Coexistence of site- and bond-centered electron localization in the high-pressure phase ofLuFe2O4

Magnetic-electronic hyperfine interaction parameters of spectral components are obtained from in situ $⁵⁷\mathrm{Fe}$ M\"ossbauer spectroscopy pressure studies of the mixed-valence $\mathrm{LuF}{\mathrm{e}}₂{\mathrm{O}}₄$ multiferroic, up to $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and on recovered high-pressure phase samples. Temperature-dependent M\"ossbauer spectra of the low-pressure phase show that $\mathrm{F}{\mathrm{e}}²⁺$ and $\mathrm{F}{\mathrm{e}}³⁺$ 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 $P>8\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, 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 ${\mathrm{Fe}}^{(2+\mathrm{\ensuremath{\Delta}}+)}\ensuremath{\Leftrightarrow}{\mathrm{Fe}}^{(3\ensuremath{-}\mathrm{\ensuremath{\Delta}})+}$, where the minority-spin electron deconfinement coefficient is $\mathrm{\ensuremath{\Delta}}=0.3\ensuremath{-}0.4$. 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 $\mathrm{LuF}{\mathrm{e}}₂{\mathrm{O}}₄$ in its high-pressure phase as a new charge order instigated (electronic) ferroelectric.