Effect of alkali cations on structure and stability of aluminum-oxygen-fluorine complex ions in aluminum electrolytes

Abstract

In the aluminium electrolytes, the structure and stability of different complex ions directly determine the properties of the melts, and further affect the process and technical-economic indicator for aluminum electrolysis. Herein, we investigate the effect of alkali cations on structure and stability of aluminum-oxygen-fluorine complex ions ([Al₂OF₆]²⁻ and [Al₂O₂F₄]²⁻) using structural characteristics, charge population analysis, Raman spectrum, cationic binding free energy, density of state based on density functional theory calculation. The research indicates the [Al₂OF₆]²⁻ ion is more stable than [Al₂O₂F₄]²⁻ ion for isolated Al-O-F ions. When adding the alkali cations, the cations with larger sizes enhance the stability of [Al₂OF₆]²⁻ and [Al₂O₂F₄]²⁻ ions, implying the Al-O-F ions are easier to form and exist in cryolite electrolytes with a sequence of K₃AlF₆>Na₃AlF₆>Li₃AlF₆. We can observe intuitively the part of electrons of Al atoms and cation disappear and then gather around O and F atoms from electron density difference diagrams. The calculated Raman frequencies of the Al-O-F ions are greatly consistent with the published literature value.

Keywords

aluminum-oxygen-fluorine complex ions / density functional theory / alkali cations / aluminum electrolytes / structure and stability

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