Coexistence of site- and bond-centered electron localization in the high-pressure phase of LuF e2 O4

Magnetic-electronic hyperfine interaction parameters of spectral components are obtained from in situ Fe57 Mössbauer spectroscopy pressure studies of the mixed-valence LuFe2O4 multiferroic, up to ∼30GPa and on recovered high-pressure phase samples. Temperature-dependent Mössbauer spectra of the low-pressure phase show that Fe2+ and Fe3+ sites are discernible, consistent with known site-centered charge order in the triangular (frustrated) Fe sublattice network. Magnetic spectra of the high-pressure phase, stabilized in a rectangular Fe sublattice network at P>8GPa, exhibit fingerprints of iron in an intermediate valence state only. Temperature-dependent resistivity pressure studies evidence thermally activated small polaron motion in the high-pressure phase. These experimental signatures, complemented by ab initio calculations of electronic structure, are considered evidence of asymmetric dimer formation Fe(2+Δ+)Fe(3-Δ)+, where the minority-spin electron deconfinement coefficient is Δ=0.3-0.4. Bragg satellites discerned in electron diffraction patterns of the metastable high-pressure phase possibly stem from this admixture of site- and bond-centered localization (intermediate-state charge order) in a magnetic background. This breaks inversion symmetry and potentially renders LuFe2O4 in its high-pressure phase as a new charge order instigated (electronic) ferroelectric.