Structural transitions in interionic force models of liquid AlCl₃

Abstract

We present molecular dynamics simulation of an interionic force model of liquid AlCl₃ along a few paths in the temperature-density plane. These paths include (1) an isobar and an isochore starting from the experimental standard freezing point (SFP), and (2) high-temperature isotherms starting from the isochore passing through the SFP. Our calculations show: the dissociation of dimers and higher molecular clusters into monomers with increasing temperature both along the experimental atmospheric pressure isobar and along the SFP isochore; and the pressure-induced molecular-to-ionic (MI) transition accompanied by, or followed by, solidification on increasing density along the two isotherms. The high-pressure solid structure is of the same layer type, with 6-fold coordinated metal ions, met at standard pressure. Crossing of the mean square displacements of the two ionic species provides a clear signal of the MI transition in the liquid. We discuss the consistency of our results with recent X-ray diffraction experiments on AlCl₃ under pressure.

Keywords:

• molten salts
• molecular liquids
• pressure induced transitions
• polarisable models

Acknowledgements

M.P.T. thanks Prof. V.E. Kratsov and the Condensed Matter and Statistical Physics Section of the Abdus Salam International Center for Theoretical Physics in Trieste for their hospitality.