Oxidation kinetic method in studying the defect structure and transport properties of non-stoichiometric manganous sulphide

Abstract

Thermodynamics and kinetics of point defects in metal-deficient manganous sulphide (Mn 1− y S) have been studied using Rosenburg's two-stage kinetic method. Sulphidation rate measurements have been carried out in a new microthermogravimetric apparatus, enabling the mass changes of the sulphidized sample to be determined as a function of time with the accuracy of the order of 10 −7 g. In one series of two-stage kinetic experiments the concentration of cation vacancies and their mobility in Mn 1− y S have been determined as a function of temperature (1073–1273 K) and sulphur activity (10 −1−10 4 Pa). In agreement with the literature data it has been found that cation vacancies in this sulphide are doubly ionized and their concentration is the following function of equilibrium sulphur pressure and temperature: [ V Mn ″]=5.41 10 −2 p S 2 1/6exp((−42kJ/mol)/RT). The chemical diffusion coefficient, in turn, has been found to be pressure independent and is the following function of temperature: D̃=3.88 10 −2 exp((−81.5 kJ/mol)/RT). Recalculation of this coefficient into a defect diffusion coefficient, which is a direct measure of their mobility, yields the following empirical equation: D V=1.29 10 −2 exp((−81.5 kJ/mol)/RT). Using these data self-diffusion coefficient of cations in Mn 1− y S has been calculated as a function of temperature and sulphur activity: D Mn =6.98 10 −4p s 1/6 exp((−123.5 kJ/ mol)/ RT ). Enthalpies and entropies of formation and migration of point defects in metal-deficit manganous sulphide have been reported.