1. J. B. TASK and D. J. YOUNG,Ceram. Bull. 61 (1982) 7.
2. D. S. BUIST,Min. Mag. 36 (1968) 676.
3. A. L. GENTILE and W. R. FOSTER,J. Amer. Ceram. Soc. 46 (1963) 74.
4. R. W. NURSE, J. H. WELCH and A. J. MAJUMDAR,Trans. Br. Ceram. Soc. 64 (1965) 409.
5. E. CRIADO, P. PENÃ and A. CABALLERO, in ?Science of Ceramics-14?, edited by D. Tallor (The Institute of Ceramics, Stoke-on-Trent, 1987) p. 193.