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)||C7-272 - C7-272|
J. Phys. Colloques 37 (1976) C7-272-C7-272
HYDROSTATIC-PRESSURE-TUNING FROM CLASSICAL TO TUNNELING MOTION AND FROM OFF- TO ON-CENTER POTENTIALS FOR Ag+, Cu+ AND Li+ DEFECTSU. HOLLAND and F. LÜTY
Physics Department, University of Utah, Salt Lake City, Utah, U.S.A.
The delicate balance among various defect-lattice interaction energies, producing off-center potentials for certain small point-defects, can be drastically upset by increasing the repulsion energy through hydrostatic pressure application. Consequently, the multiwell off-center potential becomes more shallow and transforms eventually into a centro-symmetric single-well potential. This pressure tuning from off- to on-center behaviour has been studied for Ag+, Cu+ and Li+ defects in various hosts through the temperature range 1.5 to 300 K. Dielectric loss measurements show for the classically reorienting Ag+ and Cu+ defects a gradual decrease of the reorientation barrier and eventually a change to tunneling behaviour under pressure, and for the quantum-mechanical KCl : Li+ system a strong increase of the tunneling splitting. The size of the off-center dipole moment < p > was monitored by the real (ε ∝ < p >2/T) and integrated imaginary dielectric response, and - for Cu+ and Ag+ ions - by the strength of the parity-forbidden d → s transition which becomes partially allowed in off-center potentials. Both optical and dielectric data indicate a rather constant dipole moment, which collapses abruptly at a critical characteristic pressure in a way suggesting a local phase-transition picture for the off- to on-center symmetry change.