VIBRATIONAL AND ELASTIC PROPERTIES OF ALKALI-HALIDES NEAR MELTING

The phonon dispersion relations and the elastic constants are evaluated near the melting point for the alkali halides composed of isoelectronic ions, using a density-functional approach which relates the renormalized force constants of the hot crystal to the partial structure factors of the liquid phase and to the Debye-Waller factors. The input on liquid structure is obtained from a symmetric model of charged hard spheres, modified to account for dielectric screening from ionic core polarizabilities. Core polarization affects mainly the correlations between ionic charge-density fluctuations at long wavelengths and hence the longitudinal optical phonon branches. Major softening from anharmonicity is found for all the branches of the dispersion relation, this result being confirmed for the acoustic branches by the reasonable agreement obtained with data on the elastic constants at high temperatures and for the optical branches by consistency with the scanty experimental evidence concerning the dependence of zone-centre frequencies on temperature. The model also accounts for the deviations from the Cauchy relation between the elastic constants.