Interplay between pressure, anion order, and octahedral tilts in perovskite oxyfluorides

Mechanisms governing octahedral rotations in homoanionic perovskites(ABO₃,ABF₃)is well understood;cation size mismatch drive tilts of BO₆ and BF₆ octahedra with angles that linearly depend on the electron population in the π-bonding t_2gorbital of the transition metal[1].However, the applicability of this mechanism to octahedral tilting in heteroanionic materials is less established.Here we use a combination of symmetry analysis and density functional theory calculations to investigate how anion order affects octahedral tilting under hydrostatic pressure in the anion ordered double perovskite KNaNbOF₅.Consistent with earlier reports, we find that the multiple connected NaOF₅ and NbOF₅ anionic groups exhibiting tilt distortions expands the phase space from 12 to 14[2].We also find that KNaNbOF₅ undergoes multiple pressure-driven tilt transitions, which we explain based on the aforementioned mechanism for octahedral tilting.This would show a path to create a pressure-stabilized phase that may exhibit interesting properties.
[1]P.Garcia-Fernandez, et al.J.Phys.Chem.Lett.,1 ,647–651(2010).
[2]C.J.Howard,et al.ActaCryst.B59,463-471(2003).