First-Principles Molecular Dynamics Simulation on High Silica Content Na₃AlF₆-Al₂O₃-SiO₂ Molten Salt

A new method for the disposal of the spent refractory materials by adding them directly to electrolytic cells requires our better knowledge of the Na₃AlF₆-Al₂O₃-SiO₂ melt system. The development of computational materials science offers us a new way to avoid the limitation of the experiment under a strong corrosive environment at high temperatures. First-principles simulation is applied to study the structure information, electronic properties, and transport properties of the system. The study reveals that the main Si and Al ions in the melt are complex ion groups such as [SiF₂O₂]^2-, [SiFO₃]^3-, [SiF₃O₂]^3-, [AlF₂O₂]^3-, [AlF₃O]^2-, and [AlF₄O]^3-. Tangled structures like [SiAlO₃F₅]^4- also exist in the melt. The average coordination number of Al-F and Si-F is 3.21 and 2.45, respectively. O ions mainly act as bridge ions in the melt. The bonding ability of Al with O ions is stronger than that of Si with O ions. Moreover, the Al-O bond is mainly covalent, while the Al-F bond is basically ionic characters. The order of diffusion ability of ions from large to small is Na, F, Al, Si, and O. Addition of SiO₂ into the Na₃AlF₆-Al₂O₃ molten salt causes an increase of the viscosity and a decrease of ionic conductivity.