Spin-phonon interactions revisited: Far-infrared emission, Raman scattering, and high-resolution x-ray diffraction at the Néel temperature in LaFeO 3

Abstract

We report on the structural evolution, spin-phonon interactions, and magnetoelastic effects in bulk LaFeO₃ perovskite across its antiferromagnetic transition. Our investigation is motivated by anomalies in the zone-center phonon bandwidth and peak positions, which, as superexchange precursors, deviate in far-infrared emission and high-temperature Raman scattering spectra from the behavior expected in a purely anharmonic lattice. High-resolution synchrotron X-ray diffraction (SXRD) patterns provide new and intriguing insights into the interplay between the perovskite lattice and the magnetic transitionWe found that while the lattice constants exhibit slight deviations at the Néel temperature (TN), due to spin-phonon interactions, individual interatomic bonds display distinct behavior. The octahedral B-site basal plane, involving oxygen ion pairs that mediate superexchange interactions, displays bond contraction (or elongation) assimilable to the observed in X-ray diffraction for negative thermal expansion. These bond deviations reach a singular point near TN, where a sharp change in bond length is observed. A similar abrupt change is also detected in the nearest-neighbor La–O2 basal plane distances, which we interpret as arising from ion size differences within the 3d–2p hybridized states associated with the superexchange mechanismHigh-resolution SXRD measurements of the c-axis Fe³⁺–O1 and La³⁺–O1 apex distances reveal a distinct inflection at the Néel temperature. This behavior supports the role of lattice distortions and crystal field modifications in the emergence of non-collinear weak ferromagnetism in RFeO₃ (R = rare earth) perovskites.Furthermore, our SXRD patterns show an increase in diffraction line intensities peaking at TN, suggesting a straightforward indicator for spin-phonon interactions that also resolves into lattice metastability deduced by nonlinear thermal expansion in the paramagnetic phase. Preliminary results across the full RFeO₃ (R = rare earth) series indicate a shared structural framework among all members, potentially offering insights into previously inconclusive structural interpretations in oxides with common octahedral sublattices.