1. K. OKADA and N. ŌTSUKA,J. Am. Ceram. Soc., in press.
2. T. SAKURAI, RSLC-3 (UNICS) Univeral Crystallographic Computation Program System (Crystallographic Society of Japan).
3. W. E. CAMERON,Ceram. Bull. 56 (1977) 1003.
4. W. M. KRIVEN and J. A. PASK,J. Am. Ceram. Soc. 66 (1983) 649.
5. G. W. BRINDLEY and M. NAKAHIRA,ibid. 43 (1959) 319.