Analysis of the microscopic properties and Raman spectra of molten NaF-AlF₃-CaF₂-Al₂O₃ using first-principles molecular dynamics

Abstract First-principles molecular dynamics (FPMD) and Voronoi tessellation have been employed to investigate the microscopic properties and Raman spectra of molten NaF-AlF 3 -CaF 2 -Al 2 O 3 systems with varying cryolite ratios (molar ratio of NaF to AlF 3 ) and alumina concentrations. The findings reveal that Na + and Ca 2+ are predominantly present in a free state, whereas Al 3+ , F − and O 2− form complex ion groups. Al–F and Al–O ion pairs have an identical average bond length of 1.725 Å, unaffected by cryolite ratio or alumina concentration. The Na–F and Ca–F pairs have a common bond length of 2.275 Å. Increasing alumina concentration decreases the coordination number of Al–F while increasing that of Al–O. The self-diffusion coefficient of Na + (13.9E-5 cm 2 s −1 ) exceeds that of O 2− (8.85E-5 cm 2 s −1 ), with F − diffusing slightly faster than Al 3+ . Both cryolite ratio and alumina concentration influence ion diffusion. Fluctuations in atomic charges and spin populations were observed, but these were minimally affected by cryolite ratio or alumina concentration. Al 3+ and F − primarily form single bonds while double bonding in Al–F is observed in the 2.6NaF-AlF 3 -CaF 2 -4wt%Al 2 O 3 system. Raman spectra indicate that the Al–F cluster primarily consists of [AlF 4 ] − , [AlF 5 ] 2− and [AlF 6 ] 3− , while Al–O–F complex ion groups occurs with the incorporation of O 2− ions in Al–F ion clusters or replacement of F − ions in the complexes. As the cryolite ratio increases, the dominant Al–F ion cluster shifts from [AlF 4 ] − to [AlF 6 ] 3− . Concurrently, the concentration of Al–O–F complex increases with increasing alumina concentration with a higher proportion of O 2− in these Al–O–F clusters. This study establishes the relationship between the microscopic properties and the composition of aluminum electrolytes, demonstrating the suitability of First-Principles Molecular Dynamics (FPMD) combined with Voronoi tessellation for probing the microstructural properties of aluminum electrolytes.