1. F. A. Kröger, Some Aspects of the Luminescence of Solids, Elsevier Publ. Company, Amsterdam, 1948.
2. G. Blasse and A. Bril, J. Solid-State Chem. 2, 291 (1970).
3. G. Blasse and A. F. Corsmit, J. Solid-State Chem., in press.
4. A. F. Corsmit, H. E. Hoefdraad and G. Blasse, J. Inorg. Nucl. Chem., in press.
5. G. Blasse and A. Bril, Z. physik. Chemie N. F. 57, 187 (1968).