Chemical diffusion and defect mobility in non-stoichiometric manganous sulphide

Abstract

The chemical diffusion coefficient, D ̃ , in metal-deficit manganous sulphide, Mn 1−y S, has been determined as a function of temperature (700–1273 K) and sulphur activity (10 4–10 −1 Pa) from re-equilibration kinetics. It has been found that D ̃ does not depend on p(S 2) and can be described as a function of temperature by the following empirical equation. D ̃ =5.9×10 −2 exp((−83.4 kJ/mol)/ RT). Recalculation of these results into the defect (vacancy) diffusion coefficient, D V, the direct measure of defect mobility, showed that this coefficient is also pressure independent and can be described by the following relationship. D V =1.97×10 −2 exp((−83.4 kJ/mol)/ RT). These results may be considered as the first experimental proof that the mobility of cation vacancies in metal-deficit manganous sulphide does not depend on their concentration. Using this relationship and non-stoichiometry data reported by Rau [H.Rau, J. Phys. Chem. Solids, 39 (1978) 3339] the self-diffusion coefficient of cations in Mn 1−y S has been calculated as a function of temperature and sulphur activity: D Mn =9.4×10 −4p S 2 1/6 exp((−124.9 kJ/mol)/ RT). The result of these calculations, being in good agreement with recent self-diffusion data obtained with direct tracer methods, clearly indicates that chemical diffusion measurements may offer important information on transport properties of non-stoichiometric metal sulphides.