Ionic to molecular transition in AlCl3:: an examination of the electronic structure

AlCl3 crystallizes as an ionic solid but melts to form a molecular liquid consisting of Al2Cl6 units. In order to see if this transition involves a major change in electronic structure, the dimer and crystal of AlCl3 are examined with the aid of generalized gradient corrected density functional theory (GGC-DFT) calculations and the electronic wavefunctions examined with the aid of a Wannier localisation transformation of the Kohn-Sham eigenfunctions. The change from octahedral to tetrahedral coordination of Al which is observed on melting has been induced by simply rescaling the unit cell parameters, and the variations in the hybridization of the Wannier orbitals across the transition have been analysed thoroughly. The predominantly ionic character of the interactions across the ionic to molecular transition is confirmed, validating the use of ionic interaction potentials to represent AlCl3 and related systems. Dipole moments of the single ions have been estimated from the position of the Wannier function centres and they are found to reproduce with remarkable accuracy the values predicted by a polarizable ionic interaction potential. A similar analysis has been carried over to the AlBr3-Al2Br6 system, which is known to consist of molecular units in both crystal and gas phase.