Thermal Conductivity in Mixed Alkali Halides: KCl:Li and KBr:Li

Phonon resonances have been observed in both KCl:Li and KBr:Li through measurements of the low-temperature thermal conductivity between 0.05 and 100^∘{}K. Their frequencies are 1.2 and 63 cm^-1 in KCl, and 3.2 and 39 cm^-1 in KBr. Static electric fields are used to change the low-frequency resonances and to help in their identification. The 1.2-cm^-1 resonance in KCl is ascribed to the tunneling motion of the Li⁺, very similar to the tunneling of CN^- observed earlier. The frequency dependence of the relaxation rate associated with this resonance is best described with a Lorentzian resonance denominator of the form ω²(ω₀²-ω²)², which varies as ω² at low frequencies, in disagreement with existing theories. The 3.2-cm^-1 resonance in KBr is ascribed to an oscillation involving the motion of the Li⁺ plus its six nearest-neighbor Br^- ions. It is speculated that this LiBr₆ molecule ion has a central instability. The high-frequency resonances are attributed to impurity modes involving the oscillations of the ions surrounding the Li⁺. The 39-cm^-1 resonance in KBr does not show an isotope effect. It is not possible to describe the resonant scattering by these high-frequency resonances with the existing theory of elastic scattering. The phonon-defect relaxation rate must contain a temperature dependence. It is believed that inelastic three-quantum processes are important in these two systems.