Structure and Raman Spectra in Cryolitic Melts: Simulations with an ab Initio Interaction Potential

The Raman spectra of cryolitic melts have been calculated from molecular dynamics computer simulations using a polarizable ionic potential obtained by force-fitting to ab initio electronic structure calculations. Simulations which made use of this ab initio derived polarizable interaction potential reproduced the structure and dynamical properties of crystalline cryolite, Na3AlF6, rather well. The transferability of the potential model from solid state to the molten state is tested by comparing results for the Raman spectra of melts of various compositions with those previously obtained with empirically developed potentials and with experimental data. The shapes of the spectra and their evolution with composition in the mixtures conform quite well to those seen experimentally, and we discuss the relationship between the bands seen in the spectra and the vibrational modes of the AlFn(3-n) coordination complexes which are found in the NaF/AlF3 mixtures. The simulations thus enable a link between the structure of the melt as derived through Raman spectroscopy and through diffraction experiments. We report results for quantities which relate to the degree of cross-linking between these coordination complexes and the diffusive properties of ions.