Structure of molten lanthanum and cerium tri-halides by the method of isomorphic substitution in neutron diffraction

The total structure factors of the molten trivalent metal halides MX₃, where M³⁺ denotes La³⁺ or Ce³⁺ and X^- denotes Cl^-, Br^- or I^-, have been measured by using neutron diffraction. Difference function methods were then applied on assuming that the LaX₃ and CeX₃ melts for a given halide ion are isomorphic. The results which follow from this assumption show that the first sharp diffraction peak in the measured total structure factors arises from cation correlations and its movement to lower scattering vector values with increasing anion size is consistent with an enhanced separation in real space of cation centred polyhedra. On melting the MX₃ salts exhibit a decrease in the coordination number of both the cations and anions. In the liquid state the M-X coordination environment is asymmetric with M-Cl, M-Br and M-I nearest-neighbour distances of 2.93(2) Å, 3.01(2) Å, 3.18(2) Å and M-Cl, M-Br and M-I coordination numbers of 8.2(2), 7.4(2), 6.7(2) respectively. The Cl-Cl, Br-Br and I-I nearest-neighbour distances are 3.58(3) Å, 3.76(3) Å, 4.13(2) Å respectively and there is a significant penetration of the X-X partial pair distribution function into the first peak of the M-X partial pair distribution function for all three anions. The Cl-Cl, Br-Br and I-I coordination numbers are 9.2(2), 8.7(2) and 8.2(2) respectively if the M-M coordination number is two.