Scanning tunneling spectroscopy study of the electronic structure of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi mathvariant="normal">Fe</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>surfaces

Jordan, Kenneth D.; Cazacu, A.; Manai, Giuseppe; Ceballos, S. F.; Murphy, S.; Shvets, I. V.

doi:10.1103/physrevb.74.085416

Cited by 74 publications

(59 citation statements)

References 32 publications

(44 reference statements)

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“…Low-energy ion scattering experiments reveal an increase in backscattered Ar + ions below T V , which is attributed to differences in the neutralization probability and the reduced 27 transparency of the sample in the Verwey phase [104][105][106]. In scanning tunnelling spectroscopy, Jordan et al observe a 200 meV gap at both room temperature and 95 K [58]. In 2007, Lodziana described the (√2×√2)R45° reconstruction of Fe 3 O 4 as a "surface Verwey transition" [69], and suggested that the surface remains in the insulating state well above the bulk Verwey transition temperature.…”

Section: Surface Verwey Transitionmentioning

confidence: 99%

“…Jordan et al [58] observed a small band gap at room temperature and 100 K, as they did for the (100) surface, but Shimizu et al [107] report metallic behaviour at room temperature, and a small gap at 78 K. In the latter study, the authors propose that the discrepancy arises from surface defects, and that the Jordan et al result is less reliable because many different surface sites (including defects) were averaged to create the STS IV spectra. For further details regarding the Verwey transition and charge ordering in low dimensions, the reader is referred to the very recent topical review by BernalVillamil and Gallego [108].…”

Section: Surface Verwey Transitionmentioning

confidence: 99%

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Iron oxide surfaces

Parkinson

2016

Surface Science Reports

501

463

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The current status of knowledge regarding the surfaces of the iron oxides, magnetite (Fe 3 O 4 ), maghemite (γ-Fe 2 O 3 ), hematite (α-Fe 2 O 3 ), and wüstite (Fe 1-x O) is reviewed. The paper starts with a summary of applications where iron oxide surfaces play a major role, including corrosion, catalysis, spintronics, magnetic nanoparticles (MNPs), biomedicine, photoelectrochemical water splitting and groundwater remediation. The bulk structure and properties are then briefly presented; each compound is based on a close-packed anion lattice, with a different distribution and oxidation state of the Fe cations in interstitial sites. The bulk defect chemistry is dominated by cation vacancies and interstitials (not oxygen vacancies) and this provides the context to understand iron oxide surfaces, which represent the front line in reduction and oxidation processes. The atomic-scale structure of the low-index surfaces of iron oxides is the major focus of this review. Fe 3 O 4 is the most studied iron oxide in surface science, primarily because its stability range corresponds nicely to the ultra-high vacuum environment, and because it is an electrical conductor, which makes it straightforward to study with the most commonly used surface science methods such as photoemission spectroscopies (XPS, UPS) and scanning tunneling microscopy (STM). The impact of the surfaces on the measurement of bulk properties such as magnetism, the Verwey transition and the (predicted) half-metallicity is discussed.The best understood iron oxide surface at present is probably Fe 3 O 4 (100); the structure is known with a high degree of precision and the major defects and properties are well characterised. A major factor in this is that a termination at the Fe oct -O plane can be reproducibly prepared by a variety of methods, as long as the surface is annealed in 10 Such straightforward preparation of a monophase termination is generally not the case for other iron oxide surfaces. All available evidence suggests the oft-studied (√2×√2)R45° reconstruction results from a rearrangement of the cation lattice in the outermost unit cell in which two octahedral cations are replaced by one tetrahedral interstitial, a motif conceptually similar to well-known Koch-Cohen defects in Fe (0001) is the most studied hematite surface, but 2 difficulties preparing stoichiometric surfaces under UHV conditions have hampered a definitive determination of the structure. There is evidence for at least three terminations: a bulk-like termination at the oxygen plane, a termination with half of the cation layer, and a termination with ferryl groups. When the surface is reduced the so-called "bi-phase" structure is formed, which eventually transforms to a Fe 3 O 4 (111)-like termination. The structure of the bi-phase surface is controversial; a largely accepted model of coexisting Fe 1-x O and α-Fe 2 O 3 (0001) islands was recently challenged and a new structure based on a thin film of Fe 3 O 4 (111) on α-Fe 2 O 3 (0001) was proposed. The merits of the compet...

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Section: Surface Verwey Transitionmentioning

confidence: 99%

Section: Surface Verwey Transitionmentioning

confidence: 99%

Iron oxide surfaces

Parkinson

2016

Surface Science Reports

501

463

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“…In turn, the robust insulating surface layer, which seems to be a universal feature of magnetite even under metastable terminations 17,18 , has an important local effect on the bulk CO. This better manifests at the Fe ure 7 evidences that also at the Fe 2+ termination those trimerons closer to the surface are slightly affected by it: the Fe 3+ -Fe 2+ distances between L3 and L5 are moderately enlarged, which introduces an asymmetry in the Fe chain weakening the charge sharing in its upper branch.…”

Section: The Fe3o4(001) Surface Above Tvmentioning

confidence: 99%

Charge order at magnetite Fe₃O₄(0 0 1): surface and Verwey phase transitions

Bernal-Villamil

Gallego

2014

J. Phys.: Condens. Matter

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At ambient conditions, the Fe3O4(001) surface shows a ( √ 2 × √ 2)R45 o reconstruction that has been proposed as the surface analog of the bulk phase below the Verwey transition temperature, TV . The reconstruction disappears at a high temperature, TS, through a second order transition. We calculate the temperature evolution of the surface electronic structure based on a reduced bulk unit cell of P 2/m symmetry that contains the main features of the bulk charge distribution. We demonstrate that the insulating surface gap arises from the large demand of charge of the surface O, at difference with that of the bulk. Furthermore, it is coupled to a significant restructuration that inhibits the formation of trimerons at the surface. An alternative bipolaronic charge distribution emerges below TS, introducing a competition between surface and bulk charge orders below TV .

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“…Its (100) surface has been extensively studied. [3][4][5][6][7][8][9][10][11][12][13][14][15] Instead of the (1 × 1) bulklike termination, it reconstructs into a structure with a larger ( √ 2 × √ 2)R45 • unit cell. The atomic structure of this reconstruction has been painstakingly unraveled by density functional theory (DFT), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM) 4 -the surface is terminated by octahedrally coordinated iron atoms arranged in rows, as shown in Fig.…”

mentioning

confidence: 99%

Order-disorder phase transition on the (100) surface of magnetite

et al. 2013

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Using low-energy electron diffraction, we show that the room-temperature ( √ 2 × √ 2)R45 • reconstruction of Fe 3 O 4 (100) reversibly disorders at ∼450 • C. Short-range order persists above the transition, suggesting that the transition is second order and Ising-like. We interpret the transition in terms of a model in which subsurface Fe 3+ is replaced by Fe 2+ as the temperature is raised. This model reproduces the structure of antiphase boundaries previously observed with scanning tunneling microscopy, as well as the continuous nature of the transition. To account for the observed transition temperature, the energy cost of each charge rearrangement is 82 meV. Metal oxides are often useful because of their stability at high temperatures. An example is magnetite, Fe 3 O 4 . In catalytic applications such as the water-gas shift reaction, 1 magnetite is used at temperatures between 300 and 500 • C. Furthermore, magnetite's high Curie temperature of 580 • C allows spintronic applications. 2 Since such applications frequently depend on surface properties, a natural question arises-how does the surface structure change with temperature?The room-temperature properties of magnetite's surfaces are complex. Its (100) surface has been extensively studied. [3][4][5][6][7][8][9][10][11][12][13][14][15] Instead of the (1 × 1) bulklike termination, it reconstructs into a structure with a larger ( √ 2 × √ 2)R45 • unit cell. The atomic structure of this reconstruction has been painstakingly unraveled by density functional theory (DFT), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM) 4 -the surface is terminated by octahedrally coordinated iron atoms arranged in rows, as shown in Fig. 1(a). The observed periodicity results from small displacements of the iron atoms perpendicular to the rows [see Fig. 1(b)]. The driving force for the reconstruction is believed to be ordering of the charge state of the iron in octahedral sites beneath the surface. 15,16 In bulk magnetite at room temperature the average charge state of octahedral iron is +2.5e. The subsurface charge ordering involves disproportionation of this charge into more positively and negatively charged sites. It has been proposed 15,16 that the disproportionation is greatest in the plane of octahedral iron beneath the top layer. Figure 1(c) sketches the proposed charge order in this subsurface layer. In Fig. 1(c), and in our discussion below, the two charge states in the second layer are labeled by their nominal oxidation state Fe 3+ and Fe 2+ , although the charges are estimated to only differ by 0.2-0.4 e rather than e. 15,17 The top layer octahedral iron is displaced in the surface plane to decrease the distance to the nearest subsurface Fe 2+ , giving the undulating rows of surface octahedral iron observed by LEED and STM.What might happen to such a structure when the temperature is raised? One possibility is that the high-temperature, high-entropy surface differs significantly in stoichiometry and termination from the charge ordered ( √...

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Scanning tunneling spectroscopy study of the electronic structure ofFe3O4surfaces

Cited by 74 publications

References 32 publications

Iron oxide surfaces

Iron oxide surfaces

Charge order at magnetite Fe₃O₄(0 0 1): surface and Verwey phase transitions

Order-disorder phase transition on the (100) surface of magnetite

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Scanning tunneling spectroscopy study of the electronic structure ofFe3O4surfaces

Cited by 74 publications

References 32 publications

Iron oxide surfaces

Iron oxide surfaces

Charge order at magnetite Fe3O4(0 0 1): surface and Verwey phase transitions

Order-disorder phase transition on the (100) surface of magnetite

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Charge order at magnetite Fe₃O₄(0 0 1): surface and Verwey phase transitions