Vapor Pressures of the Neon Isotopes

Abstract

The ratio of the vapor pressures of Ne²⁰ and Ne²² have been measured in the temperature range 16–30°K. An accuracy of about 1% in the logarithm of the vapor‐pressure ratio has been achieved. The latter is checked by comparisons of (1) triple‐point pressures, (2) equilibrium fractionation experiments, and (3) the entropy difference of the condensed phases with thermal measurements. The Debye temperature for the solid derived from the ratio of the vapor pressures, 74.6°K, is in good agreement with values derived from the Debye‐Waller temperature factor and theoretical calculations by Bernardes. The discrepancy between the characteristic temperatures derived from measurements of free energy with those from heat capacities is attributed to the role of the anharmonicity of the lattice. It is shown that the difference in the thermodynamic properties of solid and liquid neon in the vicinity of the triple point can be attributed to the change in coordination number.