Evidence of octahedral local distortion across T_N in RFeO₃ (R=rare earth)

We report phonon infrared emission of non-Jahn Teller Pbnm RFeO₃ (R=rare earth) G-type compounds as a local probe in the far infrared, highlighting anomalies across T_N (the Néel temperature) in the breathing (stretching) vibrational profile. In this frequency range, the dielectric deconvolution reveals high temperature orbital octahedral local site distortions. High resolution X-ray diffraction patterns of RFeO₃ (R=La, Er, Lu), obtained at the ID22 ESRF beamline (Grenoble), reveal modifications in the FeO₆ octahedra across T_N. For instance, for R=La, the room temperature delta distortion (i.e., deviation of individual distances from the average value) is 0.992x10^-4, whereas it decreases to 0.106x10^-4 at 1200 K, denoting an increment of the octahedral symmetry while keeping the Pbnm space group unchanged. This implies intrinsic changes in bond lengths and angles involving Fe-O bonds with oxygen 2p also contributing to the magnetostructural coupling (yielding an anomalous anisotropic expansion of the unit-cell parameters) and spin-exchange interaction pathways. Site distortion brings up a new extra perturbation in the superexchange interaction M-O-M determining antiferromagnetic transitions accounting T_N rare earth ion radius dependence [1].

[1] J.-S. Zhou and J. B. Goodenough, PRB B 77, 132104 (2008).