Oxo- and Oxofluoroaluminates in the RbF-Al₂O₃ System: Synthesis and Structural Characterization

Precise research on the RbF-Al₂O₃ system was carried out by means of combining X-ray powder diffraction, high-field solid-state NMR spectroscopy, and thermal analysis methods. α-Rb₃AlF₆, RbAlO₂, Rb₂Al₂₂O₃₄, and new phase, Rb₂Al₂O₃F₂, were identified in the system. The structure of this new rubidium oxofluoroaluminate was determined. It is built up from single layers of oxygen-connected AlO₃F tetrahedra, those layers beeing separated by fluorine atoms. This type of structure exhibits a decent ionic conductivity at ambient temperature, 1.74 × 10^-6 S cm^-1. The similar structural arrangement of O₃Al-O-AlO₃ and FO₂Al-O-AlO₂F tetrahedra of the conduction planes in Rb₂Al₂₂O₃₄ and Rb₂Al₂O₃F₂ were confirmed by ²⁷Al NMR measurements. A thermal analysis of the RbF-Al₂O₃ system revealed that it can be defined as a pseudobinary subsystem of the more general quaternary RbF-AlF₃-Al₂O₃-Rb₂O phase diagram. From a phase analysis of individual phase fields, the mutual metastable behavior of all founded phases can be considered. It was observed that fluoro- and oxoaluminates exist together. Rb₂Al₂O₃F₂ is more stable under high temperature. Rubidium fluoro- and oxoaluminates are metastable precursors of the thermodynamically more stable structure of rubidium oxofluoroaluminate.