MÉTHODES DE RELAXATION DIPOLAIREORIENTATIONAL DEFECT RELAXATION BY CLASSICAL AND TUNNELING PROCESSES

Nous donnons une revue des principaux aspects expérimentaux et théoriques et des récents développements ayant trait à la relaxation de défauts de basse symétrie à basse température. Les systèmes en question sont les défauts moléculaires substitutionnels (tels OH-, CN-,... ) et des ions "off-center" (tels Li+, Ag+, Cu+) dans différents réseaux d'halogénures alcalins. Quelques techniques expérimentales importantes pour mesurer la relaxation a basse température (pertes diélectriques, techniques électro-caloriques, ITC et RPE) et des résultats marquants sur les défauts OH- dans différents réseaux seront discutés.A survey is given on the main experimental and theoretical aspects and recent developments, related to the orientational relaxation of low symmetry defects at low temperatures. The systems in question are substitutional molecular defects (like OH-, CN-, ... ) and off-center point-ions (like Li+, Ag+, Cu+) in various alkali-halide lattices. Some important experimental techniques for measuring low temperature relaxation (dielectric loss, electro-caloric, ITC, and PER techniques) and some representative results on OH- defects in various host lattices will be discussed. The results will be compared to the various available relaxation models (classical rate process, phonon-assisted tunneling, "dressed" tunneling).The latter model will be particularly used to explain the peculiar static and dynamic of-center properties of Ag+ ions in RbCl and RbBr, measured recently. It will further be shown how a combination of stress and field application can be used to produce "stress-tunable Orbach relaxation", which allows an accurate determination of the relative magnitudes in the rates of relaxation through different possible angles between dipole states. Trends in the observed off-center effects of Ag+ and Cu+ ions and in the transition from quantum-mechanical to classical reorientation behavior will be discussed