1. 1. R.Truell, C.Elbaum, B.Chik, Ultrasonic Methods in Solid State Physics, Mir, Moscow (1972) [in Russian].
2. 2. A.A.Botaki, A.A.Vorobjev, V.L.Uljanov, Radiation Physics of Ionic Crystals, Atomizdat, Moscow (1980) [in Russian].
3. 3. A.Granato, K.Lucke, J. Appl. Phys., 27, 583 (1956).
4. 4. G.G.Knab, A.A.Urusovskaja, Crystallography, 17, 828 (1972).
5. 5. I.A.Parfianovich, E.E.Penzina, Electronic Color Centers in Ionic Crystals, East-Siberia Book Publishing House, Irkutsk (1977).