Signatures of Orbital Order and Disorder in Fluoro-Perovskites with t2g Electronic Degeneracies

A detailed high-resolution, variable temperature powder diffraction study of the fluoro-perovskites NaFeF₃ and NaCoF₃ is performed to probe their orbital ordering transitions. Through analysis of the symmetry adapted macrostrains and atomic distortions, we show that NaFeF₃ undergoes a C-type orbital order transition associated with the t2g⁴ states of Fe²⁺. Counter-intuitively, the phase transition leading to the orbital order appears second order-like, which contradicts the thermodynamic requirements for electronic and isosymmeric phase transitions, implying that there must be an associated hidden symmetry breaking. On the other hand, for NaCoF₃, consideration of the symmetry adapted strains allows us to confidently rule out the occurrence of any long-range orbital orders down to 4 K. Since NaCoF₃ is an insulator with quenched orbital angular momentum at this temperature, our findings point towards a novel kind of orbital disorder associated to the t2g⁵ electronic degeneracy.