Common polytypes of PbI₂ at low and high temperatures and the 2H-12R transformation

PbI₂ single crystals grown in gel and grown from vapour were studied by X-ray diffraction. The structure analysis of the type 2H with space group Pm1 has revealed that the z coordinate of the I ion is 0.268 and the thermal vibration of the Pb ion in its own layer is considerably smaller as compared with that perpendicular to the layer. From the dependence of the frequencies of types 2H and 12R[13]₃ on temperature of crystal growth, it has been shown that type 2H is the most common type at room temperature and type 12R[13]₃ the most common type at high temperatures. Observations on the 2H-12R transformation by annealing have revealed that this transformation is reversible.