1. The article by A. Bansil in this volume.
2. J.S. Faulkner, in Progress in Material Science, edited by T. Massalski (Pergamon, New York, 1982) Vol. 27.
3. A. Bansil, in Positron Annihilation, edited by P.G. Coleman, S.C. Sharma and L. Diana (North-Holland Amsterdam, 1982).
4. The article by R.S. Rao in this volume.
5. For other reviews see Refs. 3,6, and 7.