Structure of molten trivalent metal chlorides studied by using neutron diffraction: The systems TbCl₃, YCl₃, HoCl₃ and ErCl₃

The total structure factors of the molten trivalent metal chlorides MCI₃, where M denotes Tb, Y. Ho or Er, were measured by using neutron diffraction. All four salts melt with a small volume change of ≤5% from structures comprising MCI₆ ^3- octahedra to form liquids which show intermediate-range ionic ordering as manifest by a first sharp diffraction peak at ≈ 1 Å^-1 that moves to smaller scattering vector values with decreasing cation size. In the liquid state, the M-C1 distances are comparable with the sum of the ionic radii and the results are consistent with the existence of distorted MCI₆ ^3- octahedral units The implications of the results for a polarizable ion interaction model used in recent molecular dynamics calculations are briefly discussed.