Structural Distortion, Spin-Phonon Coupling, Interband Electronic Transition, and Enhanced Magnetization in Rare-Earth-Substituted Bismuth Ferrite

Rare-earth ions (RE = La³⁺, Nd³⁺, and Er³⁺) substituted BiFeO₃ (BFO) ceramics were synthesized by a conventional solid-state sintering procedure. X-ray diffraction patterns and Raman spectra confirm a rhombohedral R3c symmetry in all samples with significant distortion in FeO₆ octahedron, as well as the occurrence of remarkable spin-phonon coupling and notable change in magnetic transition temperature induced by RE dopants. The enhanced magnetization was observed in all RE-doped BFO ceramics, unveiling that the spatial spin structure of BFO should be perturbed by RE dopants. Diffuse reflectance spectra show a conspicuous evolution of interband electronic transitions in RE-doped BFO ceramics. Especially, the two crystal-field d-d band transitions (⁶A_1g→⁴T₁ and ⁶A_1g→⁴T_2g) exhibit a linear red-shift behavior with the reduction in the cell volume, which is well-linked with a linear tendency of increased magnetization. On the basis of these investigations, a possible mechanism was proposed in this paper to demonstrate the correlation between the structural distortion, interband electronic transitions, and magnetic properties in RE-doped BFO ceramics.