Phase transformations in PbI2 crystals with temperature

A systematic study of phase transformation with temperature has been undertaken in single crystals of lead iodide grown in gel. The crystals of the polytype 2H, which is known to be the common modification of PbI₂ at room temperature, have been finally found to transform into polytype 12R after heating at 150°C. During the intermediate period of heating the crystals show features of disorder, viz. streaking or arcing or both, on their X-ray diffraction photographs. The higher polytypes 12H and 16H do not show any change even after prolonged heating at 150°C. If silver iodide is added in a minute quantity during crystallization, the process of phase transformation is appreciably altered. The results have been discussed.