Anharmonic interactions in alkali halides. II

Abstract

Numerical calculations have been made of the anharmonic contributions to the free energy, entropy, and specific heat of potassium bromide and sodium iodide. The frequencies and eigenvectors used in the calculations were obtained from harmonic shell models of the crystals. The frequency distribution functions of these harmonic models have been calculated by an extrapolation method giving 17496000 frequencies in the Brillouin zone, and the harmonic contributions to the thermodynamic functions calculated from them . Corrections to these results have been made to allow for the fact that the models were fitted to experimental results obtained at finite temperatures. The Debye-Waller factors of the ions have been calculated in the harmonic approximation and the lowest order anharmonic contributions also evaluated. Where possible the results are compared with experimental results and with other calculations.