Influence of doping on the surface feature and formation of PbI2 polytypes

A study of phase transformation at elevated temperature has been undertaken on 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 found to transform into well‐ordered crystals of the polytype 12R after heating for nearly 8 h at 150°C. During the intermediate period of heating the crystals show features of disorder, viz. streaking, arcing, or both, on their x‐ray diffraction photographs. On the other hand, the higher polytypes do not show any change in cell dimensions even after prolonged heating. In addition, it was found that if silver iodide is added in extremely minute quantities as an impurity during crystallization the process of phase transformation in the as‐grown crystals appreciably accelerates. Inspection by optical microscopy of these crystals has revealed the existence of a wide variety of growth features on their basal surfaces. The results have been discussed in detail. Copyright © 2004 John Wiley & Sons, Ltd.