Structure of molten MCI₃ systems from a polarizable ion simulation model

Computer simulations of a range of molten salts of stoichiometry MX₃ using a polarizable, formal charge ionic interaction model are described. The systems studied - LaCl₃, TbCl₃, and YCl₃ -span a range of cation sizes and the interaction model is a "generic" one, in the sense that the cation size is the only parameter in the interaction potential which distinguishes one system from another. The liquid structures predicted from the simulations are compared with recently obtained neutron diffraction data. Excellent agreement is found, except that the first coordination shell seems to be too tightly bound in the computer simulations. The cation in LaCl₃ is found to be 7-8 fold coordinate, and the coordination number drops to 6 for the smallest cation considered (Y³⁺), so that the coordination number in these systems does not change substantially on melting, in contrast to earlier reports. The polarization effects promote a significant degree of edge-sharing between these coordination polyhedra relative to predictions of a simple ionic pair potential. Associated with these changes is a shift to smaller scattering vectors of the prepeak in the structure factor and an increase in the fluidity.