Anharmonic temperature dependences of the reststrahlen frequencies and the dielectric properties of potassium halides

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Abstract
The anharmonic temperature dependence of the reststrahlen frequency and the complex frequency-dependent index of refraction are calculated for KBr, KCl, and KI. Starting with harmonic phonons obtained from the breathing shell model (BSM) with input parameters appropriate to the crystal at 0 K, volume-dependent quasiharmonic phonons were calculated at different temperatures. These quasiharmonic phonons were then used to compute the self-energies of the zone-center TO modes, from which the frequency-dependent dielectric functions as well as the reststrahlen frequencies were obtained. The anharmonic force-constant parameters, assumed to be derivable from a central potential and restricted to nearest neighbors, were obtained by a fit of the measured coefficient of linear expansion and the "bare harmonic" reststrahlen frequency extrapolated from experiment. The calculated dielectric properties and the temperature dependence of the reststrahlen frequencies are in good agreement with experiment. As an alternative procedure for obtaining the anharmonic parameters a Born-Mayer potential was employed, but the results so obtained are in poor agreement with experiment. Different computional methods are studied and discussed.