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

Abstract

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.

X-ray Diffraction, NMR Studies, and DFT Calculations of the Room and High Temperature Structures of Rubidium Cryolite, Rb3AlF6

“…The synthesis protocol of rubidium cryolite was identical to that described in our previous work. 19 Rubidium tetrafluoroaluminate was prepared by heating a stoichiometric mixture of RbF and AlF3. Two grams of mixture were mechanically homogenized in a glovebox under inert atmosphere (Ar -Messer, 99.999 % purity), placed in a Pt crucible and heated in a tightly closed vertical resistance furnace with water-cooling from room temperature to 600 °C, at a rate of 5 °C/min.…”

“…The classical thermal analysis was performed as described by Šimko et al 19 in a Pt crucible with a lid. The cooling rate was 1.4 °C/min.…”

“…Room temperature solid-state NMR experiments were carried out on a Bruker Avance III NMR spectrometers operating at 9.4 T and 20.0 T, using a 2.5 mm resonance probe at MAS frequency varying between 30 and 34 kHz. In addition, to enhance resolution in some of the 19 F solid state NMR experiments, we used a 1.3 mm probe to be able to rotate samples up to 67 kHz. 27 Al NMR spectra were acquired using a double frequency sweep (DFS) pulse due to a possibility of signal enhancement.…”

“…The parameters of the dipolar 87 Rb-19 F HMQC 21,22 experiments were identical to our previous study. 19 87 Rb MQMAS spectra were acquired with Z-filtering 23 and hypercomplex (States) 24 phase detection. A sweep width of 60 kHz and 120 kHz (4νr) was used in the direct and indirect dimension, respectively.…”

“…In all in-situ HT experiments, the MAS frequency was 5000 Hz. Because of the broad NMR patterns of 19 F and 87 Rb resulting from large chemical shift anisotropy and low MAS rate, two-dimensional (2D) NMR techniques, MATPASS 25 and QMATPASS, 26 for 19 F and 87 Rb respectively that able to separate the isotropic and the anisotropic chemical shifts in two different dimensions were employed, as ideal approach to study our samples at very high temperature. They yield high-resolution NMR isotropic peaks free of CSA-related broadening in one dimension and the corresponding anisotropic powder sideband pattern for each isotropic peak, in the second dimension.…”