1. Landholt-B�rnstein, in ?Group III Crystals and Solid State Physics? Vols III and IX, edited by K. Hellwege (Springer-Verlag, Berlin, 1969).
2. H. Igaraski, K. Matsunaga, T. Tanioi andK. Okazaki,Amer. Ceram. Soc. Bull. 57 (1978) 815.
3. T. Takenaka, K. Shoji, H. Takai andK. Sakata,Proc. Japan Cong. Mat. Res. 19 (1976) 230.
4. M. Granaham, M. Holmes, W. A. Schultze andR. E. Newham,J. Amer. Ceram. Soc. 4 (1981) C68.
5. L. D. Landau andE. M. Lifshits, ?Statistical Physics? (Pergamon Press, Oxford, 1958) p. 183.