First-principles molecular dynamics investigation on Na3AlF6 molten salt

Abstract

Abstract Local structure and transport properties of Na 3 AlF 6 molten salt were investigated by First-principles molecular dynamics (FPMD) simulation. For Na 3 AlF 6 molten salt, the local ionic structure is governed by five-coordinated [AlF 5 ] 2− and six-coordinated [AlF 6 ] 3− . Coulomb force dominates the interionic interactions for Na 3 AlF 6 molten salt. The first-shell average coordination number (CN) of Na-F, Al-F in the Na 3 AlF 6 molten salt is 6.03, 5.45, respectively and the F-Al-F bond angles are mainly located at 87°, 124° and 171°. The percentage of bridging F b is small about 1–2%, while the free F f is up to 26%, suggesting the polymerization degree of local structure is lower. Al-F bonds of the [AlF x ] 3−x groups in Na 3 AlF 6 molten salt have ionic characters as well as partial covalent characters due to the hybridization of F-2p and Al-3s (3p) orbitals, while the Na-F and F-F bonds are mainly ionic. The order of ion diffusion ability was found as Na + > F − > Al 3+ . Calculated results of viscosity and ionic conductivity are in good agreement with the experimental results, generally within 7%.