Field-Frustrated Cooperative Distortions: Suppressing Jahn-Teller Ordering via Microwave Annealing

Cooperative Jahn-Teller (CJT) distortions in correlated oxides couple local electronic degeneracy to long-range lattice symmetry breaking, giving rise to functional properties such as orbital ordering and magnetoelastic effects. In copper ferrite (CuFe₂O₄), this coupling drives a well-known cubic-to-tetragonal phase transition under equilibrium thermal processing. Here, we investigate how microwave (MW) annealing modifies oxidation, local structure, and global symmetry in CuFe₂O₄ using synchrotron X-ray diffraction, pair distribution function analysis, and X-ray photoelectron spectroscopy. We find that MW processing results in complete oxidation to Cu²⁺ and restoration of short-range oxygen polyhedral order, yet the crystal structure remains cubic, in contrast to furnace-annealed samples that undergo CJT-driven symmetry lowering. Notably, three samples with identical cubic symmetry, nanocrystalline, MW-annealed nanocrystalline, and samples relaxed from a tetragonal CJT state under MW exposure, exhibit distinct Cu 2p satellite intensities and binding energies, indicating different local electronic environments. These results demonstrate that MW annealing decouples local structural relaxation from long-range cooperative ordering. Using a custom-built pulsed MW system with spatially resolved infrared thermography, we show that this behavior cannot be attributed to rapid quenching. The findings establish MW fields as a pathway to stabilize symmetry-frustrated states in complex oxides.