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 AlX4-, Al2X7- , Al3X10- . 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 AlX4-, Al2X7- , and Al3X10- 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 Al2X7-, 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-degrees. Aluminum atoms are found to be tetrahedrally coordinated by the halide atoms, and information on the orientational correlations of neighboring Al2X7- is derived.