Thermodynamics and kinetics of point defects in manganous sulphide

Abstract

The defect equilibria and diffusion kinetics of point defects and manganese ions in manganese sulphide have been discussed with the background of available literature data. It has been shown that the behaviour of defects over the whole phase field of this compound can be satisfactorily described in terms of point defect thermodynamics. Over the major part of the homogeneity range, doubly ionized cation vacancies and electron holes are the dominating defects, manganous sulphide being a metal deficit p-type semiconductor (Mn1−yS). Near the Mn/MnS phase boundary, on the other hand, doubly ionized interstitial cations and quasi-free electrons prevail, the sulphide being thus a metal excess n-type semiconductor (Mn1+yS). The enthalpy and entropy of cation vacancy formation in Mn1−yS and those of interstitial cations in Mn1+yS, have been calculated. Both near and at the stoichiometric composition, the defect structure of manganous sulphide is more complex and strongly dependent on temperature. Below about 1100 K, intrinsic Frenkel electronic disorder dominates. In agreement with the defect model it has been shown that the self-diffusion of cations in Mn1−yS proceeds by a simple vacancy mechanism via doubly ionized cation vacancies, randomly distributed in the crystal lattice. The activation enthalpy and entropy of this process have been calculated. On the other hand, diffusion of manganese ions in Mn1+yS proceeds by an interstitial mechanism, in which the cation enters from its normal lattice site into a neighbouring interstitial position. In agreement with theoretical predictions, the activation enthalpy of this process has been found to be considerably higher than that of the vacancy diffusion. It has been demonstrated that the minimum rate of manganese self-diffusion at a given temperature corresponds very well to the stoichiometric composition of the sulphide, which is again in agreement with the defect model.