Growth and Polytype Transformation of AgI-Doped and Undoped PbI₂ Crystals

Both undoped and AgI-doped single crystals of lead iodide have been grown by the vapour technique and characterized by X-ray diffraction (XRD) to study their polytype formation. All undoped crystals are exclusively found to contain the stable high-temperature modification, polytype 12R, while nearly 2/3 of the doped crystals are found to consist additionally of the polytype 4H and the room-temperature modification 2H. The AgI doping is held to be responsible for the formation of 2Hat the prevalent high temperature of crystal growth. The crystals were subsequently stored for nearly seven months and re-examined through XRD to study possible polytype transformation in them. The undoped crystals did not transform at all but all the doped crystals transformed into the stable room-temperature polytype 2H. It is concluded that only those crystals that initially have an appreciable number of 2H nuclei present eventually transform into the stable 2H structure after prolonged storage.