The optical absorption of iron oxides

Balberg, I.; Pinch, H. L.

doi:10.1016/0304-8853(78)90138-5

Cited by 109 publications

(83 citation statements)

References 13 publications

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“…A strong absorption corresponding to this 3d(Fe)-2p(O) → 3d(Fe) transition is expected. Considering shorter Fe-O bond length in Mw compared to that in pure FeO, the calculated gap of 3 eV is in reasonable agreement with an experimental absorption with stronger line around 2.4 eV in FeO [26]. The large mixing between majority Fe d states and O p states over a wide range of energies and the small contribution of O at the top of the valence band are also similar to these features observed in the density of states of FeO [12].…”

supporting

confidence: 87%

Pressure induced high spin to low spin transition in magnesiowüstite

Tsuchiya

Wentzcovitch

Silva

et al. 2006

Physica Status Solidi (b)

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PACS 71.15. Mb, 71.27.+a, 75.20.Ck Using a rotationally invariant formulation of LDA + U, we report a successful study of the high spin (HS)/ low spin (LS) transition in low solute concentration magnesiowüstite (Mw), (Mg 1-x Fe x )O, (x < 20%), the second most abundant phase in Earth's lower mantle. The HS state crosses over smoothly to the LS state passing through an insulating mixed spins state where properties change continuously, as seen experimentally. These encouraging results indicate this method should enable first principles studies of strongly correlated iron-bearing minerals, a major class of mineral physics problems. Magnesiowüstite (Mw), (Mg 1-x Fe x )O, is believed to be the major mineral phase in Earth's lower mantle (LM) after ferrosilicate perovskite, (Mg 1-x Fe x )SiO 3 (hereafter Pv) [1]. The effect of iron incorporation on the thermodynamic and elastic properties of these minerals is a crucial issue in mineral physics. High spin to low spin (HS/LS) transitions in iron have been observed by in situ X-ray emission spectroscopy (XES) and Mössbauer spectroscopy from 40 to 70 GPa in Mw [2,3] and from 70 and 120 GPa in Pv [4][5][6] at room temperature. This transition is accompanied by volume reduction [3] and changes in these minerals' optical absorption spectrum. These can produce seismic velocity anomalies, variations in MgFe 2+ partitioning between Mw and Pv, changes in radiative heat conductivity, and compositional layering [2,4,7,8]. The elastic signature of this transition has now been partially explored [3], but there is still much to be quantified to pin down the effects of this spin transition on these properties. At the same time, the strongly correlated behavior of iron oxide has deterred the quantification of these changes by density functional calculations based on the local spin density (LSDA) and spin-polarized generalized gradient approximations (σ-GGA). These approaches and phenomenological models [9] produce incorrectly a metallic HS ground state and then successive spin collapses across the transition [10,11].Here we use a rotationally invariant version of the LDA + U approach implemented in the planewave pseudopotential method, where U is calculated in an internally consistent way [12]. This approach has been very successful in describing the electronic and structural properties of FeO, wüstite, under pressure [12], an antiferromagnetic insulator with a Néel temperature of 198 K. Computations were performed using the local density approximation (LDA) [13,14]. The oxygen pseudopotential was generated by the

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supporting

confidence: 87%

Pressure induced high spin to low spin transition in magnesiowüstite

Tsuchiya

Wentzcovitch

Silva

et al. 2006

Physica Status Solidi (b)

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“…The hematite was modelled using the full frequency-dependent dielectric function 51 , Au and titanium using the data of Palik 52 and SiO 2 using a refractive index of 1.4. The reported values of bulk Fe 2 O 3 vary depending on the synthesis procedure 53 . For the simulation, an absorption coefficient of 5 Â 10 4 at 500 nm was used.…”

Section: Methodsmentioning

confidence: 99%

Plasmon-induced photonic and energy-transfer enhancement of solar water splitting by a hematite nanorod array

et al. 2013

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Plasmonic metal nanostructures offer a promising route to improve the solar energy conversion efficiency of semiconductors. Here we show that incorporation of a hematite nanorod array into a plasmonic gold nanohole array pattern significantly improves the photoelectrochemical water splitting performance, leading to an approximately tenfold increase in the photocurrent at a bias of 0.23 V versus Ag|AgCl under simulated solar radiation. Plasmoninduced resonant energy transfer is responsible for enhancement at the energies below the band edge, whereas above the absorption band edge of hematite, the surface plasmon polariton launches a guided wave mode inside the nanorods, with the nanorods acting as miniature optic fibres, enhancing the light absorption. In addition, the intense local plasmonic field can suppress the charge recombination in the hematite nanorod photoanode in a photoelectrochemical cell. Our results may provide a general approach to overcome the low optical absorption and spectral utilization of thin semiconductor nanostructures, while further reducing charge recombination losses.

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“…However, surveying the present literature gives clear hints that the consistent description of TM compounds is difficult within a single approach: For example, the G 0 W 0 (HSE) approach (denoting a G 0 W 0 calculation based on initial HSE calculation of wavefunctions and eigenvalues using the hybrid functional of Ref. [34]), that is considered to be reliable in main group compounds [15] gives a much too large band gap of 4.0 eV in Fe 2 O 3 [31], compared to the experimental gap at 2.1 eV [35]. The G 0 W 0 (LDA) variant, which underestimates the gap of ZnO by as much as 1 eV [7,22], already overestimates the gap by 0.3 eV in TiO 2 [30] and by 0.6 eV in SrTiO 3 [36].…”

Section: Introductionmentioning

confidence: 99%

Band-structure calculations for the 3dtransition metal oxides inGW

2013

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Many-body GW calculations have emerged as a standard for the prediction of band-gaps, bandstructures, and optical properties for main-group semiconductors and insulators, but it is not well established how predictive the GW method is in general for transition metal (TM) compounds. Surveying the series of 3d oxides within a typical GW approach using the random phase approximation reveals mixed results, including cases where the calculated band gap is either too small or too large, depending on the oxidation states of the TM (e.g., FeO/Fe 2 O 3 , Cu 2 O/CuO). The problem appears to originate mostly from a too high average d-orbital energy, whereas the splitting between occupied and unoccupied dsymmetries seems to be reasonably accurate. It is shown that augmenting the GW self-energy by an attractive (negative) and occupation-independent on-site potential for the TM d-orbitals with a single parameter per TM cation can reconcile the band gaps for different oxide stoichiometries and TM oxidations states. In Cu 2 O, which is considered here in more detail, standard GW based on wavefunctions from initial density or hybrid functional calculations yields an unphysical prediction with an incorrect ordering of the conduction bands, even when the magnitude of the band gap is in apparent agreement with experiment. The correct band ordering is restored either by applying the d-state potential or by iterating the wave functions to self-consistency, which both have the effect of lowering the Cu-d orbital energy. While it remains to be determined which improvements over standard GW implementations are needed to achieve an accurate ab initio description for a wide range of transition metal compounds, the application of the empirical on-site potential serves to mitigate the problems specifically related to d-states in GW calculations.2

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The optical absorption of iron oxides

Cited by 109 publications

References 13 publications

Pressure induced high spin to low spin transition in magnesiowüstite

Pressure induced high spin to low spin transition in magnesiowüstite

Plasmon-induced photonic and energy-transfer enhancement of solar water splitting by a hematite nanorod array

Band-structure calculations for the 3dtransition metal oxides inGW

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