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Magnetic-electronic studies of mixed-valence Fe 2 OBO 3 have shown that ionic charge order (CO) is disrupted at ß16 GPa. The pertinent minority-spin carrier exhibits persistent intersite electron exchange Fe 2+ ⇔ Fe 3+ to well beyond this pressure. Temperature-dependent electrical transport measurements over an extended pressure range presented here demonstrate that the electronic structure remains gapped to well beyond 16 GPa. Extrapolation of data to higher pressure suggests that metallization will only prevail at P > 50 GPa. Both the persistent gapped electronic state across the CO instability and signature of carrier confinement to Fe-Fe dimers in the Fe 2+ ⇔ Fe 3+ electron exchange are rationalized as crossover from a Wigner crystal (site centered) insulator to a dimer Mott (bond centered type) insulator-"Wigner-Mott transition" at ß16 GPa. The dimer insulating state is a consequence of modulation of the relevant hopping parameter t in quasi-low-dimensional features in the structure (ribbons and chains). Complementary structural studies suggest that the a axis is appreciably more compressible than other crystallographic directions of the original monoclinic unit cell. Therefore, such a modulation in t may arise from Peierls type distortions along the a axis or else stems from intrinsic modulation in the c axis direction of the unit cell. This is aided by a monoclinic (P 2 1 /c) → orthorhombic (Pmcn) structural adjustment that is concurrent across the electronic transition. Pressure tuning of relative values of on-site U /t and intersite V /t Coulomb interaction parameters of the quasi-low-dimensional features evolve the system from site-centered to dimer-centered electron localization.

Emission Mössbauer spectroscopy on 57 Fe fed by 57 Mn ions implanted in the metal oxides ZnO, MgO and Al 2 O 3 has been performed. The implanted ions occupy different lattice sites and charge states. A magnetic part of the spectra in each oxide can be assigned to Fe 3+ ions in a paramagnetic state with unusually long relaxation time observable to temperatures up to several hundreds Kelvin. Earlier expectations that the magnetic spectra could correspond to an ordered magnetic state could not be confirmed. A clear decision for paramagnetism and against an ordered magnetic state was achieved by applying a strong magnetic field of 0.6 Tesla.The relaxation times deduced were compared to spin-lattice relaxation times from electron paramagnetic resonance (EPR).

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Magnetic-electronic hyperfine interaction parameters of spectral components are obtained from in situ 57 Fe Mössbauer spectroscopy pressure studies of the mixed-valence LuFe 2 O 4 multiferroic, up to ∼30 GPa and on recovered high-pressure phase samples. Temperature-dependent Mössbauer spectra of the low-pressure phase show that Fe 2+ and Fe 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 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 Fe (2+ +) ⇔ Fe (3−)+ ,w h e r e the minority-spin electron deconfinement coefficient is = 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 LuFe 2 O 4 in its high-pressure phase as a new charge order instigated (electronic) ferroelectric.