The structure of liquid AlCl₃and structural modification in AlCl₃-MCl (M=Li, Na) molten salt mixtures

Abstract

The scattering from pure liquid AlCl3 and AlCl3-MCl (M=Li, Na) binary molten salt mixtures has been measured by time-of-flight neutron diffraction. Reverse Monte Carlo simulation of the AlCl3 data whilst confirming a regular tetrahedral local geometry for the Al3+ ion, challenges the accepted view of the structure as consisting (predominantly) of discrete Al2Cl6 dimers. The data for the mixtures suggest that the connectivity in the structure of pure AlCl3, formed by Al-Cl-Al bridges, is proportionately reduced upon the addition of alkali halide as isolated AlCl4- units are created. In addition, a gradual shift in the position of the first sharp diffraction peak to higher Q is observed, implying a systematic reduction in the length scale of intermediate-range order. The findings for the mixtures are generally consistent with those from previous structural studies using X-ray diffraction and also with Raman and electrochemical measurements.