1. J. Chen, H. M. Chan and M. P. Harmer, J. Am. Ceram. Soc. 72 (1989) 593.
2. F. Galasso and J. Pyle, J. Phys. Chem. 67 (1963) 1561.
3. K. S. Hong, I. T. Kim and C. D. Kim, J. Am. Ceram. Soc., submitted.
4. R. A. Swalin, ?Thermodynamics of Solids?, 2nd Edn (Wiley, New York, 1972) p. 153.
5. H. P. Klug and L. E. Alexander, ?X-ray Diffraction Procedures?, 2nd Edn (Wiley, New York, 1974).