From molecular clusters to liquid structure in AlCl3 and FeCl3

Abstract

Melting of aluminum and iron trichloride is accompanied by a Structural transition front sixfold to fourfold coordination of the trivalent metal ions, and a widely accepted interpretation of the Structure of their melts near freezing is that they mainly consist of strongly correlated dimers formed from two edge-sharing tetrahedra. We carry Out classical molecular dynamics simulations to examine how a polarizable-ion force law, determined on isolated molecular monomers and dimers ill the gaseous phase of these compounds, Fares in accounting for the pair Structure Of their liquid phase and for mean square displacements and diffusion coefficients of the two species ill each melt. The model reproduces the main features of the neutron diffraction Structure Factor, showing peaks due to intermediate range order and to charge and density short-range order, and accounts for the experimental data at a good semi-quantitative level. We find agreement with the neutron and X-ray diffraction data oil metal-halogen and Cl-Cl bond lengths in the melt, and demonstrate the high sensitivity of the results for the width of the first-neighbor shell to truncation ill obtaining it by Fourier transform or the neutron-weighted structure factor in momentum space. We also report comparisons with it recent first-principles Study of the structure of the AlCl3 melt by the Car-Parrinello method. Finally, we demonstrate break-Up of dimers into monomers upon raising the liquid temperature ill the case of AlCl3.