On the Phase Transition in Cuprous Iodide

The phase transition and the crystal structures of cuprous iodide (CuI) have been studied by the X-ray and calorimetric measurement Two phase transitions at 369°C and 407°C were found and the corresponding energy and entropy changes were estimated from the specific heattemperature curve. The crystal lattice of γ, β and α-phases were determined to be the cubip (ZnS type), hexagonal (ZnO type) and cubic, respectively, by the X-ray powder method. The intensity change with temperature was measured in detail by a new method by which the begins temperature factor can be taken into account more exactly. From this measurement, it was found that the crystal structure of this substance begins to change anomalously already at about 200°C, and finally transforms into the high temperature form having an average structure. That is to say, there are some metastable positions around every copper position of the low temperature form and the probability of finding the Cu-atoms in these positions gradually increases with temperature and finally, in the α-Phase region, the atoms are equally distributed over all the positions. It was concluded that these probable positions are 4( c ): (1/4 1/4 1/4) and 16( e ): ( x x x , x \bar x \bar x , \bar x x \bar x , \bar x \bar x x ) of T a 2 with the parameter x ≈0.34±0.01. The entropy change expected from this proposed structure change is fairly in agreement with the value obtained by the calorimetric measurement.