View Publication | CEMHTI CNRS UPR3079 Orléans France

G.King, A.Rakhmatullin, F.Šimko, 'The Role of Dynamic Polyhedral Rotations in the Phase Transitions of Rb3AlF6 and Cs3AlF6', Inorg. Chem. (2025) doi:10.1021/acs.inorgchem.5c02352

The structures and phase transitions of cryolite-related rubidium and cesium aluminum fluoride phases are investigated and related to the dynamic motions of AlF6 octahedra. Low-temperature synchrotron powder diffraction data were collected on Rb3AlF6 and Cs3AlF6 to search for new polymorphs, and MAS NMR was used to study changes in dynamic motion occurring across the phase transitions. Rb3AlF6 is found to undergo a subtle phase transition around 270 K and then a second, more substantial phase transition around 250 K. The low temperature polymorphs maintain essentially the same framework as the room temperature Fddd structure, but are more complex due to an ordered freezing of dynamic rotations of AlF6 octahedra. Freezing of octahedral rotations is observed in Cs3AlF6 around room temperature, but the same C2/m structure is maintained down to 80 K without any phase transitions. The dynamic motions and local structure in the high-temperature cubic polymorph of Cs3AlF6 are also examined jointly by MAS NMR and reverse Monte Carlo modeling of X-ray pair distribution function data.