First-principles molecular dynamics investigation on Na3AlF6 molten salt

Local structure and transport properties of Na3AlF6 molten salt were investigated by First-principles molecular dynamics (FPMD) simulation. For Na3AlF6 molten salt, the local ionic structure is governed by five-coordinated [AlF](2-) and six-coordinated [AlF6](3-). Coulomb force dominates the interionic interactions for Na3AlF6 molten salt. The first-shell average coordination number (CN) of Na-F, Al-F in the Na3AlF6 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 [AlFx](3-x) groups in Na3AlF6 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 Ne+ > F- >Al3+. Calculated results of viscosity and ionic conductivity are in good agreement with the experimental results, generally within 7%. (C) 2016 Elsevier B.V. All rights reserved.