J. Phys. Colloques
Volume 37, Numéro C7, Décembre 1976Second International Conference on Lattice Defects in Ionic Crystals / Seconde Conférence Internationale sur les Défauts de Réseau dans les Cristaux Ioniques
|Page(s)||C7-271 - C7-271|
J. Phys. Colloques 37 (1976) C7-271-C7-271
OFF-CENTER FLUORIDE DEFECTS IN ALKALI-BROMIDES AND -IODIDESJ. WAHL1, A. GONGORA2 and F. LÜTY3
1 MPI Festkörperforschung, Stuttgart, Germany.
2 Instituto Politecnico Nacional, Mexico.
3 Physics Department, University of Utah, Salt Lake City, Utah, U.S.A.
A comprehensive study was performed in the sodium-, potassium-, and rubidium bromides and -iodides for possible off-center effects of substitutional F- defects, using dielectric and Kerr-effect techniques. While normal centro-symmetric positions were found for F- in NaI, KBr and RbBr, a paraelectric behaviour - scaling with the F- content - was detected in KI, RbI and NaBr , indicating off-center positions for the fluoride ions. A < 110 > symmetry for the off-center defects was verified in all three cases by the anisotropy of the Kerr-effect and by dielectric measurements under stress of various symmetry. The temperature dependence of the optical and dielectric results yields a Curie-law (E/T) behaviour of the dipole alignment. The reorientation kinetics of the F- off-center ions was studied in detail by the rise- and decay-time of the Kerr-effect and by dielectric loss. The results show in all three cases predominant 90° reorientation at the lowest temperatures, which changes for higher T to a 60° reorientation process. A discussion in terms of the elastic dressing model will be given.