1. W. Van Gool, Phillips Res. Repts. Suppl. No. 3. 1961 (Contains many references),
2. R. S. Title, in Physics and Chemistry of II-VI Compounds, Edited by Aven and Prener, Publ. by North Holland, 1967, p. 298.
3. H. H. Woodbury in Ref. 2, p. 240.
4. S. Shionoya, in Luminescence in Inorganic Solids, Edited by P. Goldberg, Publ. by Acad. Press 1966. p. 225.
5. W. Lehmann, Private communication.