Structural studies of phase transitions in crystalline and liquid Halides (ZnCl, AlCl) under pressure

Brazhkin, V. V.*; Katayama, Yoshinori; Lyapin, A. G.*; Popova, S. V.*; Inamura, Yasuhiro*; Saito, Hiroyuki; Utsumi, Wataru

The results of investigating the phase diagrams of ZnCl and AlCl halides, as well as the structure of the short range 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 ZnCl crystal is compressed, a phase transition occurs from the phase (HgI structure type) to the phase (distorted CdI structure, WTe type). The structural studies of the liquid state of ZnCl and AlCl 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 ZnCl and AlCl, 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 AlCl occurs at about 4 GPa and is a sharp first order transition, whereas the transition in ZnCl occurs more smoothly in a pressure range of 2-4 GPa with a maximum intensity near 3 GPa. Thus the AlCl and ZnCl compounds exemplify the existence of two phenomena, gradual decay of intermidiate-range structural correlations and a sharper liquid-liquid coordination transition.