1. E. M. Voronkova, B. N. Grechushnikov, G. I. Distler, and I. P. Petrov, Optical Materials for Infrared Technology [in Russian], Nauka, Moscow (1965).
2. A. I. Kotova and V. S. Rubanov, in: Proceedings of the Conference on the Electronics of Gas Lasers [in Russian],2(18) (1970), p. 35.