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

Abstract

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.

New examples of non-cooperative octahedral tilting in a double perovskite: phase transitions in K3GaF6

“…Similar behavior was proposed for -K 3 AlF 6 and verified by modeling pair distribution function data (King et al, 2011). Dynamic octahedral rotations were also observed in several polymorphs of K 3 AlF 6 and Rb 3 AlF 6 using nuclear magnetic resonance spectroscopy (Rakhmatullin et al, 2020). Such rotations imply that the actual bond lengths are a little different than those given by the average crystal structure.…”

“…It undergoes three phase transitions at higher temperature to structures where the number of AlF 6 rotated by $45 is one fifth, then one sixth, ISSN 2052-5206 # 2020 International Union of Crystallography and finally the cubic double perovskite structure with no octahedral tilting (King et al, 2011). Very recently Rb 3 AlF 6 was reported to have the same structure as -K 3 AlF 6 at room temperature (Rakhmatullin et al, 2020). There are also a few oxide and oxyfluoride compounds which have structures based on non-cooperative octahedral tilting (Bramnik et al, 2000;King et al, 2010King et al, , 2012King et al, , 2017Stö ger et al, 2010;Fry & Woodward, 2013).…”

Sodium Site Exchange and Migration in a Polar Stuffed-Cristobalite Framework Structure

“…Variable temperature 23 Na MAS NMR data (up to 730 °C) were acquired on a Bruker AVANCE III 750 MHz NMR spectrometer using the diode laser-heated NMR probe head described previously. 21 The sample was sandwiched between two layers of ground KBr which allowed for monitoring of the effective sample temperature through the shift of 79 Br resonance signal of KBr. 22 In the high temperature laser heating setting, the sample is contained in an aluminum nitride container placed in a zirconia rotor and immobilized with two O-rings made from the sodium-containing ceramic that give some secondary signals in the high temperature experiments.…”

Polymorphs of Rb3ScF6: X-ray and Neutron Diffraction, Solid-State NMR, and Density Functional Theory Calculations Study

“…Room temperature solid-state MAS NMR spectra were recorded on a Bruker Avance III HD 17.6 and Avance NEO 20 T spectrometers using 1.3 and 2.5 mm resonance probes at MAS frequency of 60 or 30 kHz, respectively. 19 F, 45 Sc, and 87 Rb RF strengths were 190, 464, and 83 kHz, respectively. 87 Rb MQMAS spectra were acquired by two different ways, i.e.…”

“…Different U values were used for previously studied fluoroscandates. 12,28 The correlations between calculated isotropic 45 Sc and 19 F magnetic shielding constants (σ iso ) and experimentally determined isotropic (δ iso ) chemical shift values has been plotted and fitted by linear regressions (Figure S1). From these fits, R-squared values were obtained.…”