Local Coordination and Medium Range Order in Molten Trivalent Metal Chlorides: The Role of Screening by the Chlorine Component

Abstract

Earlier work has identified the metal ion size R_M as a relevant parameter in determining the evolution of the liquid structure of trivalent metal chlorides across the series from LaCI₃(R_M ≍ 1.4 A) to AICI₃(R_M ≍ 0.8 A). Here we highlight the structural role of the chlorines by contrasting the structure of fully equilibrated melts with that of disordered systems obtained by quenching the chlorine component. Man attention is given to how the suppression of screening of the polyvalent ions by the chlorines changes trends in the local liquid structure (first neighbour coordination and partial radial distribution functions) and in the intermediate range order (first sharp diffraction peak in the partial structure factors). The main microscopic consequences of structural quenching of the chlorine component are a reduction in short range order and an enhancement of intermediate range order in the metal ion component, as well as the suppression of a tendency to molecular-type states at the lower end of the range R_M.

Key Words:

• Coordination number, partial structure factors
• screening by chlorine.