1. V. N. Sigaev, A. A. Loshmanov, I. I. Yamzin, et al.,Kristallografiya,22, 408–410 (1977).
2. M. C. Wittels,Phil. Mag.,2, 1445–1461 (1957).
3. A. R. Silin, L. N. Skuja, and A. N. Trukhin,J. Non-Cryst. Solids,38, 195–200 (1980).
4. Yu. D. Glinka, S. N. Naumenko, V. M. Ochenko, et al.,Opt. Spektrosk.,71, 431–433 (1991).
5. D. Grasse, O. Kocar, and J. Peisl,Phys. Rev. Lett.,46, 261–264 (1981).