1. G. Heiland and Y. Ibach, Solid State Commun. 4(3), 353 (1966).
2. G. A. Smolenskiĭ, I. G. Siniĭ, A. K. Tagantsev, S. D. Prokhorova, V. D. Mikvabiya, and W. Windsch, Zh. Éksp. Teor. Fiz. 88(3), 1020 (1985) [Sov. Phys. JETP 61 (3), 599 (1985)].
3. N. D. Gavrilova, E. N. Maksimov, V. K. Novik, and S. N. Drozhdin, Fiz. Tverd. Tela (Leningrad) 27(9), 2597 (1985) [Sov. Phys. Solid State 27 (9), 1559 (1985)].
4. V. K. Novik and N. D. Gavrilova, Fiz. Tverd. Tela (St. Petersburg) 42(6), 961 (2000) [Phys. Solid State 42 (6), 991 (2000)].
5. Yu. V. Shaldin, Kristallografiya 47(3), 531 (2002) [Crystallogr. Rep. 47 (3), 484 (2002)].