Atomistic Simulation of Defect Structures and Ion Transport in α‐Fe2O3 and α‐Cr2O3

Catlow, C. Richard A.; Corish, J.; Hennessy, John; Mackrodt, W. C.

doi:10.1111/j.1151-2916.1988.tb05758.x

Cited by 70 publications

(47 citation statements)

References 19 publications

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“…If this model is valid, a ratio S can be expected where further removal of OH results in major structural rearrangement (to hematite). A temperature of 400 K, however, is probably too low to enable sufficient cation diffusion, because of a very high activation energy (390-580 kJ/mole in hematite; Catlow et al, 1988). A plateau is therefore expected, at which the dehydration reaction comes almost to a stop.…”

Section: Discussionmentioning

confidence: 99%

The effect of dry heating on the chemistry, surface area, and oxalate solubility of synthetic 2-line and 6-line ferrihydrites

Stanjek

Weidler

1992

Clay miner.

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Synthetic 2-line and 6-line ferrihydrites were heated to 400 K, 500 K and 600 K, under dry conditions. Heating to 400 K lowered the ratios of structural OH to Fe2O3 without a phase transformation. The lowest ratios observed at 400 K were 1·23 (2-line) and 0·85 (6-line ferrihydrite) after 1150 h. For 6-line ferrihydrites heated to 400 K, the solubility in oxalate and the BET surface areas decreased with increasing heating time, whereas the unit-cell parameters, the relative peak intensities and the mean coherence lengths did not change significantly. Heating the 2-line ferrihydrites to 400 K produced microporosity.

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Section: Discussionmentioning

confidence: 99%

The effect of dry heating on the chemistry, surface area, and oxalate solubility of synthetic 2-line and 6-line ferrihydrites

Stanjek

Weidler

1992

Clay miner.

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“…According to Tamman's theory (cf. Gregg, 1953), however, volume diffusion only starts above the "Tamman temperature", which should be at approximately half the melting point of 1838 K. This leads to a very high activation energy of 390-580 kJ/mol for cation diffusion in hematite (Catlow et aL, 1988).…”

Section: Discussionmentioning

confidence: 99%

The Influence of Aluminum on Iron Oxides. Part XVI: Hydroxyl and Aluminum Substitution in Synthetic Hematites

Stanjek

Schwertmann

1992

Clays and clay miner.

102

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--Synthetic hematites with A1 substitutions between 0 and 18 mol % were synthesized at different temperatures and water activities. The cell-edge lengths a for different synthesis conditions decreased linearly with increasing A1 substitution. The regression lines, however, had different slopes and intercepts: the series with the highest synthesis temperature (1270 K) had the most negative slope. With increasing AI substitution, the hematites contained increasing amounts of non-surface water. Significant correlations were found between these chemically determined water contents and the deviations of the unit-cell parameters a, c, and V relative to the corresponding 1270 K regression lines. To explain the measured X-ray peak intensities, structural OH had to be included into the theoretical calculations. From intensity ratios normalized to Il ~ 3, it is possible to determine the structural OH separately from the AI substitution, which can be assessed by the shift of the cell-edge lengths relative to the 1270 K regression lines. The incorporation of A1 and OH into the hematite structure induces strain, which was quantified by X-ray diffraction.

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“…In general it seems that both V O and interstitial Fe can form, but Catlow et al [172] proposed that electronic disorder far outweighs ionic diffusion and is responsible for electric conductivity. Defective α-Fe 2 O 3 prepared under O 2 -poor conditions has been proposed to possess V O s, which leads to enhanced carrier concentration [173].…”

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confidence: 99%

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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Atomistic Simulation of Defect Structures and Ion Transport in α‐Fe₂O₃ and α‐Cr₂O₃

Cited by 70 publications

References 19 publications

The effect of dry heating on the chemistry, surface area, and oxalate solubility of synthetic 2-line and 6-line ferrihydrites

The effect of dry heating on the chemistry, surface area, and oxalate solubility of synthetic 2-line and 6-line ferrihydrites

The Influence of Aluminum on Iron Oxides. Part XVI: Hydroxyl and Aluminum Substitution in Synthetic Hematites

Iron oxide surfaces

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