Molecular-network-ionic structure transitions in liquid AlCl3 and ZnCl2 halogenides under pressure

We present the in situ high-pressure-high-temperature x-ray diffraction study of the liquid AlCl3 and ZnCl2 halogenides having a quasi-molecular network structure in liquid state at normal pressure. These liquids are intermediate between pure covalent and ionic melts. Structural study of these liquid halogenides is indicative of a rapid and strong breakdown of an intermediate-range order in a tetrahedral network of melts for the initial pressure range, 0-2.5 GPa for AlCl3 and 0-1.8 GPa for ZnCl2, and points to rather sharp transitions in liquids with the formation of a short-range order structure similar to ionic melt structures around 4 GPa for AlCl3 and 3 GPa for ZnCl2. Thus, pseudo-covalent liquid halogenides like AlCl3 and ZnCl2 provide testimony to two phenomena under high pressures, namely, a gradual decay of structural correlations in the tetrahedral network of the melt and a sharp transition from molecular-network to ionic structure in liquid on further compression. Such a two-stage structural transformation under pressure is the general feature for a wide class of simple melts, including most of the pseudo-covalent halogenides.