PO4 )3 were obtained by the least squares method from observed d-value and h k l data corrected with those of ␣-Al2 O3 using the internal standard method. The mean particle size and the size distribution were estimated by measuring the diameters of 100 particles on the SEM photographs. The sample composition was confirmed by X-ray fluorescence analysis (Rigaku ZSX100e). The photoluminescent (PL) excitation and emission spectra of the Eu0.5 Zr2 (PO4 )3 phosphor were obtained at room temperature with a fluorescence spectrophotometer (Shimadzu RF-5300PC).
74
T. Masui et al. / Journal of Alloys and Compounds 418 (2006) 73–76 Table 1 Comparison of the lattice parameters of the NASICON-type phosphates
Fig. 1. Structure of MII 0.5 Zr2 (PO4 )3 phosphates (M = Sr, Eu, and Ba).
3. Results and discussion X-ray powder diffraction (XRD) pattern for Eu0.5 Zr2 (PO4 )3 is shown in Fig. 2. The XRD pattern of the sample is in good agreement with a single phase of the NASICON-type structure [6–9], and there is no diffraction peak corresponding to any impurities in the XRD patterns. The cell parameters of the Eu0.5 Zr2 (PO4 )3 phosphor data are a = 0.8693 nm and c = 2.3388 nm, and the indexation of all reflections is consistent with the R3 space group. The comparison of these parameters with those of Ca0.5 Zr2 (PO4 )3 [8], Sr0.5 Zr2 (PO4 )3 [7], and Ba0.5 Zr2 (PO4 )3 [9] are summarized in Table 1. The ionic radii of Ca2+ , Eu2+ , Sr2+ , and Ba2+ are 0.100 nm, 0.117 nm, 0.118 nm, and 0.135 nm for six coordination, respectively [10]. It can be seen from Table 1 that in the series of the M0.5 Zr2 (PO4 )3 compounds with M = Ca, Eu, Sr, and Ba, the a-parameter decreases and the c-parameter increases with an increase in the ionic radius of the M cation. These tendencies in the behavior of the unit-cell parameters have been interpreted
Fig. 2. X-ray powder diffraction pattern of the Eu0.5 Zr2 (PO4 )3 sample.
Sample
a (nm)
c (nm)
Ca0.5 Zr2 (PO4 )3 [8] Eu0.5 Zr2 (PO4 )3 (present study) Sr0.5 Zr2 (PO4 )3 [7] Ba0.5 Zr2 (PO4 )3 [9]
0.8781 0.8693 0.8693 0.8646
2.2675 2.3388 2.3389 2.3984
from the viewpoint of the structural modification [11]. In the NASICON-type M0.5 Zr2 (PO4 )3 , M2+ cations occupy the positions inside the columns between two faces of the neighboring ZrO6 -octahedra located along the c-axis, which is elongated by the introduction of a larger M2+ cation. In addition, the correlated rotation of zirconium octahedra and phosphorous tetrahedra connecting the parallel columns is also caused by the introduction of the M2+ cation, which in turn, decreases the a-parameter. The photoluminescent (PL) excitation (left) and emission (right) spectra of the Eu0.5 Zr2 (PO4 )3 phosphor are depicted in Fig. 3. This phosphor exhibits a broad band emission which is well-known characteristic of the Eu2+ blue emission, corresponding to the transition from 4f6 5d1 to 4f7 . In the structure of Eu0.5 Zr2 (PO4 )3 , there are two different divalent cation sites: M(1) and M(2) [3–5]. The former site is elongated antiprism formed by triangular faces of two ZrO6 octahedra, while the latter is a large 10-fold coordinated site created by the phosphozirconate framework. Since it is suggested that all Eu2+ ions occupy the one-half of the M(1) sites [4,5,12–15], only one Eu2+ emission center should be expected in the Eu0.5 Zr2 (PO4 )3 phosphor. However, the broad emission bands in the range between 420 nm and 640 nm can be separated into two Gaussians with maxima at about 457 nm and 490 nm, respectively. This result suggests that there are two M(1) sites [3a (0 0 0) and 3b (0 0 1/2)] in Eu0.5 Zr2 (PO4 )3 as reported for Mn0.5 Zr2 (PO4 )3 [15]. Although the crystal environments of the Eu2+ ions on the (3a) and the (3b) sites are very similar, the splitting of 5dexcitation level of the Eu2+ ion in the solid-state compounds strongly depends on the strength of the crystal field around the
Fig. 3. PL excitation (λem = 450 nm) and emission (λex = 254 nm) spectra of the Eu0.5 Zr2 (PO4 )3 phosphor; broken lines represent the Gaussian fit of the 460 nm emission band.
T. Masui et al. / Journal of Alloys and Compounds 418 (2006) 73–76
75
Fig. 5. Particle size distribution histograms of the Eu0.5 Zr2 (PO4 )3 (open bars) and the commercial BaMgAl10 O17 (closed bars) phosphors.
the optimization of the synthesis procedure such as the utilization of a flux and the modification of the particle morphologies. Furthermore, it is advantageous that the phosphor particle is as small as possible because this potentially results in higher packing density and better paste rheology leading to high resolution in displays and lower loading in lamps. In addition, the granular shape of the phosphor particles is available to minimize the quantity of binder as well as vehicles required in the paste used in the process of making back plates of several displays by screen-printing or ink-jet printing. Fig. 4. Scanning electron microscopic images of the Eu0.5 Zr2 (PO4 )3 (a) and the commercial BaMgAl10 O17 (b) phosphors.
Eu2+ ion. When the crystal environments are analogous, the Eu2+ center with a shorter Eu2+ –O2− distance will give a longer wavelength emission. Since the distance of M2+ –O2− on the (3a) site is longer than that on the (3b) site [15], we ascribed the 457 nm emission to Eu2+ ions on the (3a) sites, and the 490 nm emission to those on the (3b) sites, respectively. Similar phenomena have been reported in the BaMgSiO4 :Eu2+ phosphor [16]. SEM photographs and the particle size distribution histograms of the Eu0.5 Zr2 (PO4 )3 and the commercial BaMgAl10 O17 (BAM) phosphors are shown in Figs. 4 and 5, respectively. It is obvious that the particle size of the present Eu0.5 Zr2 (PO4 )3 is considerably smaller than that of the commercial one. The present Eu0.5 Zr2 (PO4 )3 particles distribute in the range of 0.3–1.7 m and the average size was 0.76 m, while the BaMgAl10 O17 particles distribute in the range of 1.0–5.7 m and the average size was 2.6 m. In addition, the particle morphology of the Eu0.5 Zr2 (PO4 )3 is granular and that of the BaMgAl10 O17 is plate-like. Although the emission intensity of the Eu0.5 Zr2 (PO4 )3 phosphor was 15.1% of the commercial BaMgAl10 O17 one, this material is expected to be applicable for some optical devices by
4. Conclusions A new blue emitting phosphor, Eu0.5 Zr2 (PO4 )3 , was synthesized for the first time by the solid-state reaction method. The sample developed in the present study has the stable NASICONtype structure in a single phase with a high crystallinity, and the photoluminescent emission spectrum shows a typical emission from 4f6 5d1 to 4f7 of Eu2+ . Although the emission intensity of this phosphor under UV excitation is not sufficient in the present stage, improvement of the luminescent property is expected by the optimization of the preparation process and the modification of the morphology. Acknowledgements The authors sincerely thank Dr. Ken-ichi Nakayama and Prof. Dr. Masaaki Yokoyama (Osaka University) for their assistance with the SEM data collection. This work was supported by the Industrial Technology Research Grant Program in 02 (Project No. 02A27004c) from the New Energy and Industrial Technology Development Organization (NEDO) based on funds provided by the Ministry of Economy, Trade and Industry, Japan (METI).
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T. Masui et al. / Journal of Alloys and Compounds 418 (2006) 73–76
References [1] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer Verlag, Berlin, 1994. [2] K. Hirao, S. Todoroki, D.H. Cho, N. Soga, Opt. Lett. 18 (1993) 1586–1587. [3] V.P. Gapontsev, S.M. Matitsin, A.A. Isineev, V.B. Kravchenko, Opt. Laser Technol. 14 (1982) 189–196. [4] L. Hagman, P. Kierkegaard, Acta Chem. Scand. 22 (1968) 1822– 1832. [5] J.B. Goodenough, H.Y.-P. Hong, J.A. Kafalas, Mater. Res. Bull. 11 (1976) 203–220. [6] S. Subramaniam, D.P. Stinton, O.B. Cavin, C.R. Hubbard, P.F. Becher, S.Y. Limaye, Powder Diffr. 9 (1994) 111–114.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Nat. Bur. Stand. (U.S.) Monogr. 25 (1984) 20–94. J. Alamo, J.L. Rodrigo, Solid State Ionics 63–65 (1993) 678–683. Nat. Bur. Stand. (U.S.) Monogr. 25 (1983) 20–17. R.D. Shannon, Acta Crystallogr. A32 (1976) 751–767. V.I. Pet’kov, V.S. Kurazhkovskaya, A.I. Orlova, M.L. Spiridonova, Crystallogr. Rep. 47 (2002) 736–743. A. Mouline, M. Alami, R. Brochu, R. Olazcuaga, C. Parent, G. Le Flem, J. Solid State Chem. 152 (2000) 453–459. M. Alami Talbi, R. Brochu, J. Solid State Chem. 110 (1994) 350–355. M. Alami, R. Brochu, C. Parent, L. Rabardel, C. Delmas, G. Le Flem, J. Alloys Compd. 118 (1992) 117–119. A. Mouline, M. Alami, R. Brochu, R. Olazcuaga, C. Parent, G. Le Flem, Mater. Res. Bull. 35 (2000) 899–908. M. Peng, Z. Pei, G. Hong, Q. Su, J. Mater. Chem. 13 (2003) 1202–1205.