1. See, for example,J. H. Schulman andW. D. Compton:Color Centers in Solids, Chap. IV (London, 1962), and references therein.
2. W. Känzig andT. O. Woodruff:Journ. Phys. Chem. Solids,9, 70 (1958).
3. N. Itoh:Journ. Phys. Chem. Solids,27, 197 (1966).
4. G. Giuliani: submitted for publication in theSolid-State Comm..
5. J. L. Kolopus, C. J. Delbecq, E. L. Yasaitis andP. M. Yuster:Bull. Am. Phys. Soc.,10, 1110 (1965).