Isotopic differences in the 0  K volume and sublimation energies of solid neon and argon

Calculations are presented of the relative volume differences and sublimation energy differences of solid neon and argon isotopic lattices, assuming isotopic invariance of the intermolecular forces, which are chosen to be of the Lennard-Jones type. The calculations, performed at 0 °K on 20 Ne and 22 Ne, and 36 Ar and 40 Ar, make use of the anharmonic potential parameters presented by Brown in 1966. It is found that the relative volume and sublimation energy differences in solid neon and argon are well accounted for by the inclusion of cubic and quartic anharmonic terms at 0 °K. Comparison of our calculated sublimation energy differences with experiment is not possible owing to the lack of any measurements, and the only measured volume difference which is available is that of solid neon. It is suggested that the accuracy of the measurements has advanced to a point where non-adiabatic contributions may contribute slightly.