Structure of molten ScCl3 and ScI3 studied by using neutron diffraction

The structure of the salts ScX3, where X- denotes Cl- or I-, was investigat ed by using neutron diffraction. On heating ScCl3, a phase transition from the FeCl3-type crystal structure to possibly the YCl3-type crystal structur e was observed whereas ScI3 remained in its room temperature FeCl3-type cry stal structure up to the melting point. On melting, there is a reduction in the Sc-X coordination number from six to 4.8(2) for ScCl3 and from six to 4.7(2) for ScI3 and the Sc-Cl and Sc-I nearest-neighbour distances in the l iquid are 2.48(2) Angstrom and 2.76(2) Angstrom respectively. The measured total structure factors of molten ScCl3 and ScI3 each have a prominent firs t sharp diffraction peak at a scattering vector value of 0.75(5) Angstrom(- 1). The results are discussed in the context of those available for several other trivalent metal halide systems which also melt from the closely rela ted YCl3-type and FeCl3-type crystal structures.