1. L. M. BELYAEV, B. V. VITOVSKY and G. F. DOBRZHANSKY, in ?Growth of Crystals: Report at the First Conference on Crystal Growth?, edited by A. V. Shubnikov and N. N. Sheftal (1956).
2. F. A. KRÖGER, ?Some Aspects of the Luminescence Solids? (Elsevier Publishing Company, New York, 1948) Ch. 3 and 6.
3. W. VAN LOO, Thesis Utrecht (1975).
4. Idem, Phys. Stat. Solidi (a)27 (1975) 565.
5. Idem, ibid. 28 (1975) 227.