Diffusion of sulfur 35 in single crystals of lead sulfide as a function of stoichiometry and doping additions

Abstract

Self-diffusion of sulfur 35 in single crystals of lead sulfide has been studied as a function of temperature, sulfur pressure and foreign atom additions. The primary conclusions of these studies are: (a) sulfur diffuses as rapidly as or more rapidly than lead in single crystals of PbS, (b) sulfur does not diffuse via jumps into sulfur vacancies. In addition sulfur was inferred to diffuse via several simultaneous mechanisms, the predominant one depending on the experimental variables. The probable predominant mechanisms in undoped PbS are : (1) In crystals with an excess of sulfur, sulfur diffuses as an atomic rather than as an ionic species, by an interstitial mechanism, or via electrically neutral vacancies in the lead sublattice.; (2) In crystals with an excess of lead, sulfur diffuses via a neutral defect pair. Sulfur diffuses through lead sulfide heavily doped with the acceptor silver by the same mechanisms that are predominant in undoped PbS. This is evidenced by the lack of influence of silver concentration on diffusivities in the temperature range 500–750°C, which serves to substantiate the neutral electrical nature of the defects responsible for diffusion in undoped PbS. For lead sulfide doped with the donor bismuth, sulfur diffusion at 700°C occurs by the same mechanisms as are predominant in undoped PbS over the entire homogeneity range for the compound. At lower temperatures, jumps of sulfur into ionized lead vacancy acceptors, such as V' Pb may contribute appreciably to the total diffusion.