Non-Central-Force Model of LiH: Phonon Dispersion Curves and He Migration

Abstract

Two models for calculating defect properties of LiH (${}_{}{}^7{}_{}{}^{}\mathrm{Li}$D) are obtained using noncentral interionic potentials of the form $f(\theta , \varphi )\phi \mathrm{}\left(r\right)\mathrm{}$, where $f(\theta , \varphi )$ has cubic symmetry and $\theta \mathrm{}\left(r\right)\mathrm{}$ is a Born-Mayer repulsion taken from quantum-mechanical calculations. The parameters of the models were varied to give the best fit to the interionic spacing, the binding energy, and the elastic coefficients of LiH. Phonon dispersion curves, which can be obtained from only one of the models, are compared with experimental results. Calculations with the two models give 0.69 and 0.87 eV for the activation energy for migration of an interstitial He atom in LiH.