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, wher e M3+ denotes La3+ or Ce3+ and X- denotes Cl-, Br- or I-, have been measure d by using neutron diffraction. Difference function methods were then appli ed on assuming that the LaX3 and CeX3 melts for a given halide ion are isom orphic. The results which follow from this assumption show that the first s harp diffraction peak in the measured total structure factors arises from c ation correlations and its movement to lower scattering vector values with increasing anion size is consistent with an enhanced separation in real spa ce 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 s tate the M-X coordination environment is asymmetric with M-CI, M-Br and M-I nearest-neighbour distances of 2.93(2) Angstrom, 3.01(2) Angstrom, 3.18(2) Angstrom 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) Angstrom, 3.76(3) Angstrom, 4.13(2) Angstrom, respectively and the re is a significant penetration of the X-X partial pair distribution functi on into the first peak of the M-X partial pair distribution function for al l 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.