1. M. O. Henry, J. P. Larkin and G. F. Imbusch, Phys. Rev. B 13 (1976) 1893.
2. R. A. Weeks, J. Gastineau and E. Sonder, Phys. Status Solidi (a) 61 (1980) 265.
3. J. Corish, P. W. M. Jacobs and S. Radha-krishna, in ?Surface and Defect Properties of Solids?, Vol. 6, edited by M. W. Roberts and J. M. Thomas, (The Chemical Society, London, 1977) Ch. 5.
4. B. J. Wuensch, ?Mass Transport Phenomena in Ceramics?, edited by A. R. Cooper and A. H. Heuer, Materials Science Research, Vol. 9 (Plenum, New York, 1975) p. 211.
5. R. Freer, J. Mater. Sci. 15 (1980) 803.