The migration of inert gases in ionic crystals

Abstract

The migration and trapping of inert gases in ionic solids is of practical interest in reactor materials and of scientific interest as a result of experiments designed to simulate practical situations on better-defined systems. Many of these experiments have been carried out with crystals of alkali halides and alkaline earth fluorides. The present paper extends the classical Born model of an ionic crystal to the calculation of the energies of migration and trapping of the inert gases Ar and Kr in K and Rb halides. The energies of solution and migration of interstitial Ar and Kr atoms are evaluated as well as their binding energies into ionic vacancies and vacancy pairs. The results are in good general agreement with the observations on gas release from these crystals after neutron irradiation. In particular the differing behaviour of Ar in KF and in the other K halides is accounted for in terms of a strong trapping of the Ar atoms into vacancy pairs which occurs in KF. It is predicted that the behaviour of Kr in RbF should be in even more striking contrast to its behaviour in the other Rb halides. Finally the paper touches on the implications for geologic dating of natural sylvine by determination of its ⁴⁰Ar content.