Nucleation of Ice on Lead Iodide

Abstract

A cold box was constructed to study ice crystal growth on single crystals of lead iodide. When the temperature of the PbI2 substrate was slowly lowered past the temperature of the ice surface, crystal growth was observed first at the edges of the crystal and then at certain preferred sites on the PbI2 surface. If water drops appeared on the crystals, there was a reduction in the number of active, nucleating sites. This was especially noticed at the edges of the crystal. It was noted that the threshold temperature for ice crystal growth, as determined by visual observation, was ?6C. From the temperature-vapor pressure curve, it appears that PbI2 acts as a freezing nucleus from ?6C to about ?12C to ?14C and a sublimation nucleus at temperatures lower-than ?12C to ?14C.