1. Y. HIGUCHI and H. KATSUBE, US patent 4 665 041 (1987).
2. Y. PARK, N. H. CHO and Y. H. KIM, US patent 176 770 (1994).
3. Y. PARK, N. H. CHO and Y. H. KIM, US patent 370 580 (1994).
4. G. BLASSE,J. Anorg, Chem., 345 (1966) 222.
5. R. CHRISTOFERSON and P. K. DAVIS,J Amer. Ceram. Soc. 75 (1992) 563.