The identification and some properties of point defects and non-basal dislocations in molybdenite surfaces

Abstract

From electron and optical microscopic studies of the topography of oxidized and cleaved basal faces of naturally occurring single crystals of molybdenite, quantitative estimates of both the density of (surface) point-defects and non-basal dislocations have been made. The point defects, probably vacancies, were rendered detectable by a combination of anisotropic oxidation (which ‘expands’ the defects in two dimensions parallel to the basal plane) and gold-decoration. The conditions for both the oxidation, to yield volatile products and triangular depressions, and the decoration, to facilitate the surface migration of gold, are critical. The gold-decoration technique is shown to be capable (i) of revealing ‘monomolecular’ (i. e. unit layer thickness of 6.15 Å ) steps and triangular depressions, (ii) of delineating pinned basal-plane dislocations lying close to the surface, and (iii) of discriminating, in principle, between ‘surface’ and ‘bulk’ defect concentration. For samples of molybdenite emanating from Australia, Korea and the United States, no significant differences in point-defect concentrations are detectable; and the density of defects at the surface (10 7 cm -2 ) is roughly equal to that in the bulk. Terraced depressions, penetrating ten to twelve layers, are thought to be associated with the termini of non-basal edge dislocations, the geometry of which is outlined. Non-basal screw dislocations are detected entirely by optical microscopy, chiefly because the Burgers vectors of such dislocations fall in the range 300 to 4000 Å, a situation reminiscent of natural graphite. Their density is ca . 10 3 cm -2 and many are inclined to the c -axis. From the dimensions of pits and depressions produced by oxidation it is possible to make order of magnitude estimates of the degree of kinetic anisotropy associated with the reaction at specific surface locations. The values of the ratio of the rate of oxidation along the c -axis to that along the basal planes, R c/a , at 600 °C, is found to be 10 -1 at (emergent) non-basal screw-dislocations, 10 -2 at non-basal edge dislocations, and 10 -11 at ‘ideal’ sites in the basal surface. The preferred direction of cleavage steps, which originate at the cores of forest screw dislocations, is <112̄0>, indicating that the interfacial energy, in air, of the {101̄l} face is smaller than that of {112̄l}. The techniques described should now be applicable to the study of defects in other layer structures.