A crystallographic approach incorporating multinuclear high field solid state NMR (SSNMR), X-ray structure determinations, TEM observation, and density functional theory (DFT) was used to characterize two polymorphs of rubidium cryolite, Rb3AlF6. The room temperature phase was found to be ordered and crystallizes in the Fddd (n°70) space group with a = 37.26491(1) Å, b = 12.45405(4) Å, c = 17.68341(6) Å. Comparison of NMR measurements and computational results revealed the dynamic rotations of the AlF6 octahedra. Using in-situ variable temperature MAS NMR measurements, the chemical exchange between rubidium sites was observed. The β-phase, i.e. high temperature polymorph, adopts the ideal cubic double-perovskite structure, space group 3 ̅ , with a = 8.9930(2) Å at 600 °C. Additionally, a series of polymorphs of K3AlF6 has been further characterized by high field high temperature SSNMR and DFT computation.
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.
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