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

The crystal structures of three polymorphs of Rb3ScF6 have been determined through a combination of synchrotron, laboratory X-ray, and neutron powder diffraction, electron diffraction, and multinuclear high-field solid-state NMR studies. The room temperature (RT; alpha) and medium-temperature (beta) structures are tetragonal, with space groups I4(1)/a (Z = 80) and I4/m (Z = 10) and lattice parameters a = 20.2561(4) angstrom, c = 36.5160(0) angstrom and a = 14.4093(2) angstrom, c = 9.2015(1) angstrom at RT and 187 degrees C, respectively. The high-temperature (gamma) structure is cubic space group Fm (3) over barm (Z = 4) with a = 9.1944(1) angstrom at 250 degrees C. The temperatures of the phase transitions were measured at 141 and 201 degrees C. The three alpha, beta, and gamma Rb3ScF6 phases are isostructural with the alpha, beta, and delta forms of the potassium cryolite. Detailed structural characterizations were performed by density functional theory as well as NMR. In the case of the beta polymorph, the dynamic rotations of the ScF6 octahedra of both Sc crystallographic sites have been detailed.