Defect and transport properties of nonstoichiometric cobaltous sulfide

Abstract

Deviations from stoichiometry and chemical diffusion in cobaltous sulfide, Co _1−y S, have been studied as a function of temperature (923–1073 K) and sulfur activity (1–10 ³ Pa) using a thermogravimetric technique. It has been shown that in agreement with Libowitz's model, the unusual dependence of nonstoichiometry in Co _1−y S on equilibrium sulfur pressure and temperature may be interpreted in terms of repulsive interaction of cation vacancies in this compound. The chemical diffusion coefficient has been found to be essentially pressure-independent and can be described as a function of temperature by the equation:

$$\tilde D = 0.29 exp\left( { - \frac{{{{110 \pm 8.4kJ} \mathord{\left/ {\vphantom {{110 \pm 8.4kJ} {mol}}} \right. \kern-\nulldelimiterspace} {mol}}}}{{RT}}} \right)$$

Recalculation of these results shows that the mobility of cation vacancies in cobaltous sulfide decreases with an increase in their concentrations, probably due to the site-blocking effect. The self-diffusion coefficient of cobalt in Co _1−y S, calculated from defect-diffusion coefficients and nonstoichiometry data, has been found to be very weakly dependent on sulfur activity, and its pressure and temperature dependence can be described by the following empirical equation:

$$D_{Co} = 8.9 \times 10^{ - 4} p_{s_2 }^{{1 \mathord{\left/ {\vphantom {1 {20}}} \right. \kern-\nulldelimiterspace} {20}}} exp\left( { - \frac{{{{93.6 \pm 7.5kJ} \mathord{\left/ {\vphantom {{93.6 \pm 7.5kJ} {mol}}} \right. \kern-\nulldelimiterspace} {mol}}}}{{RT}}} \right)$$