X-ray absorption study of molten yttrium trihalides

The local structure and structural changes in molten YCl₃-LiCl-KCl and molten YBr₃-LiBr systems have been investigated by using a high-temperature extended X-ray absorption fine structure (EXAFS) technique. The behaviour of octahedral coordination of the halide ion (Cl^- and Br^-) around the Y³⁺ ion has been studied by EXAFS of the Y K-absorption edge. The nearest Y³⁺-Cl^- and Y³⁺-Br^- distances and coordination numbers of halide ions around the Y³⁺ ion do not change by mixing with the alkali halides. The stabilization of the (YCl₆)^3- and (YBr₆)^3- octahedral coordination by adding alkali halides was suggested by decreasing the Debye-Waller factor and the anharmonicity in the nearest Y³⁺-Cl^- and Y³⁺-Br^- interactions. The bridging structure of the (YBr₆)^3- octahedra sharing a Br^- ion in the molten YBr₃-LiBr system was studied by EXAFS of the Br K-absorption edge. The coordination number of Y³⁺ around the Br^- ion decreases from 2 in the pure melt to almost 1 in the 30mol% and 15mol% YBr₃ melts. This suggests that the bridging is almost broken and the stable octahedron exists freely in the LiBr-rich melts.