Pressure tuning of the motional behavior ofLi+,Ag+, andCu+ions in shallow off-center potentials of alkali halides

Abstract

The delicate balance of interaction energies, which creates in certain alkali halides a shallow off-center potential for small substitutional defect ions, can be drastically altered by application of relatively small ( 7 kbar) hydrostatic pressure (corresponding to about 1% lattice compression). This has been investigated for KCl: ${\mathrm{Li}}^{+}$, NaBr: ${\mathrm{Cu}}^{+}$, RbCl: ${\mathrm{Ag}}^{+}$, and RbBr: ${\mathrm{Ag}}^{+}$ by measurements of the real and imaginary dielectric response of the off-center dipoles, and by absorption-strength measurements of the ${\mathrm{Ag}}^{+}$ and ${\mathrm{Cu}}^{+}$ forbidden $d\ensuremath{\rightarrow}s$ uv transitions (which become partially allowed in the odd-parity off-center potential). Applied pressure reduces gradually the central and rotational barriers of the multiwell potential and, while keeping the off-center dipole moment constant, drastically increase the dipolar reorientation rates. As a consequence, the observable range of relaxation frequencies (${10}^{2}$-${10}^{5}$ Hz) shifts under pressure to low temperatures, tuning through regimes of thermally activated motion with decreasing barriers into regimes of tunneling motion. Similar to the situation in the superionic conductors, a particular match between cavity and point-ion size produces an optimum local ionic mobility at lowest temperatures. Slightly higher lattice compression leads to the disappearance of this ionic mobility, due to a change of the defect potential into a centrosymmetric one. This off- to on-center transition (a localized analog to pressure-induced displacive phase transitions in ferroelectrics) is discussed in terms of its first- or second-order character. The question of whether pressure induced changes in the phonon dressing (polaron effect) of the dipoles can contribute to the observed relaxation changes will be discussed, too.