Thermodynamic properties of the rare-gas solids Ar, Kr, and Xe are calculated using phenomenological two-body central potentials. The cubic and quartic anharmonic terms in the crystal Hamiltonian have been included, using conventional perturbation theory. Although some definite discrepancies with experiment exist for $T<\mathrm{}\frac{1}{3}{T}_{\mathrm{melting}}$ these simple models give a fair over-all account of both the volume and temperature dependence of the Helmholtz energy. However, for $T>\mathrm{}\frac{1}{3}{T}_{\mathrm{melting}}$ (rms deviations greater than ∼6% of the nearest-neighbor distance), anharmonic contributions to the thermodynamic properties become unrealistically large, indicating, we believe, the breakdown of perturbation theory to the order considered in this work.

• Received 13 January 1969

DOI:https://doi.org/10.1103/PhysRev.184.968