On the structure of crystalline and molten cryolite: Insights from the ab initio molecular dynamics in NpT ensemble

Ab initio molecular dynamics simulations in isobaric-isothermal ensemble have been performed to study the low- and the high-temperature crystalline and liquid phases of cryolite. The temperature induced transitions from the low-temperature solid (alpha) to the high-temperature solid phase (beta) and from the phase beta to the liquid phase have been simulated using a series of MD runs performed at gradually increasing temperature. The structure of crystalline and liquid phases is analysed in detail and our computational approach is shown to reliably reproduce the available experimental data for a wide range of temperatures. Relatively frequent reorientations of the AIF(6) octahedra observed in our simulation of the phase beta explain the thermal disorder in positions of the F- ions observed in X-ray diffraction experiments. The isolated AlF63-, AlF4-, as well as the bridged Al2Fm6-m ionic entities have been identified as the main constituents of cryolite melt. In accord with the previous high-temperature NMR and Raman spectroscopic experiments, the compound AIF(5)(2-) has been shown to be the most abundant Al-containing species formed in the melt. The characteristic vibrational frequencies for the AlFn3-n species in realistic environment have been determined and the computed values have been found to be in a good agreement with experiment. (C) 2016 AIP Publishing LLC.