Influence of Charged Defects on Mechanical Properties of Lead Sulfide

A chemical etch, suitable for revealing dislocation etch pits on cleaved crystals of PbS, has been utilized to reveal rosette patterns formed upon indentation of specimens whose charge-carrier concentration had been fixed by high-temperature equilibration in controlled sulfur pressure. Hardness and rosette wing length are nearly independent of concentration of free electrons in the range 2×1017 to 3×1019 cm−3. This is true for n-type material prepared by introduction of an excess of lead or by addition of a high concentration of bismuth donors to the PbS. Similar measurements on p-type, sulfur excess crystals, pure or containing silver as an acceptor addition, reveal a marked dependence of hardness and wing length on concentration of holes. For hole concentrations above 8×1017 cm−3, rapid hardening is observed with an attendant decrease in rosette size. It is suggested that this behavior results from an electrostatic interaction between charged dislocations and acceptor point defects.