Spin-phonon interactions at LaFeO₃ Néel temperature revealed by high-resolution X-ray diffraction

Supported by our high-temperature phonon far infrared emission and Raman scattering about T_N 740 K, we report using synchrotron X-ray radiation subtle changes in the A-O and

B-O perovskite sublattice bond lengths due to magnetic ordering away of thermal anharmonic phonon softening in LaFeO₃ . While Rietveld refinement yields only weak magnetic-induced lattice changes, a closer examination of bond lengths reveals otherwise. The paramagnetic spin-lattice interactions induce deformations that, starting from 800 K show non-Jahn Teller Pbnm octahedral Fe-O₂ bonds undergoing a correlated contraction and elongation amounting to an octahedral basal area sudden contraction of ~0.09 Ų. At this temperature, the La-O distances also change by ~0.12 Å minimizing the energy in accordance with the abrupt onset of Fe³⁺-O-Fe³⁺ superexchange interactions (Fe³⁺ t³e² (S = 5/2)), leading to G-type Γ₄ (Gx, Ay, Fz) magnetism. As the temperature decreases, the La-O bonds gradually return to values similar to those above T_N within fluctuating librational dynamics associated with negative thermal expansion. This motion ceases at ~650 K leading to Raman phonon splitting. Notably, the X-ray diffraction lines peak at the antiferromagnetic transition, suggesting a simpler and more unequivocal technique for detecting spin-phonon interactions in oxides.