1. Levy, D., Reisfeld, R., and Avnir, D., Chem. Phys. Lett. 109, 595 (1984).
2. Devlin, K., O'Kelly, B., Tang, Z.R., McDonagh, C., and McGilp, J.F., J. of Non-Crystalline Solids 135, 8 (1991).
3. M. Ferrari, R. Campostrini, G. Carturan and M. Montagna, Phil. Mag. B, 65, 251 (1992).
4. Dahmouche, K., Bovier, C., Boukenter, A., Dumas, J., Duval, E., Mai, C., and Serughetti, J., J. de Physique IV C2, 127 (1992).
5. Weber, M.J., in Optical properties of ions in crystals, edited by H.M. Crooswhite and H.W. Moos (Interscience Publishers, New York, 1967), p. 467.