Self-Diffusion of Lead 210 in Single Crystals of Lead Sulfide as a Function of Stoichiometry and Doping Additions

Self-diffusion of lead 210 in single crystals of lead sulfide has been studied in the temperature range of 500° to 800°C. Measurements were made on undoped crystals and on crystals doped with Bi2S3 or Ag2S under well-defined sulfur pressures and under an inert gas. Results show that increasing the cation vacancy concentration by means of excess sulfur or by bismuth additions increases the diffusion coefficient, whereas decreasing the concentration of cation vacancies by means of excess lead or by silver additions also increases the diffusion coefficient over those of stoichiometric PbS. In all cases the electron concentration is accounted for according to: [interstitial lead atoms]∝pPb/[excess electrons],and [cation vacancies]∝[excess electrons]/pPb,assuming singly ionized interstitial lead atoms and singly ionized cation vacancies. The activation energies decrease in the order, stoichiometric PbS, lead excess PbS and sulfur excess PbS. Results support a Frenkel disorder of the cation type.