The quaternary fluoride LiNaCa2Al2F12 in aluminum electrolytes: synthesis, structure, thermal stability

Abstract

The quaternary fluoride phase LiNaCa2Al2F12 has been obtained by quenching a melt consisting of a stoichiometric mixture of the corresponding simple fluorides at a temperature of 850 ​°C in air. Previously, the phase has been observed by authors as unknown impurity in cooled samples of aluminum production electrolyte. It has been established that the phase does not form solid solution regions with the initial components. The crystal structure has been solved by X-ray powder diffraction technique: a ​= ​5.053(2) Å, c ​= ​17.546(7) Å, V ​= ​448.02 ​Å3, Z ​= ​2, SG. P-421c. Calcium cations and isolated octahedral [AlF6]3- anions form mixed layers, which are separated by flat nets of lithium and sodium cations. Lithium cations occupy positions with a tetrahedral environment of fluorine atoms. The short interatomic distance Li⋯F 1.89 ​Å makes the position unavailable for other cations. Thermal decomposition has been studied by high-temperature X-ray diffraction and thermal analysis. The LiNaCa2Al2F12 phase is stable up to 600 ​°C in air, and then decomposes in the solid state. The final products of decomposition are fluorite and aluminum oxides and aluminates. It has been concluded that the phase was metastable and it was the result of rapid cooling of the melt. The stability at low temperatures has explained by high potential barrier for transformation. The formation of LiNaCa2Al2F12 indicates the presence in the melt with fluorine deficiency of isolated [AlF6]3- anions, the concentration of which increases due to the existence of coordinated Li+-[AlF6]3- pairs in the melt. It is suggested that the presence of [AlF6]3- anions in acidic electrolyte melts ensures the dissolution of alumina.