1. V. K. Malinovskii, O. A. Gudaev, V. A. Gusev, et al., Photoinduced Phenomena in Sillenites (Nauka, Novosibirsk, 1990).
2. T. V. Panchenko and N. A. Truseeva, Ukr. Fiz. Zh. 34, 1495 (1989).
3. T. V. Panchenko and N. A. Truseeva, Ferroelectrics 129, 113 (1992).
4. B. Briat, H. J. Reyher, A. Hamri, et al., J. Phys.: Condens. Matter 7, 6951 (1995).
5. J. Foldvari, L. E. Halliburton, G. J. Edwards, et al., Solid State Commun. 77, 181 (1991).