Defect Structure and Transport Properties of Nonstoichiometric Metal Sulphides

The present paper is an attempt to discuss the results of studies on the defect structure and transport properties of nonstoichiometric metal sulphides, obtained in the recent decade. These physicochemical properties of metal sulphides remain still less known than those of corresponding oxides. Such situation results mainly from much greater experimental difficulties in studying the high-temperature heterogeneous reactions in sulphur-containing atmospheres. Sulphur vapours are, namely, extremely aggressive at high temperatures and as a consequence, all standard thermogravimetric and other equipments commonly used in oxidation studies are not applicable under such conditions. This experimental restriction has been over came recently by the development of novel microthermogravimetric assemblies, enabling the determination of the kinetics of mass changes of a given sample with the accuracy of about two orders of magnitude higher than in all other thermogravimetric equipments. In addition, these assemblies make it possible to study the concentration and the mobility of point defects in metal sulphides showing not only large, but also very low deviation from stoichiometry. As a consequence, important in formation on physicochemical properties of metal sulphides can be obtained in rather simple way by two microthermogravimetric techniques, i.e. re-equilibration and two-stage kinetic methods, with the accuracy difficult to attain using other, much more complicated and time consuming methods. It has been shown that the results obtained using both these methods may be useful in studying transport properties of nonstoichiometric metal sulphides and thereby in designing novel high temperature alloys, better resistant to sulphide corrosion.