1. Woodward PM (1997) Acta Crystallogr Sect. B 53:44
2. Kim JS, Kang SJL (1999) J Am Ceram Soc 82:119, and references therein
3. Gränicher H, Jakits O (1954) suppl. de nuovo cimento (9).11, No.3 480
4. McQuarrie MC (1955) J Am Cer Soc 38, No.12 144
5. Smolenskii GA, Isupov VA, Agranovskaia AI, Sholokhova ED (1957) Sov Phys Techn Phys 2:2351