SELF-DIFFUSION AND HETERODIFFUSION IN CRYSTALLINE SOLID-SOLUTIONS

We present a self-consistent analysis of a complete set of atomic diffusion data in monocrystalline alkali halide mixed crystals. New measurements of the heterodiffusion coefficients of Na+ and Rb+ in KClzBr(1-z), are combined with previous data for the self-diffusion coefficients of the three host ions constitutive of the solid solutions: KClzBr(1-z), KzRb(1-z), Cl and KzRb(1-z)I. The correlation effects have been accounted for by making a systematic use of Manning's equations of the correlation factors for vacancy diffusion in random alloys for unbound free vacancies. The justification and generalization of those equations is discussed in the light of recent simulation and analytical studies. The diffusivity is increased in the mixed crystals. The enhancement effect for both the self- and heterodiffusion coefficients reaches a factor of about 2 around the middle composition (z approximately 0.5) in the three systems. The major part of the enhancement originates from an increase of the vacancy concentration relative to the pure end members. This effect is qualitatively interpreted in thermodynamic terms. The present conclusions derived in alkali halides may be extended to other similar solids: metallic alloys, solid solutions, etc.