- Email: [email protected]
Bridgman single crystal growth of Ce-doped (Lu1−xYx)AlO3
Journal of Crystal Growth 198/199 (1999) 492—496
Bridgman single crystal growth of Ce-doped (Lu Y )AlO \V V A.G. Petrosyan *, G.O. Shirinyan , K.L. Ovanesyan , C. Pedrini, C. Dujardin Institute for Physical Research, Armenian National Academy of Sciences, 378410 Ashtarak-2, Armenia LPCML, Unite Mixte de Recherche CRNS 5620, Universite´ Claude Bernard Lyon 1, 69622 Villeurbanne, France
Abstract Ce-doped single crystals of (Lu Y )AlO solid solutions (x"0.15—0.80) of large size and optical quality were grown \V V using the vertical Bridgman process. Thermal stability of solid solution single crystals increases compared to that of LuAlO . 1999 Elsevier Science B.V. All rights reserved. PACS: 81.10.Fq; 81.10.!h Keywords: (LuY)AlO ; Ce; Scintillator; Single crystal; Perovskite; Bridgman; Thermal stability
1. Introduction Dense Ce-activated oxide single crystals possessing a perovskite-type structure are of increasing importance for development of improved scintillators for high energy physics, medical imaging and applications requiring short decay and a good combination of the stopping power and the light yield. YAlO —Ce> (YAP) and LuAlO —Ce> (LuAP) are well-known perovskite-type scintillator materials with efficient dPf emission of Ce> ions [1—3]. Both materials however, have several deficiencies. YAP has a low density which strongly limits its wide application. Both YAP and LuAP suffer from broad band UV absorption overlapping the Ce> emission and leading to a strong (especially in
* Corresponding author. E-mail: [email protected].
LuAP) thickness dependence of the light yield. The light yield of the presently available LuAP is essentially lower than that of YAP. Finally, LuAP is unstable above 1200°C so that heat treatments towards understanding the parasitic absorption or performance improvement may damage the specimens. Relevant crystallographic and physical properties of YAlO and LuAlO are listed in Table 1. YAP has a well-developed technology and is commonly grown by the Czochralski or horizontal Bridgman techniques [4]. LuAP is developed only recently and presently is grown by Czochralski [2] and vertical Bridgman [5] techniques. In the present work Ce-doped single crystals of (Lu Y )AlO (LuYAP) were grown using the \V V vertical Bridgman technique. Solid solution compositions with x(0.5 still possess a high density (*6.8 g/cm). Depending on x, they provide also for variation of the lattice unit cell parameters and,
0022-0248/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 0 9 2 - 6
A.G. Petrosyan et al. / Journal of Crystal Growth 198/199 (1999) 492–496 Table 1 Crystallographic and physical properties of YAlO and LuAlO single crystals from Refs. [1—9]
Space group Unit cell parameters (A> )
Density (g/cm) Melting point (°C) Ce-partition coefficient Thermal stability Tendency for twinning Effective Z number Attenuation length (cm) Light yield (% of BGO) Band gap (eV)
YAlO
LuAlO
D a"5.176 b"5.332 c"7.356 5.35 1916 0.6 High High 33.5 2.63 300—400 8
D a"5.100 b"5.330 c"7.294 8.34 1900 0.17 Low Low 64.94 1.1 100—260 8.5
493
Table 2 Typical crystal growth parameters Crucible metal Crystal diameter (mm) Maximum crystal length (mm) Growth rate (mm/h) Atmosphere Growth direction Cooling period (h)
Mo 10—14 50 1.5—4 99.99%Ar#H Spontaneous 10—20
hence, the crystal field at Ce-sites, variation of the bandgap and that in the structure of the valence band — factors which may influence the excitation transfer efficiency. The solid solutions provide for a higher distribution coefficient of Ce ions compared to that in LuAP, so that high Ce-ion concentrations can be more readily incorporated in the crystals. Only first steps have been done in studies of LuYAP. The available published information on scintillation properties of LuYAP in the form of powders (x"0—1) and crystals (x"0.5) can be found in Refs. [6,7]. Luminescence and scintillation properties of single crystals grown in this work will be presented elsewhere.
Direct melt freezing technique employing small crucibles made of molybdenum was used to prepare polycrystalline samples of various composition (x"0; 0.25; 0.5; 0.75; 1) under conditions [10] providing for the formation of single phase material. Rapid solidification occurring at temperatures far below the melting point assumes that there is no time for rearrangement of atoms or segregation, so that the solid phase composition equals that of the melt composition. X-ray powder diffraction technique was used to check the crystal structure, to measure the lattice unit cell parameters (*a"0.005 A> , *b"0.005 A> , *c"0.002 A> ) and the unit cell volume (»). The Ce concentration measurements in single crystals were carried out by “Service central d’analyse” (Vernaison, France) with a precision of 2%. Polished single crystal sections 10 mm thick were used in annealing studies (1100—1700°C, hydrogen atmosphere at a pressure 3;10 Pa) to characterize the thermal stability of solid solutions with respect to that of the end members.
2. Experimental procedures
3. Results and discussion
Single crystals of LuYAP were grown using the vertical Bridgman process [5]. The starting oxide components were 4N to 5N pure Lu O , Y O and CeO powders and vacuum zone re-melted Ver neuil Al O . The fraction of x in the melts was 0.15—0.80. The Ce-dopant concentration was 0.3 at% with respect to the rare-earth sites. The major growth conditions are summarized in Table 2. In some cases a passive afterheater was used to decrease the temperature gradients and to prevent crack formation at the crystal cooling stage.
3.1. Crystal growth Single crystals of LuYAP grown under conditions given in Table 2 were colorless and transparent measuring up to 50 mm. Under substantially identical growth conditions, the tendency for twinning and crack formation was increasing with increasing the x. This behavior may be associated with the decreasing difference between a and b unit cell parameters in Y-rich compositions obviously leading to lower stresses in the twinning direction
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A.G. Petrosyan et al. / Journal of Crystal Growth 198/199 (1999) 492–496
required to cause twinning. Some twin boundaries and cracks were often present in Y-rich compositions unless lower growth rates and the passive afterheater were introduced. There is a 5% mismatch between ionic radii of Y> and Lu> [11] that may result in compositional variation along the length of the solid solution crystals. The segregation coefficient of Lu> in Czochralski-grown YAlO -Lu>(2%) is 0.8 [12] so that end to end variation in composition with respect to Lu/Y ratio may be considerable. Fig. 1 shows the relationship between the unit cell volume (») of (Lu Y )AlO solid solutions, prepared by \V V direct melt freezing in crucibles, and the concentration (x). The unit cell volume increases in proportion to yttrium concentration establishing validity of the volume additivity law. The chemical composition (x) of single crystal pieces cut from the grown boules can, therefore, be estimated from measurements of the unit cell volume. The distribution of Ce> ions along the single crystal height is non-uniform too, since the partition coefficient differs from unity. It was reported [13] that the partition coefficients of rare-earth
Fig. 1. Relationship between the unit cell volume (») and the fraction (x) in (Lu Y )AlO solid solutions. \V V
ions in orthoaluminate hosts differ from unity in proportion to the mismatch in ionic radii between the host and dopant ions. Hence, the distribution coefficient of Ce> in LuYAP solid solutions may vary in between those in YAP (0.6) and LuAP (0.17) in proportion to (x). Top to tail measurements of the Ce content in a LuYAP (x"0.45) single crystal and evaluation of the distribution coefficient (k"0.3$0.03) from the normal freeze curve have confirmed applicability of the relationship [13] to LuYAP solid solutions. At Ce concentrations of 0.3 at%, high optical quality of the crystal was maintained up to the growth rate of 4 mm/h. It is therefore clear that high Ce concentrations can be introduced in LuYAP more easily, as compared to LuAP [5]. 3.2. Thermal stability Fig. 2 shows time-annealing temperature of perovskite-to-garnet transformation in two LuYAP
Fig. 2. Time-annealing temperature dependence of perovskite to garnet phase transformation at different temperatures under reducing atmosphere in: LuAlO —Ce (*), Lu Y AlO —Ce (䉭) and Lu Y AlO —Ce (䊐). * 䉭 and 䊐 — no change of the surface state; and — greenish coloration of the surface; 䢇 䉱 䊏 — greenish coloration plus milky-ceramic surface; — clear degradation of the surface 0.5 mm thick.
A.G. Petrosyan et al. / Journal of Crystal Growth 198/199 (1999) 492–496
single crystals of different compositions (x"0.15 and x"0.7) under reducing atmosphere. For comparison, results on thermal behavior of LuAP for short duration heat treatments are also included. It is clearly seen that in terms of temperatures and annealing durations, the solid solutions are essentially more stable compared to LuAP. Similar to LuAP [13], the first signs of the perovskite phase decomposition appear as a greenish coloration of the specimen, which may be due to Ce> ions in the garnet phase separated in a thin surface layer. The green surface coloration was found to be characteristic also to YAP single crystals subjected to reducing heat treatments above 1200°C. Physical properties of solutions generally vary in between those of the end-members, except for thermal conductivity or some specific material characteristics. Thermal stability of the rare-earth orthoaluminates decreases upon decreasing the ionic radii of the rare-earth element. This is commonly referred to as the preference of smaller rareearth ions for 8-fold coordination, rather than 12-fold which occur in the perovskite-type structure. The end members LuAlO , YbAlO and TmAlO are unstable above 1200°C, while the neighboring ErAlO is stable up to the melting point [9]. However, the observed increase in thermal stability of LuYAP solid solutions, with respect to that of LuAP, cannot be explained in terms of the average ionic radii. Since the size variation with the coordination number is the same for all ions in the first approximation, the available values for 8-fold coordination [11] can be used giving the average rare-earth ionic radius in LuYAP (x"0.15) equal to 0.977 A> , which is only in between of those of Lu> (0.97 A> ) and Yb> (0.98 A> ).
4. Summary Single crystals of Ce-doped (Lu Y )AlO solid \V V solutions were grown by the vertical Bridgman technique. Growth conditions were optimized to provide twin and crack-free optical quality single crystals with composition in the range between x"0.15 and 0.8. The actual chemical composition of crystals with respect to Lu/Y ratio can be deter-
495
mined from measurements of the unit cell volume. Due to increase of the Ce distribution coefficient in solid solutions possessing larger unit cell volumes, as compared to that of LuAlO , high dopant con centrations can be introduced more readily. Thermal stability of solid solutions drastically increases with respect to that of LuAlO providing for possi bilities of material post-growth heat treatments at high temperatures.
Acknowledgements This work was partially supported by the Ministry of Science of Armenia (grant C96-763), NATO grant HTECH.LG 971230 and “Crystal Clear” collaboration. The travel grant awarded by the “Open Society Institute Assistance Foundation — Armenia” to attend ICCG-12 is acknowledged.
References [1] W.W. Moses, S.E. Derenzo, A. Fedorov, M. Korzik, B. Minkov, V. Aslanov, IEEE Trans. Nucl. Sci. 42 (1995) 275. [2] A. Lempicki, M.H. Randles, D. Wisniewski, M. Barcerzyk, C. Brecher, A.J. Wojtowicz, IEEE Trans. Nucl. Sci. 42 (1995) 280. [3] C. Dujardin, C. Pedrini, D. Bouttet, J.W.M. Verweij, A.G. Petrosyan, A.N. Belsky, A.N. Vasil’ev, E. Zinin, P. Martin, in: P.Dorenbos, C.W.E. van Eijk (Eds.), Proc. Int. Conf. on Inorganic Scintillators and their Applications, SCINT’95, Delft University Press, Delft, 1996, p. 336. [4] V.G. Baryshevsky, M.V. Korzik, B. Minkov, S.A. Smirnova, A.A. Fedorov, P. Dorenbos, C.W.E. van Eijk, J. Phys.: Condens. Matter 5 (1993) 7893. [5] A.G. Petrosyan, G.O. Shirinyan, C. Pedrini, C. Dujardin, K.L. Ovanesyan, R.G. Manucharyan, T.I. Butaeva, M.V. Derzyan, Crystal Res. Technol. 33 (1998) 239. [6] W.P. Trower, M.V. Korzik, A.A. Fedorov, S.A. Smirnova, V.A. Aslanov, in: P. Dorenbos, C.W.E. van Eijk (Eds.), Proc. Int. Conf. on Inorganic Scintillators and their Applications, SCINT’95, Delft University Press, Delft, 1996, p. 355. [7] C. Dujardin, C. Pedrini, J.C. Gacont, A.G. Petrosyan, A.N. Belsky, A.N. Vasil’ev, J. Phys.: Condens. Matter 9 (1997) 5229. [8] A. Lempicki, C. Brecher, D. Wisniewski, E.Zych, in: P. Dorenbos, C.W.E. van Eijk (Eds.), Proc. Int. Conf. on Inorganic Scintillators and their Applications, SCINT95, Delft University Press, Delft, 1996, p. 340.
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[9] P.A. Arsen’ev, L.M. Kovba, Kh.S. Bagdasarov, Soedineniya Redkozemel’nykh Elementov. Sistemy s Oksidami Elementov I—III Grup, Nauka, Moscow, 1983, ch. 5, p. 160. [10] A.G. Petrosyan, J. Crystal Growth 139 (1994) 372. [11] R.D. Shannon, C.T. Prewit, Acta Crystallogr. B 25 (1969) 925.
[12] Li Gansheng, Guo Yibin, Lu Jian, Shi Zhenyzhu, Wu Jinhue, Chen Ying, Chen Jinfeng, J. Crystal Growth 118 (1992) 371. [13] A.G. Petrosyan, C. Pedrini, in: P. Dorenbos, C.W.E. van Eijk (Eds.), Proc. Int. Conf. on Inorganic Scintillators and their Applications, SCINT’95, Delft University Press, Delft, 1996, p. 498.
Bridgman single crystal growth of Ce-doped (Lu Y )AlO \V V A.G. Petrosyan *, G.O. Shirinyan , K.L. Ovanesyan , C. Pedrini, C. Dujardin Institute for Physical Research, Armenian National Academy of Sciences, 378410 Ashtarak-2, Armenia LPCML, Unite Mixte de Recherche CRNS 5620, Universite´ Claude Bernard Lyon 1, 69622 Villeurbanne, France
Abstract Ce-doped single crystals of (Lu Y )AlO solid solutions (x"0.15—0.80) of large size and optical quality were grown \V V using the vertical Bridgman process. Thermal stability of solid solution single crystals increases compared to that of LuAlO . 1999 Elsevier Science B.V. All rights reserved. PACS: 81.10.Fq; 81.10.!h Keywords: (LuY)AlO ; Ce; Scintillator; Single crystal; Perovskite; Bridgman; Thermal stability
1. Introduction Dense Ce-activated oxide single crystals possessing a perovskite-type structure are of increasing importance for development of improved scintillators for high energy physics, medical imaging and applications requiring short decay and a good combination of the stopping power and the light yield. YAlO —Ce> (YAP) and LuAlO —Ce> (LuAP) are well-known perovskite-type scintillator materials with efficient dPf emission of Ce> ions [1—3]. Both materials however, have several deficiencies. YAP has a low density which strongly limits its wide application. Both YAP and LuAP suffer from broad band UV absorption overlapping the Ce> emission and leading to a strong (especially in
* Corresponding author. E-mail: [email protected].
LuAP) thickness dependence of the light yield. The light yield of the presently available LuAP is essentially lower than that of YAP. Finally, LuAP is unstable above 1200°C so that heat treatments towards understanding the parasitic absorption or performance improvement may damage the specimens. Relevant crystallographic and physical properties of YAlO and LuAlO are listed in Table 1. YAP has a well-developed technology and is commonly grown by the Czochralski or horizontal Bridgman techniques [4]. LuAP is developed only recently and presently is grown by Czochralski [2] and vertical Bridgman [5] techniques. In the present work Ce-doped single crystals of (Lu Y )AlO (LuYAP) were grown using the \V V vertical Bridgman technique. Solid solution compositions with x(0.5 still possess a high density (*6.8 g/cm). Depending on x, they provide also for variation of the lattice unit cell parameters and,
0022-0248/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 0 9 2 - 6
A.G. Petrosyan et al. / Journal of Crystal Growth 198/199 (1999) 492–496 Table 1 Crystallographic and physical properties of YAlO and LuAlO single crystals from Refs. [1—9]
Space group Unit cell parameters (A> )
Density (g/cm) Melting point (°C) Ce-partition coefficient Thermal stability Tendency for twinning Effective Z number Attenuation length (cm) Light yield (% of BGO) Band gap (eV)
YAlO
LuAlO
D a"5.176 b"5.332 c"7.356 5.35 1916 0.6 High High 33.5 2.63 300—400 8
D a"5.100 b"5.330 c"7.294 8.34 1900 0.17 Low Low 64.94 1.1 100—260 8.5
493
Table 2 Typical crystal growth parameters Crucible metal Crystal diameter (mm) Maximum crystal length (mm) Growth rate (mm/h) Atmosphere Growth direction Cooling period (h)
Mo 10—14 50 1.5—4 99.99%Ar#H Spontaneous 10—20
hence, the crystal field at Ce-sites, variation of the bandgap and that in the structure of the valence band — factors which may influence the excitation transfer efficiency. The solid solutions provide for a higher distribution coefficient of Ce ions compared to that in LuAP, so that high Ce-ion concentrations can be more readily incorporated in the crystals. Only first steps have been done in studies of LuYAP. The available published information on scintillation properties of LuYAP in the form of powders (x"0—1) and crystals (x"0.5) can be found in Refs. [6,7]. Luminescence and scintillation properties of single crystals grown in this work will be presented elsewhere.
Direct melt freezing technique employing small crucibles made of molybdenum was used to prepare polycrystalline samples of various composition (x"0; 0.25; 0.5; 0.75; 1) under conditions [10] providing for the formation of single phase material. Rapid solidification occurring at temperatures far below the melting point assumes that there is no time for rearrangement of atoms or segregation, so that the solid phase composition equals that of the melt composition. X-ray powder diffraction technique was used to check the crystal structure, to measure the lattice unit cell parameters (*a"0.005 A> , *b"0.005 A> , *c"0.002 A> ) and the unit cell volume (»). The Ce concentration measurements in single crystals were carried out by “Service central d’analyse” (Vernaison, France) with a precision of 2%. Polished single crystal sections 10 mm thick were used in annealing studies (1100—1700°C, hydrogen atmosphere at a pressure 3;10 Pa) to characterize the thermal stability of solid solutions with respect to that of the end members.
2. Experimental procedures
3. Results and discussion
Single crystals of LuYAP were grown using the vertical Bridgman process [5]. The starting oxide components were 4N to 5N pure Lu O , Y O and CeO powders and vacuum zone re-melted Ver neuil Al O . The fraction of x in the melts was 0.15—0.80. The Ce-dopant concentration was 0.3 at% with respect to the rare-earth sites. The major growth conditions are summarized in Table 2. In some cases a passive afterheater was used to decrease the temperature gradients and to prevent crack formation at the crystal cooling stage.
3.1. Crystal growth Single crystals of LuYAP grown under conditions given in Table 2 were colorless and transparent measuring up to 50 mm. Under substantially identical growth conditions, the tendency for twinning and crack formation was increasing with increasing the x. This behavior may be associated with the decreasing difference between a and b unit cell parameters in Y-rich compositions obviously leading to lower stresses in the twinning direction
494
A.G. Petrosyan et al. / Journal of Crystal Growth 198/199 (1999) 492–496
required to cause twinning. Some twin boundaries and cracks were often present in Y-rich compositions unless lower growth rates and the passive afterheater were introduced. There is a 5% mismatch between ionic radii of Y> and Lu> [11] that may result in compositional variation along the length of the solid solution crystals. The segregation coefficient of Lu> in Czochralski-grown YAlO -Lu>(2%) is 0.8 [12] so that end to end variation in composition with respect to Lu/Y ratio may be considerable. Fig. 1 shows the relationship between the unit cell volume (») of (Lu Y )AlO solid solutions, prepared by \V V direct melt freezing in crucibles, and the concentration (x). The unit cell volume increases in proportion to yttrium concentration establishing validity of the volume additivity law. The chemical composition (x) of single crystal pieces cut from the grown boules can, therefore, be estimated from measurements of the unit cell volume. The distribution of Ce> ions along the single crystal height is non-uniform too, since the partition coefficient differs from unity. It was reported [13] that the partition coefficients of rare-earth
Fig. 1. Relationship between the unit cell volume (») and the fraction (x) in (Lu Y )AlO solid solutions. \V V
ions in orthoaluminate hosts differ from unity in proportion to the mismatch in ionic radii between the host and dopant ions. Hence, the distribution coefficient of Ce> in LuYAP solid solutions may vary in between those in YAP (0.6) and LuAP (0.17) in proportion to (x). Top to tail measurements of the Ce content in a LuYAP (x"0.45) single crystal and evaluation of the distribution coefficient (k"0.3$0.03) from the normal freeze curve have confirmed applicability of the relationship [13] to LuYAP solid solutions. At Ce concentrations of 0.3 at%, high optical quality of the crystal was maintained up to the growth rate of 4 mm/h. It is therefore clear that high Ce concentrations can be introduced in LuYAP more easily, as compared to LuAP [5]. 3.2. Thermal stability Fig. 2 shows time-annealing temperature of perovskite-to-garnet transformation in two LuYAP
Fig. 2. Time-annealing temperature dependence of perovskite to garnet phase transformation at different temperatures under reducing atmosphere in: LuAlO —Ce (*), Lu Y AlO —Ce (䉭) and Lu Y AlO —Ce (䊐). * 䉭 and 䊐 — no change of the surface state; and — greenish coloration of the surface; 䢇 䉱 䊏 — greenish coloration plus milky-ceramic surface; — clear degradation of the surface 0.5 mm thick.
A.G. Petrosyan et al. / Journal of Crystal Growth 198/199 (1999) 492–496
single crystals of different compositions (x"0.15 and x"0.7) under reducing atmosphere. For comparison, results on thermal behavior of LuAP for short duration heat treatments are also included. It is clearly seen that in terms of temperatures and annealing durations, the solid solutions are essentially more stable compared to LuAP. Similar to LuAP [13], the first signs of the perovskite phase decomposition appear as a greenish coloration of the specimen, which may be due to Ce> ions in the garnet phase separated in a thin surface layer. The green surface coloration was found to be characteristic also to YAP single crystals subjected to reducing heat treatments above 1200°C. Physical properties of solutions generally vary in between those of the end-members, except for thermal conductivity or some specific material characteristics. Thermal stability of the rare-earth orthoaluminates decreases upon decreasing the ionic radii of the rare-earth element. This is commonly referred to as the preference of smaller rareearth ions for 8-fold coordination, rather than 12-fold which occur in the perovskite-type structure. The end members LuAlO , YbAlO and TmAlO are unstable above 1200°C, while the neighboring ErAlO is stable up to the melting point [9]. However, the observed increase in thermal stability of LuYAP solid solutions, with respect to that of LuAP, cannot be explained in terms of the average ionic radii. Since the size variation with the coordination number is the same for all ions in the first approximation, the available values for 8-fold coordination [11] can be used giving the average rare-earth ionic radius in LuYAP (x"0.15) equal to 0.977 A> , which is only in between of those of Lu> (0.97 A> ) and Yb> (0.98 A> ).
4. Summary Single crystals of Ce-doped (Lu Y )AlO solid \V V solutions were grown by the vertical Bridgman technique. Growth conditions were optimized to provide twin and crack-free optical quality single crystals with composition in the range between x"0.15 and 0.8. The actual chemical composition of crystals with respect to Lu/Y ratio can be deter-
495
mined from measurements of the unit cell volume. Due to increase of the Ce distribution coefficient in solid solutions possessing larger unit cell volumes, as compared to that of LuAlO , high dopant con centrations can be introduced more readily. Thermal stability of solid solutions drastically increases with respect to that of LuAlO providing for possi bilities of material post-growth heat treatments at high temperatures.
Acknowledgements This work was partially supported by the Ministry of Science of Armenia (grant C96-763), NATO grant HTECH.LG 971230 and “Crystal Clear” collaboration. The travel grant awarded by the “Open Society Institute Assistance Foundation — Armenia” to attend ICCG-12 is acknowledged.
References [1] W.W. Moses, S.E. Derenzo, A. Fedorov, M. Korzik, B. Minkov, V. Aslanov, IEEE Trans. Nucl. Sci. 42 (1995) 275. [2] A. Lempicki, M.H. Randles, D. Wisniewski, M. Barcerzyk, C. Brecher, A.J. Wojtowicz, IEEE Trans. Nucl. Sci. 42 (1995) 280. [3] C. Dujardin, C. Pedrini, D. Bouttet, J.W.M. Verweij, A.G. Petrosyan, A.N. Belsky, A.N. Vasil’ev, E. Zinin, P. Martin, in: P.Dorenbos, C.W.E. van Eijk (Eds.), Proc. Int. Conf. on Inorganic Scintillators and their Applications, SCINT’95, Delft University Press, Delft, 1996, p. 336. [4] V.G. Baryshevsky, M.V. Korzik, B. Minkov, S.A. Smirnova, A.A. Fedorov, P. Dorenbos, C.W.E. van Eijk, J. Phys.: Condens. Matter 5 (1993) 7893. [5] A.G. Petrosyan, G.O. Shirinyan, C. Pedrini, C. Dujardin, K.L. Ovanesyan, R.G. Manucharyan, T.I. Butaeva, M.V. Derzyan, Crystal Res. Technol. 33 (1998) 239. [6] W.P. Trower, M.V. Korzik, A.A. Fedorov, S.A. Smirnova, V.A. Aslanov, in: P. Dorenbos, C.W.E. van Eijk (Eds.), Proc. Int. Conf. on Inorganic Scintillators and their Applications, SCINT’95, Delft University Press, Delft, 1996, p. 355. [7] C. Dujardin, C. Pedrini, J.C. Gacont, A.G. Petrosyan, A.N. Belsky, A.N. Vasil’ev, J. Phys.: Condens. Matter 9 (1997) 5229. [8] A. Lempicki, C. Brecher, D. Wisniewski, E.Zych, in: P. Dorenbos, C.W.E. van Eijk (Eds.), Proc. Int. Conf. on Inorganic Scintillators and their Applications, SCINT95, Delft University Press, Delft, 1996, p. 340.
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[9] P.A. Arsen’ev, L.M. Kovba, Kh.S. Bagdasarov, Soedineniya Redkozemel’nykh Elementov. Sistemy s Oksidami Elementov I—III Grup, Nauka, Moscow, 1983, ch. 5, p. 160. [10] A.G. Petrosyan, J. Crystal Growth 139 (1994) 372. [11] R.D. Shannon, C.T. Prewit, Acta Crystallogr. B 25 (1969) 925.
[12] Li Gansheng, Guo Yibin, Lu Jian, Shi Zhenyzhu, Wu Jinhue, Chen Ying, Chen Jinfeng, J. Crystal Growth 118 (1992) 371. [13] A.G. Petrosyan, C. Pedrini, in: P. Dorenbos, C.W.E. van Eijk (Eds.), Proc. Int. Conf. on Inorganic Scintillators and their Applications, SCINT’95, Delft University Press, Delft, 1996, p. 498.