HYDROSTATIC-PRESSURE-TUNING FROM CLASSICAL TO TUNNELING MOTION AND FROM OFF- TO ON-CENTER POTENTIALS FOR Ag⁺, Cu⁺ AND Li⁺ DEFECTS

Abstract
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.