1. J. A. Chapman and E. Lilley, J. de Phys. Supplement C9 (1973) 341.
2. D. L. Kirk and P. L. Pratt, Proc. Brit. Ceram. Soc. 9 (1969) 215.
3. R. W. Dreyfus and A. S. Nowick, Phys. Rev. 126 (1962) 1367.
4. Y. Haven, ?Defects in Crystalline Solids?, (Physical Society, London, 1955).
5. G. D. Watkins, Phys. Rev. 113 (1959) 79, 91.