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

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 (no. 70) space group with a = 37.26491(1) angstrom, b = 12.45405(4) angstrom, and c = 17.68341(6) angstrom. Comparison of NMR measurements and computational results revealed the dynamic rotations of the AIF 6 octahedra. Using in situ variable temperature MAS NMR measurements, the chemical exchange between rubidium sites was observed. The beta-phase, i.e., high temperature polymorph, adopts the ideal cubic double-perovskite structure, space group Fm (3) over barm, with a = 8.9930(2) angstrom at 600 degrees C. Additionally, a series of polymorphs of K3AlF6 has been further characterized by high field high temperature SSNMR and DFT computation.