The morphology of α-Al2O3 and α-Fe2O3: The importance of surface relaxation

Mackrodt, W. C.; Davey, Roger J.; Black, Simon; Docherty, R.

doi:10.1016/0022-0248(87)90093-5

Cited by 228 publications

(117 citation statements)

References 13 publications

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“…Hematite is isostructural with corundum (α-Al 2 O 3 ) and cromia (α-Cr 2 O 3 ), which have been studied extensively, both by means of atomistic simulation techniques and experimentally [53][54][55][56][57]63,[124][125][126]. The hematite structure contains iron and oxygen atoms arranged in a trigonal-hexagonal scalenohedral (class) structure with space group R-3c and lattice parameters a = b = 5.0356 Å, c = 13.7489 Å, with six formula units per unit cell [1].…”

Section: Bulk α-Fe 2 O 3 Structurementioning

confidence: 99%

A Density Functional Theory Study of the Adsorption of Benzene on Hematite (α-Fe2O3) Surfaces

2014

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Abstract:The reactivity of mineral surfaces in the fundamental processes of adsorption, dissolution or growth, and electron transfer is directly tied to their atomic structure. However, unraveling the relationship between the atomic surface structure and other physical and chemical properties of complex metal oxides is challenging due to the mixed ionic and covalent bonding that can occur in these minerals. Nonetheless, with the rapid increase in computer processing speed and memory, computer simulations using different theoretical techniques can now probe the nature of matter at both the atomic and sub-atomic levels and are rapidly becoming an effective and quantitatively accurate method for successfully predicting structures, properties and processes occurring at mineral surfaces. In this study, we have used Density Functional Theory calculations to study the adsorption of benzene on hematite (α-Fe 2 O 3 ) surfaces. The strong electron correlation effects of the Fe 3d-electrons in α-Fe 2 O 3 were described by a Hubbard-type on-site Coulomb repulsion (the DFT+U approach), which was found to provide an accurate description of the electronic and magnetic properties of hematite. For the adsorption of benzene on the hematite surfaces, we show that the adsorption geometries parallel to the surface are energetically more stable than the vertical ones. The benzene molecule interacts with the hematite surfaces through π-bonding in the parallel adsorption geometries and through weak hydrogen bonds in the vertical geometries. Van der Waals interactions are found to play a significant role in stabilizing the absorbed benzene molecule. Analyses of the electronic structures reveal that upon benzene adsorption, the conduction band edge of the surface atoms is shifted towards the valence bands, thereby considerably reducing the band gap and the magnetic moments of the surface Fe atoms. OPEN ACCESSMinerals 2014, 4 90

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Section: Bulk α-Fe 2 O 3 Structurementioning

confidence: 99%

A Density Functional Theory Study of the Adsorption of Benzene on Hematite (α-Fe2O3) Surfaces

2014

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“…In addition, the generalized VDOS obtained with neutron scattering 1 is depicted, which shows the same main characteristics of the curve on a qualitative level. The literature surface energies [33][34][35][36] given in Table II differ among each other which origins from the different methods used in the ab-initio approaches. Our calculation of the (0001) surface energy agrees with the value obtained in Ref.…”

Section: Validationmentioning

confidence: 99%

Simulation of crack propagation in alumina with ab initio based polarizable force field

Hocker

Beck

Schmauder

et al. 2012

The Journal of Chemical Physics

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We present an effective atomic interaction potential for crystalline α-Al 2 O 3 generated by the program potfit. The Wolf direct, pairwise summation method with spherical truncation is used for electrostatic interactions. The polarizability of oxygen atoms is included by use of the Tangney-Scandolo interatomic force field approach. The potential is optimized to reproduce the forces, energies and stresses in relaxed and strained configurations as well as {0001}, {1010} and {1120} surfaces of Al 2 O 3 . Details of the force field generation are given, and its validation is demonstrated. We apply the developed potential to investigate crack propagation in α-Al 2 O 3 single crystals.

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“…Growth morphologies are calculated from attachment energies (the energy needed to add an extra layer of unit cells to a crystal) and are considered applicable to larger crystals (Hartman & Bennema, 1980). Mackrodt et al (1987) calculated equilibrium morphologies from unrelaxed and relaxed surfaces for -Al 2 O 3 and -Fe 2 O 3 . They found substantial differences between the relaxed and unrelaxed morphologies, highlighting the importance of including surface relaxations in morphological calculations.…”

Section: Surfacesmentioning

confidence: 99%

Ferroelastic phase transitions: structure and microstructure

Salje

Hayward

Lee

2004

Acta Crystallogr A Found Crystallogr

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Landau-type theories describe the observed behaviour of phase transitions in ferroelastic and co-elastic minerals and materials with a high degree of accuracy. In this review, the derivation of the Landau potential G = 1 2 A S [coth( S /T) À coth( S /T C )]Q 2 + 1 4 BQ 4 + F F F is derived as a solution of the general 0 4 model. The coupling between the order parameter and spontaneous strain of a phase transition brings the behaviour of many phase transitions to the mean-®eld limit, even when the atomistic mechanism of the transition is spin-like. Strain coupling is also a common mechanism for the coupling between multiple order parameters in a single system. As well as changes on the crystal structure scale, phase transitions modify the microstructure of materials, leading to anomalous mesoscopic features at domain boundaries. The mesostructure of a domain wall is studied experimentally using X-ray diffraction, and interpreted theoretically using Ginzburg±Landau theory. One important consequence of twin mesostructures is their modi®ed transport properties relative to the bulk. Domain wall motion also provides a mechanism for superelastic behaviour in ferroelastics. At surfaces, the relaxations that occur can be described in terms of order parameters and Landau theory. This leads to an exponential pro®le of surface relaxations. This in turn leads to an exponential interaction energy between surfaces, which can, if large enough, destabilize symmetrical morphologies in favour of a platelet morphology. Surface relaxations may also affect the behaviour of twin walls as they intersect surfaces, since the surface relaxation may lead to an incompatibility of the two domains at the surface, generating large strains at the relaxation. Landau theory may also be extended to describe the kinetics of phase transitions. Time-dependent Landau theory may be used to describe the kinetics of order±disorder phase transitions in which the order parameter is homogeneous. However, the time-dependent Landau theory equations also have microstructural solutions, explaining the formation of microstructures such as tweed.

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The morphology of α-Al2O3 and α-Fe2O3: The importance of surface relaxation

Cited by 228 publications

References 13 publications

A Density Functional Theory Study of the Adsorption of Benzene on Hematite (α-Fe2O3) Surfaces

A Density Functional Theory Study of the Adsorption of Benzene on Hematite (α-Fe2O3) Surfaces

Simulation of crack propagation in alumina with ab initio based polarizable force field

Ferroelastic phase transitions: structure and microstructure

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