Quadratic-coupling phonon-assisted defect-tunneling theory applied to the RbCl:Ag+off-center system

A model for the $〈110〉$ off-center system RbCl: ${\mathrm{Ag}}^{+}$ including both linear and quadratic defect-lattice interactions is constructed. Model-parameter values are deduced from inelastic neutron scattering, infrared absorption, and electric field and stress-dichroism experimental results. The model is then used to study the temperature dependence of phonon-assisted tunneling rates of 60° and 90° reorientations over a wide temperature range, using a formalism that we have developed for this purpose. Special emphasis is placed on studying the role played by the quadratic defect-lattice coupling. Where possible, theoretical results are compared with experiment.