Analysis of the vapour pressure of KCl + RbCl solid solutions

Abstract

The vapour pressure of alkali halide solid solutions is described in terms of the quasi-harmonic theory of lattice vibrations. The solutions are assumed to be completely disordered and contributions from defects are neglected; the validity of these assumptions is discussed. The vapour pressures of several compositions in the KCl + RbCl systems have been analyzed and values obtained for the static lattice energy U₁₂ and the geometric mean frequency of the lattice vibrations ν_g. In addition, the thermodynamic functions of mixing have been obtained in the usual way. Appreciable fractions of dimeric species exist in the gaseous alkali halides and an essential step in the analyses is the calculation of the monomer partial pressures from the primary experimental data. The enthalpy of formation of the solutions at T∼800°K is smaller than the experimental uncertainties but is probably positive with values not exceeding a few 100 cal/mole. The excess entropy of mixing at 800 °K is also positive and of the order of a few tenths cal/mole.

Within the experimental uncertainties U₁₂ is a linear function of the composition in accordance with the small enthalpy of formation, and the excess entropy of mixing may be explained by the composition dependence of ν_g. A tentative explanation of the results is proposed, purely on the basis of quasiharmonic theory, in terms of the volume dependence of ν_g and a small positive volume of mixing.