Rhombohedral $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ has been studied by using density-functional theory (DFT) and the generalized gradient approximation (GGA). For the chosen supercell all possible magnetic configurations have been taken into account. We find an antiferromagnetic ground state at the experimental volume. This state is 388 meV/(Fe atom) below the ferromagnetic solution. For the magnetic moments of the iron atoms we obtain $3.4{\ensuremath{\mu}}_{\mathrm{B}},$ which is about $1.5{\ensuremath{\mu}}_{\mathrm{B}}$ below the experimentally observed value. The insulating nature of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ is reproduced, with a band gap of 0.32 eV, compared to an experimental value of about 2.0 eV. Analysis of the density of states confirms the strong hybridization between Fe $3d$ and O $2p$ states in $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}.$ When we consider lower volumes, we observe a transition to a metallic, ferromagnetic low-spin phase, together with a structural transition at a pressure of 14 GPa, which is not seen in experiment. In order to take into account the strong on-site Coulomb interaction U present in ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ we also performed $\mathrm{D}\mathrm{F}\mathrm{T}+U$ calculations. We find that with increasing U the size of the band gap and the magnetic moments increase, while other quantities such as equilibrium volume and Fe-Fe distances do not show a monotonic behavior. The transition observed in the GGA calculations is shifted to higher pressures and eventually vanishes for high values of U. Best overall agreement, also with respect to experimental photoemission and inverse photoemission spectra of hematite, is achieved for $U=4\mathrm{eV}.$ The strength of the on-site interactions is sufficient to change the character of the gap from $d\ensuremath{-}d$ to $\mathrm{O}\ensuremath{-}p\ensuremath{-}\mathrm{Fe}\ensuremath{-}d.$

http://www.thp.uni-duisburg.de/Paper/entel/PRB69-107-rollmann-fe2o3.pdf

First-principles calculation of the structure and magnetic phases of hematite

The effect of adding high pressure as a control parameter in solids, liquids, and gases expands opportunities to observe unexpected novel phenomena and understand matter in extreme environments. This review on high pressure science highlights subjects ranging from quantum criticality to Earth science. State-of-the-art experimental methods at megabar pressures are also discussed. The proliferation of pressure-induced phases illustrate promising new directions for this field of research.

https://link.aps.org/accepted/10.1103/RevModPhys.90.015007

Solids, liquids, and gases under high pressure

http://people.rses.anu.edu.au/oneill_h/pubs/JPCS64_2517.pdf

High pressure Raman spectroscopy of spinel-type ferrite ZnFe2O4

Raman spectra of Fe2O3 were measured to 62 GPa in a diamond anvil cell. All group theoretically predicted Raman-active phonon modes were detected to 54 GPa. In addition, some high-pressure spectra show an IR-active Eu mode (~660 cm ‐1 ), possibly induced by surface defects or stress. This mode is related by a factor of two to a mode at 1320 cm ‐1 . The assignment of the 1320 cm ‐1 mode has been controversial (two-magnon scattering or two-phonon scattering), and our observation supports the phonon assignment. All Raman-active phonons show nonlinear pressure-induced shifts. The mode Gruneisen parameters and their logarithmic volume dependences for two low-frequency phonons, A1g and Eg, become negative and infinite, respectively, near 50 GPa as a result of the instability of the corundum structure at this pressure. Using the observed Raman-active phonons together with acoustic phonons previously measured by ultrasonics, and Kieffer’s model, we calculate the phonon contribution to the thermodynamic parameters of hematite. Comparison with experimentally measured values allows an estimation of an upper bound to the magnon contribution to the heat capacity at ambient pressure. This increases continuously above the Morin temperature and reaches a maximum at the Neel temperature (~37%). The Raman spectra change dramatically at pressures greater than 54 GPa as indicated by the appearance of new peaks and a significant increase in background. Although direct structural analysis is not possible due to the low signal-to-background ratio and the lack of polarization information, we were able to examine the consistency of our Raman observation with the corundum-to-perovskite phase transformation using the results for an analog system: MgSiO3 ilmenite (ordered corundum type) and perovskite. This analysis shows that observed new features in Fe2O3 Raman spectra may not be consistent with the GdFeO3 perovskite structure.

/pdf/raman-spectroscopy-of-fe2o3-to-62-gpa-2sdfkjriyk.pdf

Raman spectroscopy of Fe2O3 to 62 GPa

High-pressure study of Cr2O3 obtained by high-temperature oxidation by X-ray diffraction and Raman spectroscopy

Electronic and structural properties of the high-pressure phase of Fe{sub 2}O {sub 3} were determined by combining the methods of M{umlt o}ssbauer spectroscopy, x-ray diffraction, and electrical resistance, R(P,thinspT) , to 80thinspthinspGPa. Because of a first-order phase transition taking place in the 50{endash}75thinspthinspGPa range and accompanied by a volume decrease of {approximately}10{percent} , a breakdown of the electronic d-d correlation occurred, leading to a Mott transition, a metallic and a nonmagnetic single Fe{sup 3+} electronic state. The high-pressure structure is of the distorted Rh{sub 2}O {sub 3}- II type. The accommodation of the denser phase within this six-coordinated structure is attributable to the metallic state. {copyright} {ital 1999} {ital The American Physical Society}

Breakdown of the Mott-Hubbard State in Fe 2 O 3 : A First-Order Insulator-Metal Transition with Collapse of Magnetism at 50 GPa

A miniature piston-cylinder diamond-anvil cell (DAC) was constructed and tested for pressure operation at and beyond 100 GPa. Its advantages compared to other piston-cylinder DACs are its compactness (22-mm diam by 21-mm high), self-contained force generator, and simple way of operation. Tungsten carbide backing plates are used for supporting the anvils; one with a hemispherical shape allowing for parallelism alignment, and one with a flat circular shape allowing for lateral alignment of the anvils’ culets. The force is generated by six M3 Allen screws and is conveyed to the piston via force rings. Pressures to 130 GPa were achieved with beveled culets having 300-μm flats and Re gaskets. Design features, mode of operation, and performance are described. The latter has been demonstrated for the particular case of Mossbauer spectroscopy in La57FeO3.

A multipurpose miniature piston-cylinder diamond-anvil cell for pressures beyond 100 GPa

The high-pressure phase of magnetite, $h\text{\ensuremath{-}}{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$, has been studied by M\"ossbauer spectroscopy, and electrical conductivity to pressure of $140\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and in the $5--573\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ temperature range. M\"ossbauer studies following annealing at $573\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $34\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ reveal three equal-abundant iron species within two magnetic sublattices, I and II: ${\mathrm{Fe}}^{3+}(I)$ sublattice with hyperfine field typical of six-coordinated ferric ions and magnetic ordering temperature ${T}_{M}(I)g600\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at $34\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and a combined ${\mathrm{Fe}}^{3+}(II)$ and ${\mathrm{Fe}}^{2+}$ species forming a magnetic sublattice II with ${T}_{M}(II)\ensuremath{\sim}300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at $34\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. Starting at $50\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ the ${\mathrm{Fe}}^{3+}(I)$ moment gradually collapses becoming nonmagnetic at $Pg80\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. This moment collapse is explained in terms of a charge-transfer $d\text{\ensuremath{-}}p$ gap closure mechanism. ${T}_{M}(II)$ decreases with pressure, and to $120\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, the highest pressure reached with M\"ossbauer spectroscopy, ${\mathrm{Fe}}^{3+}(II)$ remains magnetically ordered. Resistance studies with a nonannealed and highly stoichiometric sample at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ reveal a sharp increase in $R$ at the onset of the $h\text{\ensuremath{-}}{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ phase $(Pg25\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$, reaching a 25-fold maximum at $\ensuremath{\sim}45\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, after which it shows a precipitous decrease in the $45--70\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ range after which it decreases gradually with pressure reaching 10-fold reduction at $140\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The sharp increase in $R$ is attributed to a gap opening once $h\text{\ensuremath{-}}{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ is formed. Starting at $Pg50\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, coinciding with partial correlation breakdown of the ${\mathrm{Fe}}^{3+}(I)$ sublattice, a sluggish onset of metallization is observed manifested by a positive $d\mathrm{R}∕d\mathrm{T}$.

Mössbauer and resistivity studies of the magnetic and electronic properties of the high-pressure phase of Fe 3 O 4

A method for calculating the double-differential cross sections of the inelastic X-ray scattering (IXS) associated with low-Z-atom-K-level excitation of solids is proposed. We have included both the first- and the second-order contributions of one-electron perturbation theory. The second-order contribution is shown to be significant only for comparatively small initial photon energy (h(cross) omega 0<2 keV). The first-order term is proved to be determined by p partial local densities of electron states (p LDOS) whereas the second-order term is determined by s LDOS. An expression is proposed which enables us to determine the p and s LDOS from experimental IXS spectra measured for different initial photon energies and transferred momenta. Using available experimental IXS spectra and X-ray absorption spectra we have determined the pi * and sigma * p LDOS for graphite crystal. The results obtained are compared with the theoretical pi * and sigma * LDOS calculated using the full multiple-scattering method. The calculated and experimental LDOS are in reasonable agreement, both as regards peak positions and absolute values.

https://iopscience.iop.org/article/10.1088/0953-8984/6/50/014/pdf

Theory of core-level inelastic X-ray scattering spectra and partial local densities of states in low-Z atom crystals

Electrical, magnetic and structural properties of the antiferromagnetic semiconductor Sr3Fe2O7 (Fe4+, d4) were probed by resistance, Mossbauer spectroscopy (MS) and X-ray diffraction (XRD) measurements up to P ≈ 40 GPa using diamond-anvil cells. A sluggish pressure-induced insulator–metal (IM) transition is observed with a clear incipient metallic state at P ≥ 20 GPa. The Fe(IV) 3d magnetic moments remain unaltered across the transition as deduced from MS, and XRD studies show no structural symmetry change up to 40 GPa. The results are consistent with carrier delocalization due to p–p gap closure, e.g., ligand-to-ligand charge transfer that does not involve the d-states and structural symmetry changes. A mechanism to account for the IM transition in the strontium ferrates is discussed.

G. Yu. Machavariani

Papers

Breakdown of the Mott-Hubbard State in Fe 2 O 3 : A First-Order Insulator-Metal Transition with Collapse of Magnetism at 50 GPa

A multipurpose miniature piston-cylinder diamond-anvil cell for pressures beyond 100 GPa

Mössbauer and resistivity studies of the magnetic and electronic properties of the high-pressure phase of Fe 3 O 4

Theory of core-level inelastic X-ray scattering spectra and partial local densities of states in low-Z atom crystals

Pressure-Induced Metallization of the Perovskite Sr3Fe2O7