Orientation relations between four phases of AgI

Starting with wurtzite-type single crystals, AgI was investigated at room temperature and pressures up to 0.5 GPa by means of a polarizing light microscope and with the precession technique. Distinct orientation relations were observed between four different AgI phases that occur successively under increasing pressure. At 0.35 GPa a reverse-obverse (111) twin of zincblende-type AgI is formed with twin plane parallel to the (001) plane of the wurtzite-type structure. The antilitharge type of AgI occurs at 0.39 GPa. Its tetragonal [001] direction runs parallel to one of the (100) directions of the zincblende type. At almost the same pressure the transformation to the NaCl-type form takes place. This AgI modification shows also twinning on (111) and the basis vectors are oriented parallel to those of the zincblende type. The transition between the zincblende- and the antilitharge-type phase shows a hysteresis: The back transformation occurs at 0.35 GPa. NaCl-type AgI transforms directly to the zincblende-type form at approximately 0.2 GPa when pressure is released. The observed orientation relations between zincblende-, antilitharge- and NaCl-type AgI support transition mechanisms that have been proposed earlier.