1. E. V. Korovkin and T. A. Lebedkina, Fiz. Tverd. Tela (Leningrad) 29, 2807 (1987) [Sov. Phys. Solid State 29, 1612 (1987)].
2. R. L. Wild and F. C. Brown, Phys. Rev. 121, 1296 (1961).
3. E. V. Korovkin and T. A. Lebedkina, Fiz. Tverd. Tela (Leningrad) 33, 120 (1991) [Sov. Phys. Solid State 33, 67 (1991)].
4. E. V. Korovkin and T. A. Lebedkina, Fiz. Tverd. Tela (Leningrad) 33, 2483 (1991) [Sov. Phys. Solid State 33, 1400 (1991)].
5. E. V. Korovkin and T. A. Lebedkina, Fiz. Tverd. Tela (St. Petersburg) 37, 3536 (1995) [Phys. Solid State 37, 1945 (1995)].