1. Yu. A. Osip’yan, V. I. Tel’yanskii, and S. A. Shevchenko, Zh. Éksp. Teor. Fiz. 72, 1543 (1977) [Sov. Phys. JETP 45, 810 (1977)].
2. V. A. Grazulis, V. V. Kveder, and V. Yu. Mukhina, Phys. Status Solidi A 43, 407 (1977); 44, 107 (1977).
3. Yu. A. Osip’yan and S. A. Shevchenko, Pis’ma Zh. Éksp. Teor. Fiz. 20, 709 (1974) [JETP Lett. 20, 328 (1974)].
4. V. A. Goncharov, Yu. A. Osip’yan, and S. A. Shevchenko, Fiz. Tverd. Tela (Leningrad) 29, 1928 (1987) [Sov. Phys. Solid State 29, 1110 (1987)].
5. V. V. Kveder, R. Labusch, and Yu. A. Ossipyan, Phys. Status Solidi A 92, 293 (1985).