Molecular-network–ionic structure transitions in liquidAlCl3 and ZnCl2 halogenides under pressure

Abstract

We present the in situ high-pressure–high-temperature x-ray diffraction study of the liquidAlCl3 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 theseliquid halogenides is indicative of a rapid and strong breakdown of an intermediate-rangeorder in a tetrahedral network of melts for the initial pressure range, 0–2.5 GPa forAlCl3 and0–1.8 GPa for ZnCl2, and points to rather sharp transitions in liquids with the formation of ashort-range order structure similar to ionic melt structures around 4 GPa forAlCl3 and3 GPa for ZnCl2. Thus, pseudo-covalent liquid halogenides likeAlCl3 and ZnCl2 provide testimony to two phenomena under high pressures, namely, a gradual decay ofstructural correlations in the tetrahedral network of the melt and a sharp transition frommolecular-network to ionic structure in liquid on further compression. Such atwo-stage structural transformation under pressure is the general feature for awide class of simple melts, including most of the pseudo-covalent halogenides.