1. S. Sh. Byk, Yu. F. Makogon, and V. I. Fomina,Gas Hydrates [in Russian], Khimiya, Moscow (1980).
2. N. G. Parsonage and L. A. K. Staveley,Disorder in Crystals, Clarendon Press, Oxford (1978).
3. B. F. Ormont,Compounds of Variable Composition [in Russian], Khimiya, Leningrad (1969).
4. V. Stackelberg,Naturwiss. 36, 327 (1949).
5. H. R. Muller and M. Stackelberg, —ibid.,39, 20 (1952).