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

Abstract

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 ([AlF_x]^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).