Lead sulfide

Lead sulfide most commonly is encountered with divalent lead, with the formula PbS and the molecular weight 239.28. There is also a tetravalent form, PbS2, but that is principally synthetic. Refined lead sulfide is a dark gray to black powder, once used as a pigment. It melts at 1120 degrees Celsius.

Uses in electronics
Galena, a common ore of lead, is lead (II) sulfide, often occurs as as bright metallic cubic crystals. Even as the ore, lead sulfide exhibits important electronic properties as a semiconductor. The early "crystal radio" receivers, which could be built even without vacuum tubes, used a lead sulfide crystal as a diode. One conductor contacting the crystal was a fine wire, called a "cat's whisker", which was moved around the surface of the crystal to find the point of greatest sensitivity. Some hobbyists would add a third conductor providing current, which, without being understood, created a point-contact transistor. The amplifying transistor effect, however, was erratic, and was replaced by vacuum tubes until transistors were deliberately designed of other materials such as germanium.

Today, the most important electronic property is as a photoresistive detector of near infrared energy, just beyond the visual light spectrum. PbS infrared directors normallly must be chilled to operate. Lead sulfide is being incorporated, as nanocrystals, into polymer-based photodetector arrays.

Safety
Insoluble in water, it is one of the safer compounds of lead, but still should not be ingested.