Signatures of orbital order and disorder in fluoroperovskites with t 2 g electronic degeneracies

A detailed high-resolution, variable temperature powder diffraction study of the fluoro-perovskites {NaFeF}₃ and {NaCoF}₃ is performed to probe their orbital ordering transitions. Through analysis of the symmetry adapted macrostrains and atomic distortions, we show that {NaFeF}₃ undergoes a C-type orbital order transition associated with the {t}_2g⁴ states of {Fe}²⁺ . Counterintuitively, 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 {NaCoF}₃ , 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 {NaCoF}₃ is an insulator with quenched orbital angular momentum at this temperature, our findings point towards a novel kind of orbital disorder associated to the {t}_2g⁵ electronic degeneracy.