1. S. C. Abrahams, J. M. Reddy, and J. L. Bernstein, J. Phys. Chem. Solids 27, 997 (1966).
2. T. H. Yeom, Y. M. Chang, C. Rudowicz, and S. H. Choh, Solid State Commun. 87, 245 (1993).
3. G. I. Malovichko, V. G. Grachev, and S. N. Lukin, Fiz. Tverd. Tela (Leningrad) 28, 991 (1986) [Sov. Phys. Solid State 28, 553 (1986)].
4. V. G. Grachev, G. I. Malovichko, and V. V. Troitskii, Fiz. Tverd. Tela (Leningrad) 29, 607 (1987) [Sov. Phys. Solid State 29, 349 (1987)].
5. V. Grachev, G. Malovichko, and O. Schirmer, Ferroelectrics 185, 5 (1996).