1. L. C. Courrol, L. Gomes, and I. M. Ranieri, Phys. Rev. B 42(7), 4741 (1990).
2. B. D. Lobanov, N. T. Maksimova, E. D. Isyanova, et al., Opt. Spektrosk. 63(1), 816 (1987) [Opt. Spectrosc. 63, 485 (1987)].
3. V. I. Baryshnikov, V. A. Grigorov, B. D. Lobanov, et al., Izv. Akad. Nauk SSSR, Ser. Fiz. 54(8), 1467 (1990).
4. M. G. Abramishvili, V. G. Kvachadze, and Z. K. Saralidze, Fiz. Tverd. Tela (Leningrad) 29(1), 39 (1987) [Sov. Phys. Solid State 29, 22 (1987)].
5. M. G. Abramishvili, Z. G. Akhvlediani, T. L. Kalabegishvili, et al., Fiz. Tverd. Tela (St. Petersburg) 40(11), 2044 (1998) [Phys. Solid State 40, 1850 (1998)].