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 <i>Fddd</i> (no. 70) space group with <i>a</i> = 37.26491(1) Å, <i>b</i> = 12.45405(4) Å, and <i>c</i> = 17.68341(6) Å. Comparison of NMR measurements and computational results revealed the dynamic rotations of the AlF₆ octahedra. Using <i>in situ</i> 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 <i>Fm</i>3<i>m</i>, 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.