1. Aegerter, M.A. and Luty, F. (1970): J. Luminesc. 1,2, 624
2. Aegerter, M.A. and Luty, F. (1971): phys. stat. sol. (b) 43, 227 and 2U5
3. Beaumont, J.H., Harmer, A.L. and Hayes, W. (1972): J. Phys. C - Sol. State Phys. 257
4. Burke, W. (1968): Phys. Rev. 172, 886
5. Collins, W.C. (1973): phys. stat. sol. (b) 57, 63