ATOMIC SIZE EFFECTS ON LOCAL COORDINATION AND MEDIUM-RANGE ORDER IN MOLTEN TRIVALENT METAL CHLORIDES

Structural correlations in molten trivalent metal chlorides are evaluated as functions of the metal ion size R(M) across the range from LaCl3(R(M) almost-equal-to 1.4 angstrom) to AlCl3 (R(M) almost-equal-to 0.8 angstrom,) using a charged soft-sphere model and the hypernetted chain approximation. Main attention is given to trends in the local liquid structure (partial radial distribution functions, coordination numbers and bond lengths) and in the intermediate-range order (first sharp diffraction peak in the number-number and partial structure factors). The trend towards fourfold local coordination of the metal ions, the stabilization of their first-neighbour chlorine cage and the growth of medium-range order are found to proceed in parallel as the size of the metal ion is allowed to decrease at constant number density and temperature. A tendency to molecular-type local structure and liquid-vapour phase separation is found within the hypernetted chain scheme at small metal ion sizes corresponding to AlCl3 and is emphasized by decreasing the number density of the fluid. The predicted molecular units are rather strongly distorted Al2Cl6 dimers, in agreement with observation. The calculated structural trends for other trichlorides are compared with diffraction and transport data.