1. E. L. Belokoneva and N. V. Belov, Dokl. Akad. Nauk SSSR 260 (6), 1363 (1981).
2. B. V. Mill’, A. V. Butashin, G. G. Khodzhabagyan, et al., Dokl. Akad. Nauk SSSR 264 (6), 1385 (1982).
3. A. A. Kaminsky, B. V. Mill’, and S. E. Sarkisov, Physics and Spectroscopy of Laser Crystals (Nauka, Moscow, 1986) [in Russian], p. 197.
4. K. Marty, P. Bordet, V. Simonet, et al., Phys. Rev. B 81, 054416 (2010).
5. E. L. Belokoneva, S. Yu. Stefanovich, Yu. V. Pisarevskii, et al., Zh. Neorg. Khim. 45 (11), 1786 (2000).