Structural studies of phase transitions in crystalline and liquid halides (ZnCl2, AlCl3) under pressure

The results of investigating the phase diagrams of ZnCl2 and AlCl3 halides, as well as the structure of the shortrange order of the corresponding melts under pressures up to 6.5 GPa, by the method of energy-dispersive x-ray diffraction are reported. When a ZnCl2 crystal is compressed, a phase transition occurs from the γ phase (HgI2 structure type) to the δ phase (distorted CdI2 structure, WTe2 type). The structural studies of the liquid state of ZnCl2 and AlCl3 indicate that the intermediate-range order decreases rapidly in the tetrahedral network of both melts as the pressure increases to 1.8 and 2.3 GPa for ZnCl2 and AlCl3, respectively. With further compression, the transitions in both melts occur with a change in the structure of the short-range order and with an increase in the coordination number. In this case, the transition in AlCl3 occurs at ≈4 GPa and is a sharp first order transition, whereas the transition in ZnCl2 occurs more smoothly in a pressure range of 2–4 GPa with a maximum intensity near 3 GPa. Thus, the AlCl3 and ZnCl2 compounds exemplify the existence of two phenomena—gradual decay of intermediate-range structural correlations and a sharper liquid-liquid coordination transition.