Effect of alkaline metal cations on the ionic structure of cryolite melts: <i>Ab-initio</i> NpT MD study

Abstract

Ab initio molecular dynamics simulations in an NpT ensemble have been performed to study the role of alkaline metal cations (Me = Li, Na, K, Rb) on the structure and vibrational properties of melts of Me-cryolites (Me₃AlF₆) at T = 1300 K. In all melts examined in this work, the species AlF₅^2- has been found to be formed at the highest abundance [from 58% (Li) to 70% (Na)] among the Al-containing anionic clusters. The concentration of clusters AlF₄^- increases with the size of cations while that of anions AlF₆^3- follows the opposite trend and it becomes negligible in the melts of the K- and Rb-cryolites. The computed percentage of the Al atoms participating in the formation of dimers Al₂F_m^6-m bridged via common F atoms is significant only in the case of Li- and Na-cryolites (16% and 10%, respectively) and the formation of even larger aggregates is found to be unlikely in all four melts. The percentage of the F atoms that are not bound to Al is ∼20% in all four melts and the ions formed by Me⁺ and F^- are found to be only short-lived. Vibrational analysis has been performed using the velocity autocorrelation functions computed for the Cartesian and selected internal coordinates describing Raman-active symmetric stretching vibrations of different AlF_n species. The results of vibrational analysis allowed us to identify trends in the variation of positions and shapes of peaks corresponding to the anionic fragments AlF₄^-, AlF₅^2-, and AlF₆^3- with the size of cations, and these trends are found to be consistent with those deduced from the available Raman spectroscopy experiments. Our findings represent a new insight into the properties of cryolite melts, which will be useful for the interpretation of experimental data.