1. A. R. Hilton, C. E. Jones andM. Brau,Phys. Chem. Glasses 7 (1966) 105.
2. P. J. Webber andJ. A. Savage,J. Non-Cryst. Solids 20 (1976 271.
3. ?Crystal Data Determination Tables? (American Crystallographic Association, 1963).
4. R. W. G. Wyckoff, ?Crystal Structures?, 2nd Ed. (Interscience, New York, 1964).
5. K. J. Marsh andJ. A. Savage,Infrared Phys. 14 (1974) 85.