First-principles molecular dynamics investigation on Na3AlF6 molten salt

•Local structure and transport properties of Na3AlF6 molten salt were studied by FPMD.•Six-coordinated aluminum complexes [AlF6]3− is the major species in molten cryolite.•The polymerization degree of local structure in molten cryolite is lower.•The order of ion diffusion ability was found to be Na+ > F− > Al3+.

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 [AlF5]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 Fb is small about 1–2%, while the free Ff 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 Na+> F− > Al3+. Calculated results of viscosity and ionic conductivity are in good agreement with the experimental results, generally within 7%.

Graphical abstractFor Na3AlF6 molten salt, the local ionic structure is governed by five-coordinated [AlF5]2− and six-coordinated [AlF6]3−. Coulomb force dominates the interionic interactions for Na3AlF6 molten salt. The first-shell average coordination number (CN) of NaF, AlF in the Na3AlF6 molten salt is 6.03, 5.45, respectively.Figure optionsDownload full-size imageDownload as PowerPoint slide