A spin-crossover route to charge ordering in Fe4O5

Abstract Charge order reflects a delicate balance between localization and itinerancy, yet the microscopic parameters that control this balance remain poorly understood. Here we identify spin crossover as a route to control charge order in mixed-valence Fe 4 O 5 . Using single-crystal X-ray diffraction and synchrotron Mössbauer source spectroscopy to 112 GPa at room temperature, we reveal three previously unreported charge-ordered phases associated with two site-selective spin crossovers. The first two phases are characterized by Fe-Fe dimers, whereas the highest-pressure phase hosts isolated planar bent Fe trimers, a motif not previously reported in iron oxides. All three charge-ordered phases are confined to mixed spin state regimes: charge order emerges as an octahedral Fe site enters spin crossover, persists while high-spin and low-spin states coexist on that site, and vanishes once the crossover is complete. Between the two spin crossovers, a charge-disordered phase in which high-spin and low-spin iron occupy different sites shows that spin-state heterogeneity within a single site is required for ordering. We propose that near spin-state degeneracy, enhanced charge-lattice coupling favours localization and stabilizes charge order. Our results establish spin crossover as an externally tunable route to stabilizing and controlling charge order in mixed-valence materials.