Tracer diffusion in alkali halides

Abstract

The experimental measurement of diffusion coefficients in alkali halides has very much contributed to solid state and defect physics since the early work of Maurer in 1950. One of the major successes has been the qualitative and quantitative description of the Schottky defects in alkali halides, prior to all other solids. Simultaneously, defect calculations have been equally successful in explaining the formation of defects.

However, there still remains a little more hesitation about the description of the atom migration even in the alkali halides. We have undertaken a new vast experimental programme of measurement of the diffusion coefficient of the two monovalent cations Na⁺ and Rb⁺ in many alkali halides: NaCl, KCl, KBr, Nal and KI. The priority is given to the ‘super-accuracy’ of the experimental data. The target is to test the validity of the proposed methods of calculating the enthalpy and entropy of migration. Therefore with two ions of supposedly known ionic sizes, polarizabilities and shell constants, diffusing into single crystals where also the lattice energies, elastic, dielectric and optical constants have been accurately measured, one should expect the same degree of agreement between the measured and calculated values for all the examples.