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Signatures of orbital order and disorder in fluoroperovskites with t2g electronic degeneracies

C. A. Crawford1, C. I. Hiley1, J. Gainza2, C. Ritter3, R. I. Walton1,*, and M. S. Senn1,†

  • *Contact author: r.i.walton@warwick.ac.uk
  • †Contact author: m.senn@warwick.ac.uk
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Phys. Rev. B 112, 035150 – Published 21 July, 2025

DOI: https://doi.org/10.1103/hk2f-pd2s

Abstract

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+. 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 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.

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Phys. Rev. B 112, 035150– Published 21 July, 2025
  • Received 10 April 2025
  • Accepted 1 July 2025
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