First-Principles Molecular Dynamics Study of M3AlF6 (M= Li/Na/K) Molten Salts.

The microscopic properties and Raman spectra of molten, Li3AlF6, Na3AlF6, and K3AlF6 systems were investigated using first-principles molecular dynamics combined with the Voronoi tessellation method. The results have indicated that Li+, Na+, and K+ exist in a free state, whereas Al3+ and F- form ion clusters ([AlFx]3-x) with evidence of free F- anions. The nature of the alkali metal cation does not significantly affect the average Al-F bond length (1.775 Å). The coordination numbers of Al3+ and F- are 5.22, 5.21, and 4.95 in Li3AlF6, Na3AlF6 and K3AlF6, respectively, indicating a lower content of [AlF6]3- and [AlF5]2- in K3AlF6. The self-diffusion coefficients decrease in the order Li+ > Na+ > K+, and the trend is Na3AlF6 > Li3AlF6 > K3AlF6 for Al3+ and F-. The alkali metal cation has little effect on changes in the atomic charge and spin population of Al3+. Single bonds form between Al3+ and F- and exhibit uneven bond order. The upper limits of the HOMO-LUMO gaps for Li3AlF6, Na3AlF6 and K3AlF6 are, 4.82, 2.10, and 3.51 eV, respectively, suggesting higher conductivity of Na3AlF6 relative to Li3AlF6 and K3AlF6 under superheating conditions (40 K above the liquidus temperature).