1. 1. A.A. Botaki, A.A. Vorobe'ev, and V.P. Ul'yanov, Radiation Physics of Ionic Crystals (Atomizdat, Moscow, 1980) (in Russian).
2. 2. V.A. Zakrevskii and A.V. Shul'diner, Fiz. Tverd. Tela 41, 900 (1999).
3. 3. V.D. Kulikiv, Fiz. Tverd. Tela 43, 1580 (2001).
4. 4. G.G. Knab and A.A. Urusovskaya, Kristallogr. 16, 382 (1971).
5. 5. G.G. Knab and A.A. Urusovskaya, Kristallogr. 17, 828 (1972).