X-ray Diffraction, NMR Studies, and DFT Calculations of the Room and High Temperature Structures of Rubidium Cryolite, Rb 3 AlF 6

International audience; 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 rubid-ium 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.