1. Balzarotti, A; Piacentini, M.: Sol. Stat. Comm. 10, 421 (72) & Refs. in.
2. Mamey, R.; Cazaux, J; Levegue, G; Raisin, C: Phys. Stat. Sol. (b) 62, 201 (1974).
3. Perrin, J; Cazaux, J; Soukiassian, P: J. de Physique (to be published).
4. Williams, R. H; Williams, G. P; Norris, C.; Howells, M. R.; Munro, I. H.: Journal of Physics C. Sol. State (to be published).
5. Thomas, J. M.; Adams, I.; Williams, R. H.; Barber, M.: J. Chem. Soc. Faraday Trans I I 68, 755 (1973).