Structure of acidic haloaluminate melts: Neutron diffraction and quantum chemical calculations

The structure of molten mixtures of (KX)y(AlX3)1−y was investigated by neutron diffraction for X=Cl and Br and y=0.25 and 0.33. These melts, known as acidic haloaluminates, contain different ionic species such as AlX−4,Al2X−7, Al3X−10. The diffraction patterns were similar, although not identical, for the same halide pairs and showed features indicative of short-range and intermediate-range order. Quantum chemical calculations were carried out on AlX−4, Al2X−7, and Al3X−10 to provide information needed to interpret the structural results within the framework of the random packing of structural units model. It is shown that the bent Al–X–Al bridge for Al2X−7, obtained from the quantum chemical calculations, is necessary to represent well the structure, and that the values of the bond angles at the central halide atoms are around 110°. Aluminum atoms are found to be tetrahedrally coordinated by the halide atoms, and information on the orientational correlations of neighboring Al2X−7 is derived.