Covalent-to-ionic transition in liquid zinc dichloride

We report molecular-dynamics simulations of self-diffusion and structure in a pseudoclassical model of liquid and crystalline ZnCl₂ over a wide region of the pressure-temperature plane. The model parameters are adjusted to reproduce a liquid structure of corner-sharing ZnCl₄ tetrahedra at the standard freezing point and the measured diffusion coefficients as functions of temperature on the sfp isobar. We find that compression first weakens the intermediate-range order of the melt near freezing into a fourfold-coordinated crystal structure, and then drives at higher temperatures a novel liquid-liquid transition consisting of two broad steps: (i) a transition in which the Zn atoms start to leave their tetrahedral cages, followed by (ii) a structural transition from a covalent network of Cl atoms to a dissociated ionic liquid which then freezes into a sixfold-coordinated crystal. Good agreement is found with data from X-ray diffraction experiments under pressure.