Simulation study of Al3+ coordination in NaF-AlF3 mixtures:: development of interaction potentials

The generation of suitable interaction potentials with which to study the coordination chemistry of Al3+ ions in NaxAlF3+x mixtures by computer simulation is described. The potentials reflect an 'extended ionic' model of the interactions in this system, with formal ionic charges and many-body polarization effects included. The parameters are derived from a mixture of ab initio and empirical information. For the Al-F interactions, the structure and energetics of crystalline and molecular AlF3 form the database. This potential is then combined with an NaF potential and used to predict the crystal structures of three NaxAlF3+x crystalline compounds and also the vibrational frequencies of the NaAlF4 molecule, which have been obtained experimentally. Excellent agreement is found. The potentials will be used in simulations of the spectroscopic and structural properties of Na(x)AIF(3+x) melts.