Phase transitions in melt-grown PbI2 crystals

Employing the method of zone- refining single crystals of PbI₂ of different purities were grown. Using X-ray diffraction technique their structure was found to be of the type 12R and in some cases 12R+4H. The crystals were re-examined after prolonged storage of several months at room temperature. Neither the crystals of very high purity nor of relatively less purity transformed. But thecrystals having intermediate purities did transform from 12R to 2H. This observation establishes a clear link between impurities and phase transitions. The results have been explained on the basis of nucleation and growth mechanism of stacking faults. Also it has been concluded that the presence of impurities is a must for the formation of polytypes.