Anharmonic Contributions to the Vibratory Free Energy of a Lattice

Abstract

The anharmonic contribution to the free energy of a lattice is evaluated by means of a Bethe approximation. The approximation is accurate to within 3% for the model valuated by Maradudin, Flinn, and Coldwell· Horsfall. The anharmonic contribution to the specific heat at constant volume of a model of sodium is evaluated.The specific heat is found to increase as the temperature rises but the rate of increase is lower than the observed value