Influence of Molecular Orbitals on Magnetic Properties of FeO2Hx (original) (raw)
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Physical Review B
The application of a 3.5 GPa pressure on Fe in a H 2 environment leads to the formation of iron hydride FeH. Using a combination of high pressure x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the Fe K edge, we have investigated the modification of electronic and magnetic properties induced (i) by the transition from bcc-Fe to dhcp (double hexagonal)-FeH under pressure and (ii) by the compression of FeH up to 28 GPa. XAS and XMCD spectra under pressure have been computed in bcc-Fe and dhcp-FeH within a monoelectronic framework. Our approach is based on a semirelativistic density-functional theory (DFT) calculation of the electron density in the presence of a core hole using plane waves and pseudopotentials. Our method has been successful to reproduce the experimental spectra and to interpret the magnetic and electronic structure of FeH. In addition, we have identified a transition around 28 GPa, which is a purely magnetic transition from a ferromagnetic state to a paramagnetic state.
Physics and Chemistry of Minerals, 1985
Results of SCF-X~-SW molecular orbital calculations on (FeO,(OH)2) 7-and (FeO6) 9-clusters are used to investigate the differences between F e-O and F e-O H bonding in hydroxyl-bearing iron oxides and silicates. The Fe 3 +-O H-bond is more ionic, and has a smaller spinpolarization, then the Fe 3 +-0 2 bond. The smaller spinpolarizability of OH ligands explains why superexchange interactions between hydroxo-bridged Fe 3+ cations are much weaker than those between oxo-bridged Fe 3 + cations. Replacement of oxygens in the Fe 3 + coordination environment by O H-ligands appears to promote the covalency between Fe 3 + centers and O z-oxygens. The increased covalency lowers the effective spin of the Fe atom. This, in turn, explains the decreased magnetic hyperfine fields at the Fe nucleus in F e O O H polymorphs relative to those found in Fe 3 + oxides.
Proceedings of the National Academy of Sciences, 2009
Recent studies have shown that high pressure ( P ) induces the metallization of the Fe 2+ –O bonding, the destruction of magnetic ordering in Fe, and the high-spin (HS) to low-spin (LS) transition of Fe in silicate and oxide phases at the deep planetary interiors. Hematite (Fe 2 O 3 ) is an important magnetic carrier mineral for deciphering planetary magnetism and a proxy for Fe in the planetary interiors. Here, we present synchrotron Mössbauer spectroscopy and X-ray diffraction combined with ab initio calculations for Fe 2 O 3 revealing the destruction of magnetic ordering at the hematite → Rh 2 O 3 -II type (RhII) transition at 70 GPa and 300 K, and then the revival of magnetic ordering at the RhII → postperovskite (PPv) transition after laser heating at 73 GPa. At the latter transition, at least half of Fe 3+ ions transform from LS to HS and Fe 2 O 3 changes from a semiconductor to a metal. This result demonstrates that some magnetic carrier minerals may experience a complex sequ...
Structural and spin transitions in Fe2O3
The wide range of intriguing characteristics exhibited by Fe2O3 with pressure and temperature has renewed the attention of the scientific community in the last decade. Experimental and theoretical efforts are on to address and unravel the complexity of the system. The ambient pressure phase, hematite (alpha- Fe2O3) transforms to a new structural phase (HP1). That the HP1 phase is orthorhombic perovskite (Pbnm) or Rh2O3-II type (Pbcn) is still a debate and yet to be explored theoretically. Experimentally, the succeeding high pressure phase (HP2) was proposed to be Cmcm type post-perovskite (without any structural assignment). From the spin transition point of view, there has been a long-standing issue of an isostructural high spin (HS) to low spin (LS) transition. And experimental data till date are divided into two horizons -- one assigning the spin transition in the hematite phase and the other in the HP1 phase. In this work, motivated by these exotic unresolved controversies of th...
Physical Chemistry Chemical Physics, 2014
The renewed interest in magnetite (Fe 3 O 4 ) as a major phase in different types of catalysts has led us to study the oxidation-reduction behaviour of its most prominent surfaces. We have employed computer modelling techniques based on the density functional theory to calculate the geometries and surface free energies of a number of surfaces at different compositions, including the stoichiometric plane, and those with a deficiency or excess of oxygen atoms. The most stable surfaces are the and leading to a cubic Fe 3 O 4 crystal morphology with truncated corners under equilibrium conditions. The scanning tunnelling microscopy images of the different terminations of the and surfaces were calculated and compared with previous reports. Under reducing conditions, the creation of oxygen vacancies in the surface leads to the formation of reduced Fe species in the surface in the vicinity of the vacant oxygen. The (001) surface is slightly more prone to reduction than the (111), due to the higher stabilisation upon relaxation of the atoms around the oxygen vacancy, but molecular oxygen adsorbs preferentially at the surface. In both oxidized surfaces, the oxygen atoms are located on bridge positions between two surface iron atoms, from which they attract electron density. The oxidised state is thermodynamically favourable with respect to the stoichiometric surfaces under ambient conditions, although not under the conditions when bulk Fe 3 O 4 is thermodynamically stable with respect to Fe 2 O 3 . This finding is important in the interpretation of the catalytic properties of Fe 3 O 4 due to the presence of oxidised species under experimental conditions.
The European Physical Journal D - Atomic, Molecular and Optical Physics, 2003
We have studied structural and magnetic properties in small iron oxide clusters, FenOm (n = 1−5), by means of the first-principles calculation based on the density functional theory. We have used not only the usual spin polarized scheme, but also the scheme for noncollinear magnetism to carry out efficient optimization in magnetic structure. The result of FeOm (m = 1−4) is in good agreement with the previous work. We found the stable adduct clusters in FeO5 and FeO6. The bridge site of oxygen atom is more favorable in energy than any other site for the clusters of FenO (n = 2−5). As increasing the number of oxygen atoms, the alignment of Fe magnetic moments changes from ferromagnetic configuration to antiferromagnetic one at FenOn (n = 2−4).
Electronic structures of iron-bearing oxidic minerals at high pressure
The Scr-xa Scattered Wave Cluster Molecular Orbital Method is used to calculate the electronic structure of the square planar FeO!oxyanion, in both its quintet high spin and singlet low spin states. The order of the predominantly Fe3d crystal-field-type orbitals is calculated to be x2-yz)xylxz,yz)22 (with the ligands lying along the x,y axes), in agreement with experiment. The crystal-field-type orbitals differ substantially in ToFe character as a function of both symmetry and spin type. Calculated optical transition energies for the quintet state at R : 2.003 A are in fair agreement with one atmosphere experimental data. In the singlet state (R = 2.003 A) spectral transition energies are reduced while the Mdssbauer isomer shift increases. Studies on the quintet state at Fe-O distances of 2.03, 2.003, 1.98, and 1.93 A show a calculated increase ind*r-""-d", separation in close agreement with the crystal field R-u distance-dependence law, even though the orbitals involved are only about half metal in character. However, the separation of the spin-up and spin-down xz,yz crystal-field-type orbitals also increases as the Fe-O distance is decreased. The calculations clearly indicate that a quintetsinglet transition will not occur at Fe-O distances as small as 1.93 A. This is apparently consistent with high pressure X-ray studies showing distortion of the Fe site, although aparameterized crystal field calculation based on the high-pressure crystal-structure data does not give good agreement with the experimental optical spectra. MO results are also presented for the FeO! and FeO|o clusters at Fe-O distances of 2.17,2.06, and 1.95 A. Calculated trends in crystal field energies are again in agreement with the R-6 law while O -Fe metal charge-transfer energies are found to initially increase as Fe-O distance is reduced, but to decrease at smaller distances for FeOlo-. The covalency and width of the valence region also increase strikingly. Analysis of orbital energy trends shows that the energies of a number of different orbital sets must be considered in estimating equilibrium Fe-O distances in the Fe oxides. Differences in the energies of the various orbital sets are discussed for square planar, tetrahedral, and octahedral Fez+ at normal internuclear distances to illustrate this point. Based upon calculated trends in crystal field spectra, spin pairing in octahedral Fd+ compounds is again predicted to occur at mantle pressures. Uncertainties in the molar volumes of high and low spin Fd+, however, make prediction of the transition pressure very difficult. Finally, the large calculated changes in bond character for the compressed Fe oxides are contrasted with the very small changes calculated for compressed MgO.
Atomic structure and stability of magnetite Fe3O4(001): An X-ray view
Surface Science, 2016
The structure of the Fe 3 O 4 (001) surface was studied using surface x-ray diffraction in both ultra-high vacuum, and higher-pressure environments relevant to water-gas shift catalysis. The experimental xray structure factors from the √ 2x √ 2R45 • reconstructed surface are found to be in excellent agreement with the recently proposed subsurface cation vacancy (SCV) model for this surface (Science 346 (2014), 1215). Further refinement of the structure results in small displacements of the iron atoms in the first three double layers compared to structural parameters deduced from LEED I-V experiments and DFT calculations. An alternative, previously proposed structure, based on a distorted bulk truncation (DBT), is conclusively ruled out. The lifting of the √ 2 × √ 2R45 • reconstruction upon exposure to water vapor in the mbar pressure regime was studied at different temperatures under flow conditions, and a roughening of the surface was observed. Addition of CO flow did not further change the roughness perpendicular to the surface but decreased the lateral correlations.