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Growth of large PbWO4 single crystals by Czochralski method
Journal of Crystal Growth 226 (2001) 79–82
Growth of large PbWO4 single crystals by Czochralski method C. Yanga,*, Y. Guob, P. Shia, G. Chenc b
a Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, People’s Republic of China Department of Chemistry Engineering, Harbin Engineering University, Harbin 150001, People’s Republic of China c Beijing Glass Institute, Beijing 100001, People’s Republic of China
Received 25 November 2000; accepted 3 February 2001 Communicated by M. Schieber
Abstract Large size PbWO4 crystal was grown by Czochralski method. The dopant Fe3+ in the raw material and the annealing technology affected the transmittance at 420 nm. The transmittance at 420 nm obviously decreased when the Fe3+ content was higher than 10 ppm. The vacuum annealing improved the transmittance, whereas the oxygen-rich annealing decreased the transmittance. It was considered that the absorption at 420 nm resulted from Pb3+ in the crystal. # 2001 Elsevier Science B.V. All rights reserved. PACS: 81.10.Fq Keywords: A1. Doping; A1. Defects; A2. Czochralski method; A2. Single crystal growth; B1. Tungstates; B2. Scintillator materials
1. Introduction Lead tungstate (PbWO4) crystal is considered to be among the most promising scintillators particularly for its high density, fast response to radiation and possibly is also radiation-hard [1–3]. Actually, a systematic investigation on its growth and optical properties was developed due to its intended application at the electromagnetic calorimeter within the compact muon dolenoid (CMS) detector in the large hadron collider (LHC) experiment [4–7]. Relatively large single crystal segments having dimensions of about
*Corresponding author. Fax: +86-451-622-1048. E-mail address: [email protected] (C. Yang).
24 24 230 mm and overall volume of about 12 m3 are required for this application. PbWO4 crystal occurs in nature as two polymorphs: (1) the tetragonal stolzite which crystallizes with a scheelite type structure (space group I42/a) and (2) monoclinic raspite (space group P21/a) [8]. The PbWO4 crystal with the scheelite phase can be grown in the laboratory, but the crystal with the raspite phase cannot. The raspite phase transforms irreversibly to the stolzite one at about 4008C. PbWO4 crystal can be grown by both Czochralski and Bridgman method [9,10]. Crystal grown by Czochralski method has advantages over the Bridgman method such as higher growth rate and lower inner stress, which makes it easy to cut and polish. Of course, the difference of vaporization between PbO and WO3 in the melt
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 0 3 1 - 4
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C. Yang et al. / Journal of Crystal Growth 226 (2001) 79–82
results in the deviation from the stoichiometry along the growth direction if grown by unsealed Czochralski method [11,12], which causes the decrease of transmittance. In this paper, we studied the growth of large PWO crystals by the Czochralski method and the effect of raw material purity and annealing technology on the transmittance of the crystal.
2. Experimental procedure The starting materials were PbO and WO3. The ratio of PbO to WO3 was selected as 1 according to the phase diagram of the PbO–WO3 system, which shows a congruent melt [13]. The starting materials were loaded into the crucible and placed into a resistance heated furnace after they were mixed for 20 h. The temperature was measured by Pt–Rh thermocouples, which was placed at 20 mm below the crucible top and near the crucible wall. Both the diameter and the height of the crucible was 150 mm. The temperature was increased to 10008C at the rate of 1008C/h. Then the temperature was maintained for about 2 h in order to allow the solid phases to react completely. A raise of temperature was continued at the rate of 2008C/h until it reached 12008C, when it was maintained at this temperature for about 1 h. Then the temperature was decreased to the seeding at about 11308C, which was slightly higher than the melting point. In the meantime the seed was descended to a position of 1 mm above the melt and left for 10 min. The seed orientation was [0 0 1], with a rotation rate of 35 rpm. The axial temperature gradient was 308C/cm during the growth process. It indicated that the temperature was suitable for seeding if the seed did not melt. The seed was descended to a position of 1 mm below the melt and gradually melted. The temperature was decreased to the melting point (about 11238C) when the seed diameter was about 3 mm. Neither the seed melted nor grew at this temperature. Then the seed was slowly pulled up at a rate of 7–8 mm/h. The crystal was grown to a length of 5 mm at this higher growth rate in order to decrease its defects. First, the growth rate was lowered to 3 mm/h, while the temperature was
decreased at a rate of 0.2–0.48C/h after seeding. It took about 10 h and 18 h for the crystal to grow to the needed a diameter of 25 and 50 mm, respectively. Second, the temperature was increased slowly so that the thermal stress and defects were decreased at the edge when the diameter reached the desired size. Last, the crystal was grown at the same diameter. The float balance automatically controlled the furnace temperature and crystal diameter. The growth rate was 5 mm/h. The rotation rate was still 35 rpm. The temperature was increased at a rate of 18C/h when the crystal length reached the requirement. The crystal was drawn up from the melt at a higher rate. The elevation distance was about 5 mm. The temperature was decreased at a rate of 508C/h. The dimensions of the as grown crystal were diameter of 25 250 mm long and diameter of 40 60 mm long (Fig. 1), with nearly a uniform diameter. The thermal stress in the inner crystal is due to temperature gradient. At the same time, the chemical and structure stress existed in the crystal resulting from the interaction of dopants and composition derivation, etc. All the inner stress could be partly eliminated by annealing. The effects of vacuum annealing at 9208 for 24 h and at 4608C in oxygen for 24 h on the transmittance were studied. The transmittance analysis was carried out on the SPEX1000 M spectroscopy on samples of 10 mm thickness. The exposure time was 200 ms. The measurement pace was 5 nm. The Fe3+ content was analyzed by the ARL 9400 X-ray
Fig. 1. PbWO4 crystal grown by Czochralski method.
C. Yang et al. / Journal of Crystal Growth 226 (2001) 79–82
fluorescence. Rh being the targets, the measurement voltage was 50 kV.
3. Results and discussion 3.1. Effect of the starting material purity on the transmittance Table 1 shows the effect of the starting material purity on the transmittance of the crystal. The crystal was vacuum annealed. Usually there were two absorption bands for the PbWO4 crystal, which was at 320 and 420 nm, respectively. The absorption at 320 nm was typical of the PbWO4 crystal, which was determined by the energy required by the electron transiting from the 2p orbit of O to the space 5d orbit of W in the WO24 [14]. The absorption at 420 nm was related to the dopants, or the derivation from the stoichiometry [6], or the inner defects in the crystal, such as Pb3+, O [15,16]. It was shown that Fe3+ had little effect on the absorption at 320 nm wavelength, whereas it enhanced the absorption at the 420 nm when its concentration was higher than 10 ppm. The electronic structure of Fe3+ was 3d5. The energy level of 3d split and formed the electronic transition of d–d orbit when it entered the lattice and occupied the lattice of Pb2+ or W6+, which resulted in the absorption at 420 nm.
81
The purity of starting material was 99.995% in this experiment, in which Fe content in the starting material defined by X-ray fluorescence analysis was 10 ppm (shown in Table 1). It had been found from Table 1 that Fe3+ in the starting material had little effect of dopant Fe3+ on the absorption at 420 nm when its content was lower than 10 ppm. Thus, the effect of dopant Fe3+ on the absorption at 420 nm was excluded by using the material with high purity (99.995%) The crystal showed pale yellow color when it was annealed in the oxygenrich atmosphere, namely it had absorption at 420 nm. The transmittance at 420 nm of the crystal increased when it was vacuum annealed. In fact there were two kinds of viewpoints on the absorption at 420 nm, namely O absorption and Pb3+ absorption as mentioned above. It was reported that the concentration of F+(VS+e) center was higher than that of F (VS+2e) center at lower temperatures in CaS crystal, while the concentration of F (VS+2e) center was higher at higher temperatures [17]. There was equilibrium between F center and F+ center. We could assume that the concentration of F (VS+2e) center was higher at higher temperatures in PWO as that in CaS crystal. Otherwise, the annealing in oxygenrich atmosphere might induce the diffusion of oxygen ion into the PWO crystal thus lowering the oxygen vacancy concentration. The relative concentration of F(VO+2e) center to one of F+(VO+e) center increased to achieve the charge balance. The existence capability of O is reduced
3.2. Effect of annealing on the transmittance Fig. 2 shows the effect of different annealing conditions on the transmittance of PbWO4 crystal.
Table 1 Effect of the starting material purity on the transmittance at 420 nm of the crystal Purity of raw material (%)
Fe content in starting material (ppm)
Transmittance (%)
99.9999 99.995 99.995 99.99 99
10 10 60 (50 ppm Fe doped) 19 50
68 68 45 55 4
Fig. 2. Effect of different annealing conditions on the transmittance of PbWO4 crystal. a: crystal vacuum annealed; b: crystal unannealed; c: crystal oxygen-rich annealed; d: crystal oxygenrich reannealed.
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C. Yang et al. / Journal of Crystal Growth 226 (2001) 79–82
in the same way. Therefore, we consider that Pb3+ in the crystal resulted in the absorption at 420 nm. In fact, PbO could turn to PbO2 or Pb3O4 in air when the temperature was higher than 3908C. The volatile composition was analyzed. It was found that there were Pb3+ and Pb4+ containing compounds with the higher concentrations in the volatilized residue found on the wall. It was known that Pb4+ could not induce the absorption for its electronic structure of 4f145d10. Pb3+ could turn into Pb2+ under vacuum conditions, so the absorption at 420 nm weakened and the transmittance enhanced.
Acknowledgements This work was supported by the Scientific Research Foundation of Harbin Institute of Technology(HIT. 2000. 21), the Key Lab Foundation of Crystal Materials of Shandong University and the National Nature Sciences Foundation of China.
References [1] V.G. Barishevski, M. Korzhik, V.I. Moroz, V.B. Pavlenko, A.F. Lobko, A.A. Fedorov, V.A. Katchanov, S.G. Solovyanov, D.N. Zadneprovskii, V.A. Nefedov, P.V. Dorogovin, L.L. Nagornaya, Nucl. Instr. and Methods A 322 (1992) 231.
[2] M. Kobayashi, M. Ishii, Y. Usuku, H. Yahagi, Nucl. Instrum. and Methods A 333 (1993) 429. [3] The Compact Muon Solenoid Technical Proposal, CERN/ LHCC 94-38 LNCC/P1. [4] P. Dorenbos, C.W. Eljk (Eds.), Inorganic Scintillators and heir Applications, in: Proceedings of SCINT’95, Delft University Press, Delft, August 28–September 2, 1995. [5] B.M. Epelbaum, K. Inaba, S. Uda, T. Fukuda, J. Crystal Growth 178 (1997) 426. [6] K. Nitsch, M. Nikl, S. Ganschow, P. Reiche, R. Uecker, J. Crystal Growth 165 (1996) 163. [7] I.N. Shpinkov, I.A. Kamenskikh, M. kirm, V.N. Kolobanov, V.V. Mikhailin, A.N. Vasilev, G. Zimmerer, Phys. Stat. Sol. A 170 (1998) 167. [8] T. Fujita, I. Kawada, K. Kato, Acta Crystallogr. B 33 (1997) 162. [9] D. Yan, in: S. Baccaro, B. Borgia, I. Dafinei, E. Longo (Eds.), Proceedings of the International Workshop on Tungstate Crystal Roma, Italy, 1998, p.7. [10] P. Lecoq, in: S. Baccaro, B. Borgia, I. Dafinei, E. Longo (Eds.), Proceedings of the International Workshop on Tungstate Crystal Roma, Italy, 1998, p. 23. [11] P. Lecoq, I. Dafinei, E. Auffray, M. Schneegans, M.V. Korzhik, O.V. Missevitch, B.V. Pavlenko, A.A. Fedorov, A.N. Anenkov, V.L. Kostyle, Nucl. Instr. and Methods A 365 (1995) 291. [12] J.P. Peigneux, Nucl. Instr. and Methods A 361 (1994) 197. [13] L.L.Y. Chang, J. Amer. Ceram. Soc. 54 (1971) 357. [14] Cz. Koepke, A. Lempicki, J. Lumin. 47 (1991) 189. [15] A.N. Annenkov, A.A. Fedorov, Ph. Galez, V.A. Kachanov, M.V. Korzhik, V.D. Ligun, J.M. Moreau, V.N. Nefedov, V.B. Pavlenko, J.P. Peigneux, T.N. Timoshchenko, B.A. Zadneprovskii, Phys. Stat. Sol. A 156 (1996) 493. [16] M. Nikl, K. Nrrsch, J. Hybler, J. Chval, P. Reiche, Phys. Stat. Sol. B 196 (1996) K7. [17] J. Rennie, K. Takeuchi, Y. Kaneko, T. Koda, J. Lumin. 48&49 (1991) 787.
Growth of large PbWO4 single crystals by Czochralski method C. Yanga,*, Y. Guob, P. Shia, G. Chenc b
a Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, People’s Republic of China Department of Chemistry Engineering, Harbin Engineering University, Harbin 150001, People’s Republic of China c Beijing Glass Institute, Beijing 100001, People’s Republic of China
Received 25 November 2000; accepted 3 February 2001 Communicated by M. Schieber
Abstract Large size PbWO4 crystal was grown by Czochralski method. The dopant Fe3+ in the raw material and the annealing technology affected the transmittance at 420 nm. The transmittance at 420 nm obviously decreased when the Fe3+ content was higher than 10 ppm. The vacuum annealing improved the transmittance, whereas the oxygen-rich annealing decreased the transmittance. It was considered that the absorption at 420 nm resulted from Pb3+ in the crystal. # 2001 Elsevier Science B.V. All rights reserved. PACS: 81.10.Fq Keywords: A1. Doping; A1. Defects; A2. Czochralski method; A2. Single crystal growth; B1. Tungstates; B2. Scintillator materials
1. Introduction Lead tungstate (PbWO4) crystal is considered to be among the most promising scintillators particularly for its high density, fast response to radiation and possibly is also radiation-hard [1–3]. Actually, a systematic investigation on its growth and optical properties was developed due to its intended application at the electromagnetic calorimeter within the compact muon dolenoid (CMS) detector in the large hadron collider (LHC) experiment [4–7]. Relatively large single crystal segments having dimensions of about
*Corresponding author. Fax: +86-451-622-1048. E-mail address: [email protected] (C. Yang).
24 24 230 mm and overall volume of about 12 m3 are required for this application. PbWO4 crystal occurs in nature as two polymorphs: (1) the tetragonal stolzite which crystallizes with a scheelite type structure (space group I42/a) and (2) monoclinic raspite (space group P21/a) [8]. The PbWO4 crystal with the scheelite phase can be grown in the laboratory, but the crystal with the raspite phase cannot. The raspite phase transforms irreversibly to the stolzite one at about 4008C. PbWO4 crystal can be grown by both Czochralski and Bridgman method [9,10]. Crystal grown by Czochralski method has advantages over the Bridgman method such as higher growth rate and lower inner stress, which makes it easy to cut and polish. Of course, the difference of vaporization between PbO and WO3 in the melt
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 0 3 1 - 4
80
C. Yang et al. / Journal of Crystal Growth 226 (2001) 79–82
results in the deviation from the stoichiometry along the growth direction if grown by unsealed Czochralski method [11,12], which causes the decrease of transmittance. In this paper, we studied the growth of large PWO crystals by the Czochralski method and the effect of raw material purity and annealing technology on the transmittance of the crystal.
2. Experimental procedure The starting materials were PbO and WO3. The ratio of PbO to WO3 was selected as 1 according to the phase diagram of the PbO–WO3 system, which shows a congruent melt [13]. The starting materials were loaded into the crucible and placed into a resistance heated furnace after they were mixed for 20 h. The temperature was measured by Pt–Rh thermocouples, which was placed at 20 mm below the crucible top and near the crucible wall. Both the diameter and the height of the crucible was 150 mm. The temperature was increased to 10008C at the rate of 1008C/h. Then the temperature was maintained for about 2 h in order to allow the solid phases to react completely. A raise of temperature was continued at the rate of 2008C/h until it reached 12008C, when it was maintained at this temperature for about 1 h. Then the temperature was decreased to the seeding at about 11308C, which was slightly higher than the melting point. In the meantime the seed was descended to a position of 1 mm above the melt and left for 10 min. The seed orientation was [0 0 1], with a rotation rate of 35 rpm. The axial temperature gradient was 308C/cm during the growth process. It indicated that the temperature was suitable for seeding if the seed did not melt. The seed was descended to a position of 1 mm below the melt and gradually melted. The temperature was decreased to the melting point (about 11238C) when the seed diameter was about 3 mm. Neither the seed melted nor grew at this temperature. Then the seed was slowly pulled up at a rate of 7–8 mm/h. The crystal was grown to a length of 5 mm at this higher growth rate in order to decrease its defects. First, the growth rate was lowered to 3 mm/h, while the temperature was
decreased at a rate of 0.2–0.48C/h after seeding. It took about 10 h and 18 h for the crystal to grow to the needed a diameter of 25 and 50 mm, respectively. Second, the temperature was increased slowly so that the thermal stress and defects were decreased at the edge when the diameter reached the desired size. Last, the crystal was grown at the same diameter. The float balance automatically controlled the furnace temperature and crystal diameter. The growth rate was 5 mm/h. The rotation rate was still 35 rpm. The temperature was increased at a rate of 18C/h when the crystal length reached the requirement. The crystal was drawn up from the melt at a higher rate. The elevation distance was about 5 mm. The temperature was decreased at a rate of 508C/h. The dimensions of the as grown crystal were diameter of 25 250 mm long and diameter of 40 60 mm long (Fig. 1), with nearly a uniform diameter. The thermal stress in the inner crystal is due to temperature gradient. At the same time, the chemical and structure stress existed in the crystal resulting from the interaction of dopants and composition derivation, etc. All the inner stress could be partly eliminated by annealing. The effects of vacuum annealing at 9208 for 24 h and at 4608C in oxygen for 24 h on the transmittance were studied. The transmittance analysis was carried out on the SPEX1000 M spectroscopy on samples of 10 mm thickness. The exposure time was 200 ms. The measurement pace was 5 nm. The Fe3+ content was analyzed by the ARL 9400 X-ray
Fig. 1. PbWO4 crystal grown by Czochralski method.
C. Yang et al. / Journal of Crystal Growth 226 (2001) 79–82
fluorescence. Rh being the targets, the measurement voltage was 50 kV.
3. Results and discussion 3.1. Effect of the starting material purity on the transmittance Table 1 shows the effect of the starting material purity on the transmittance of the crystal. The crystal was vacuum annealed. Usually there were two absorption bands for the PbWO4 crystal, which was at 320 and 420 nm, respectively. The absorption at 320 nm was typical of the PbWO4 crystal, which was determined by the energy required by the electron transiting from the 2p orbit of O to the space 5d orbit of W in the WO24 [14]. The absorption at 420 nm was related to the dopants, or the derivation from the stoichiometry [6], or the inner defects in the crystal, such as Pb3+, O [15,16]. It was shown that Fe3+ had little effect on the absorption at 320 nm wavelength, whereas it enhanced the absorption at the 420 nm when its concentration was higher than 10 ppm. The electronic structure of Fe3+ was 3d5. The energy level of 3d split and formed the electronic transition of d–d orbit when it entered the lattice and occupied the lattice of Pb2+ or W6+, which resulted in the absorption at 420 nm.
81
The purity of starting material was 99.995% in this experiment, in which Fe content in the starting material defined by X-ray fluorescence analysis was 10 ppm (shown in Table 1). It had been found from Table 1 that Fe3+ in the starting material had little effect of dopant Fe3+ on the absorption at 420 nm when its content was lower than 10 ppm. Thus, the effect of dopant Fe3+ on the absorption at 420 nm was excluded by using the material with high purity (99.995%) The crystal showed pale yellow color when it was annealed in the oxygenrich atmosphere, namely it had absorption at 420 nm. The transmittance at 420 nm of the crystal increased when it was vacuum annealed. In fact there were two kinds of viewpoints on the absorption at 420 nm, namely O absorption and Pb3+ absorption as mentioned above. It was reported that the concentration of F+(VS+e) center was higher than that of F (VS+2e) center at lower temperatures in CaS crystal, while the concentration of F (VS+2e) center was higher at higher temperatures [17]. There was equilibrium between F center and F+ center. We could assume that the concentration of F (VS+2e) center was higher at higher temperatures in PWO as that in CaS crystal. Otherwise, the annealing in oxygenrich atmosphere might induce the diffusion of oxygen ion into the PWO crystal thus lowering the oxygen vacancy concentration. The relative concentration of F(VO+2e) center to one of F+(VO+e) center increased to achieve the charge balance. The existence capability of O is reduced
3.2. Effect of annealing on the transmittance Fig. 2 shows the effect of different annealing conditions on the transmittance of PbWO4 crystal.
Table 1 Effect of the starting material purity on the transmittance at 420 nm of the crystal Purity of raw material (%)
Fe content in starting material (ppm)
Transmittance (%)
99.9999 99.995 99.995 99.99 99
10 10 60 (50 ppm Fe doped) 19 50
68 68 45 55 4
Fig. 2. Effect of different annealing conditions on the transmittance of PbWO4 crystal. a: crystal vacuum annealed; b: crystal unannealed; c: crystal oxygen-rich annealed; d: crystal oxygenrich reannealed.
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C. Yang et al. / Journal of Crystal Growth 226 (2001) 79–82
in the same way. Therefore, we consider that Pb3+ in the crystal resulted in the absorption at 420 nm. In fact, PbO could turn to PbO2 or Pb3O4 in air when the temperature was higher than 3908C. The volatile composition was analyzed. It was found that there were Pb3+ and Pb4+ containing compounds with the higher concentrations in the volatilized residue found on the wall. It was known that Pb4+ could not induce the absorption for its electronic structure of 4f145d10. Pb3+ could turn into Pb2+ under vacuum conditions, so the absorption at 420 nm weakened and the transmittance enhanced.
Acknowledgements This work was supported by the Scientific Research Foundation of Harbin Institute of Technology(HIT. 2000. 21), the Key Lab Foundation of Crystal Materials of Shandong University and the National Nature Sciences Foundation of China.
References [1] V.G. Barishevski, M. Korzhik, V.I. Moroz, V.B. Pavlenko, A.F. Lobko, A.A. Fedorov, V.A. Katchanov, S.G. Solovyanov, D.N. Zadneprovskii, V.A. Nefedov, P.V. Dorogovin, L.L. Nagornaya, Nucl. Instr. and Methods A 322 (1992) 231.
[2] M. Kobayashi, M. Ishii, Y. Usuku, H. Yahagi, Nucl. Instrum. and Methods A 333 (1993) 429. [3] The Compact Muon Solenoid Technical Proposal, CERN/ LHCC 94-38 LNCC/P1. [4] P. Dorenbos, C.W. Eljk (Eds.), Inorganic Scintillators and heir Applications, in: Proceedings of SCINT’95, Delft University Press, Delft, August 28–September 2, 1995. [5] B.M. Epelbaum, K. Inaba, S. Uda, T. Fukuda, J. Crystal Growth 178 (1997) 426. [6] K. Nitsch, M. Nikl, S. Ganschow, P. Reiche, R. Uecker, J. Crystal Growth 165 (1996) 163. [7] I.N. Shpinkov, I.A. Kamenskikh, M. kirm, V.N. Kolobanov, V.V. Mikhailin, A.N. Vasilev, G. Zimmerer, Phys. Stat. Sol. A 170 (1998) 167. [8] T. Fujita, I. Kawada, K. Kato, Acta Crystallogr. B 33 (1997) 162. [9] D. Yan, in: S. Baccaro, B. Borgia, I. Dafinei, E. Longo (Eds.), Proceedings of the International Workshop on Tungstate Crystal Roma, Italy, 1998, p.7. [10] P. Lecoq, in: S. Baccaro, B. Borgia, I. Dafinei, E. Longo (Eds.), Proceedings of the International Workshop on Tungstate Crystal Roma, Italy, 1998, p. 23. [11] P. Lecoq, I. Dafinei, E. Auffray, M. Schneegans, M.V. Korzhik, O.V. Missevitch, B.V. Pavlenko, A.A. Fedorov, A.N. Anenkov, V.L. Kostyle, Nucl. Instr. and Methods A 365 (1995) 291. [12] J.P. Peigneux, Nucl. Instr. and Methods A 361 (1994) 197. [13] L.L.Y. Chang, J. Amer. Ceram. Soc. 54 (1971) 357. [14] Cz. Koepke, A. Lempicki, J. Lumin. 47 (1991) 189. [15] A.N. Annenkov, A.A. Fedorov, Ph. Galez, V.A. Kachanov, M.V. Korzhik, V.D. Ligun, J.M. Moreau, V.N. Nefedov, V.B. Pavlenko, J.P. Peigneux, T.N. Timoshchenko, B.A. Zadneprovskii, Phys. Stat. Sol. A 156 (1996) 493. [16] M. Nikl, K. Nrrsch, J. Hybler, J. Chval, P. Reiche, Phys. Stat. Sol. B 196 (1996) K7. [17] J. Rennie, K. Takeuchi, Y. Kaneko, T. Koda, J. Lumin. 48&49 (1991) 787.