Occupation Of Octahedral Sites Research Articles

A theoretical study of H surface and subsurface species on Pt(111)

Adsorption of H at a coverage ranging up to 2 monolayers with occupation of fcc and hcp sites and insertion in subsurface sites of Pt(1 1 1) is studied with use of a DFT-based ab initio method. It is shown that no important repulsion between the adatoms grows up to 1 ML, justifying the usual wisdom that 1 H atom adsorbs per Pt surface atom. Beyond this coverage, the surface occupation of both the fcc and the hcp sites become disfavoured and H starts to occupy subsurface sites. Both occupation of octahedral and tetrahedral insertion sites were studied. It is concluded that configurations involving fcc surface sites associated with tetrahedral subsurface sites are favoured. The evolution of the Pt local density of states (LDOS) with the coverage is reported. It is shown that the presence of H in the subsurface decrease the surface Pt chemical reactivity. Finally, stability curves of the Pt–H system as a function of H2 pressure and temperature are given, showing that subsurface H is likely to be present under usual catalytic reaction conditions.

Computational Study on Surface Structure and Crystal Morphology of γ-Fe2O3: Toward Deterministic Synthesis of Nanocrystals

This work is the first application of classical atomistic theory to a comprehensive treatment of γ-Fe2O3 surfaces. The surface energy and attachment energy of several low-index surfaces of γ-Fe2O3 have been calculated by the classical atomistic simulation methods. A mean-field approximation involving scaling the short-range potentials, charges, and force constants of the iron ions by a fraction corresponding to the partial occupation of octahedral iron sites in the spinel structure was employed. Reconstruction of the polar surfaces was required to remove the dipole perpendicular to the surface and to achieve structural stability. This was accomplished through the addition of vacancies. The two-dimensional periodic calculations were carried out with the MARVIN code. The (112) surface consisting of iron and oxygen ions had the smallest relaxed surface energy of 1.86 J/m2, while several others including (001), (011), and (012) were within 0.1 J/m2 of this value. The calculated surface energies of these plane...

Evaluating micas in petrologic and metallogenic aspect: I–definitions and structure of the computer program MICA +

Micas are significant ferromagnesian minerals in felsic to mafic igneous, metamorphic, and hydrothermal rocks. Because of their considerable potential to reveal the physicochemical conditions of magmas in terms of petrologic and metallogenic aspects, mica chemistry is used extensively in the earth sciences. For example, the composition of phlogopite and biotite can be used to evaluate the intensive thermodynamic parameters of temperature ( T, °C), oxygen fugacity ( fO 2), and water fugacity ( fH 2O) of magmatic rocks. The halogen contents of micas permit the estimation of the fluorine and chlorine fugacities that may be used in understanding the metal transportation and deposition processes in hydrothermal ore deposits. The Mica + computer program has been written to edit and store electron-microprobe or wet-chemical mica analyses. This software calculates structural formulae and shares out the calculated anions into the I, M, T, and A sites. Mica + classifies micas in terms of composition and octahedral site-occupancy. It also calculates the intensive parameters such as fO 2, T, and fH 2O from the composition of biotite in equilibrium with K-feldspar and magnetite. Using the calculated F–OH and Cl–OH exchange systematics and various log ratios ( fH 2O/ fHF, fH 2O/ fHCl, fHCl/ fHF, X Cl/ X OH, X F/ X OH, X F/ X Cl) of mica analyses. Mica + gives valuable determinations about the characteristics of hydrothermal fluids associated with alteration and mineralization processes. The program output is generally in the form of screen outputs. However, by using the “Grf” files that come up with this program they can be visualized under the Grapher software both as binary and ternary diagrams. Mica analyses subjected to the Mica + program were calculated on the basis of 22+ z positive charges taking into account the procedure by the Commission on New Mineral Names Mica Subcommittee of 1998.

Mossbauer studies and magnetic properties of BaFe/sub 12-X/Al/sub X/O/sub 19/ grown by a wet chemical process

Polycrystalline BaFe/sub 12-x/Al/sub x/O/sub 19/ powders (0.0/spl les/x/spl les/4.0) were fabricated by a wet chemical process. Magnetic properties and crystalline structure were investigated by using an X-ray diffractometer, vibrating sample magnetometer, and Mossbauer spectroscopy. The result of X-ray diffraction patterns shows that the a and c lattice parameters were decreased with increasing x from a=5.901 /spl Aring/ and c = 23.243 /spl Aring/ for x = 0.0, to a = 5.818 /spl Aring/ and c = 22.754 /spl Aring/ for x = 4.0. The saturation magnetization and the coercivity of BaFe/sub 10/Al/sub 2/O/sub 19/ were 30.2emu/g, 8.7 kOe, respectively, at room temperature under an applied field of 15 kOe. With increasing x, the relative intensity of octahedral sites has been obviously decreased. The authors interpreted the decreasing of relative area ratio of octahedral sites in Mossbauer spectra as closely related to the Al/sup 3+/ occupation of octahedral sites.

Preparation of zinc ferrite by high-energy ball-milling and microstructure characterization by Rietveld’s analysis

Nanocrystalline zinc ferrite is synthesized at room temperature by high-energy ball-milling the stiochiometric (1:1mol%) mixture of ZnO–α-Fe2O3. Nucleation of nanocrystalline Zn-ferrite particle takes place by solid-state diffusion between nanocrystalline ZnO and α-Fe2O3 particles. Both the particle size and collision temperature favors the formation mechanism. The process of mechanosynthesis of zinc ferrite at room temperature led to the formation of metastable inverse spinel structure. The formation of normal and inverse spinel structures is noticed after 30min and 2.5h ball-milling, respectively, and the degree of inversion increased with increasing milling time. The structural and microstructural evolution of ZnFe2O4 caused by milling is investigated by X-ray powder diffraction. The relative phase abundances of different phases, particle size, rms strain, lattice parameter change, etc. have been estimated from Rietveld powder structure refinement analysis of XRD data. A significant amount (wt.%) of nanocrystalline ZnFe2O4 is formed within 2.5h ball-milling with a very compressed lattice parameter. However, lattice parameter of 10h-milled sample annealed at 1273K for 1h agreed well the literature value and surprisingly, inverse spinel structure is completely absent in the annealed material. This leads to the conclusion that inverse spinel structure is a metastable one, formed under high-energy impact with a very small particle size (∼6nm) and high temperature annealing even for a shorter time is sufficient to reduce the disorderness in occupancy of tetrahedral and octahedral sites by Zn2+ and Fe3+ cations.

Where do the H atoms reside in PdH x systems?

We present an ab initio density-functional theory study of PdH x systems. We evaluated the total energy of PdH x systems with the H atoms occupying interstitial (octahedral and tetrahedral) sites of a Pd supercell, allowing for the relaxation of the coordinates and supercell dimensions. The majority of our calculations were based on supercells consisting of four Pd atoms, and up to four H atoms, covering the range from x = 0.25 to x = 1. In addition some larger calculations are reported. In order to compare the relative stability of systems at different values of x (at fixed pressure and temperature T = P = 0), we computed the enthalpy of formation ΔH f (x) of the (non)stoichiometric systems. In the regime x = 0 → 1, the ΔH f (x) decrease in a manner indicative of the existence of attractive interactions between the dissolved H atoms. Ideal-solution theory cannot be applied to this system. Furthermore, we find that tetrahedral occupation is favoured over octahedral occupation at high x, leading to the formation of a zincblende structure at x = 1. A preliminary vibrational analysis of normal modes has been performed. Inclusion of vibrational zero-point energies in a harmonic approximation leads us to conclude, tentatively, that the observed stability of octahedral site occupation is due to more favourable zero-point energies of the H atoms in those sites. The results indicate that a proper understanding of this system must take into account the quantum nature of the dissolved hydrogen.

Characterization of a natural magnetite

Single crystals of a rock magnetite were separated from steatite cobbles collected in a geological site near the city of Serro (18° 36′ 47′′ S 43° 22′ 46′′ W), Minas Gerais, Brazil. A typically well-shaped magnetite single crystal was characterized by chemical analysis, 57Fe Mossbauer spectrometry at 300, 77 and 4 K and under an applied magnetic field of 6 T at 10 K, magnetization measurements and electronic microprobe. From Mossbauer data, the sample is stoichiometric with a tetrahedral and octahedral site occupancy ratio of 1:2. Elemental chemical analysis and point-to-point electron microscope probing show some inclusions of lamellar ilmenite (≤ 1 mass%) randomly distributed throughout the magnetite matrix, and also that the magnetite matrix is constituted only by Fe2+ and Fe3+, with no isomorphic substitution. Results are discussed on the basis of the magnetization curve and of the temperature dependence of the AC magnetic susceptibility. The Verwey transition occurs in the temperature range of 100–115 K, observed by a sudden change in the temperature dependence of the magnetization.

Ab initiostudies of the passive film formed on iron

Passive metals are protected from the environment by a thin (3\char21{}5 nm) oxide film that forms on their surface. The corrosion rate of these materials is typically of the order of $0.1\ensuremath{\mu}\mathrm{m}/\mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r},$ and depends on the electronic structure of the oxide layer. Here we present ab initio total-energy calculations of the passive oxide film that forms on iron at anodic potentials in weakly alkaline solutions. Surface x-ray diffraction studies have revealed that this nanocrystalline passive film has a spinel structure with a fully occupied oxygen lattice [M. F. Toney, A. J. Davenport, L. J. Oblonsky, M. P. Ryan, and C. M. Vitus, Phys. Rev. Lett. $79,$ 4282 (1997)]. However, the octahedral and tetrahedral iron site occupancies are found to be reduced (approximately 80% and 66%, respectively) and partial occupancy of octahedral interstitial sites is also observed (approximately 12%). We have used total-energy pseudopotential calculations to study the energetics and electronic structure of these defects and the interrelationships between site occupancies. The calculations suggest that film is metastable and may be semi-conducting. The calculations also suggest a correlation between octahedral interstitials and tetrahedral vacancies. Finally, an estimation of energy barriers in the film suggests that cation migration through the tetrahedral sublattice dominates film growth.

Study of Thermally Treated Lithium Montmorillonite by Ab Initio Methods

The Hofmann−Klemen effect describes the observed reduction in expandable character and cation exchange capacity of lithium-saturated montmorillonite upon mild heating. Although it is widely accepted that the observed changes are due to the lithium ions becoming fixated within the layer structure of the clay mineral, some uncertainty still exists as to the precise location of the migrated lithium ions within the clay framework. It has been suggested that the cations reside in the ditrigonal cavities of the tetrahedral layer, the vacant octahedral sites, or both. We employ density functional methods to examine the phenomenon at the electronic structure level. We find that it is energetically preferable for lithium cations to reside in the vacant octahedral sites as opposed to the ditrigonal cavities due to the closer proximity to the negative charge sites in the octahedral layer increasing favorable Coulombic interactions. Occupation of octahedral sites causes structural hydroxyl groups to reorientate perpe...

Stability of subsurface oxygen at Rh(111)

Using density-functional theory (DFT) we investigate the incorporation of oxygen directly below the Rh(111) surface. We show that oxygen incorporation will only commence after nearly completion of a dense O adlayer $({\ensuremath{\theta}}_{\mathrm{tot}}\ensuremath{\approx}1.0$ monolayer) with O in the fcc on-surface sites. The experimentally suggested octahedral subsurface site occupancy, inducing a site-switch of the on-surface species from fcc to hcp sites, is indeed found to be a rather low-energy structure. Our results indicate that at even higher coverages oxygen incorporation is followed by oxygen agglomeration in two-dimensional subsurface islands directly below the first metal layer. Inside these islands, the metastable hcp/octahedral (on-surface/subsurface) site combination will undergo a barrierless displacement, introducing a stacking fault of the first metal layer with respect to the underlying substrate and leading to a stable fcc/tetrahedral site occupation. We suggest that these elementary steps, namely, oxygen incorporation, aggregation into subsurface islands and destabilization of the metal surface may be more general and precede the formation of a surface oxide at close-packed late transition metal surfaces.

Local interactions of carbon in FeAl alloys

A model study of the local interactions of carbon in a FeAl host is presented, employing the first principles density functional theory within the generalized gradient approximation. The geometric, electronic and magnetic structure of the perovskite K-carbide or Fe 3AlC is investigated for both the stoichiometric and non-stoichiometric phases. For the perovskite K-carbide, a 15-atom unit cell is used for the calculations. The bulk environment for this unit cell is simulated by including all 14 nearest neighbors for each of the six Fe-atoms on the face centers of the central unit cell. The investigations on the K-carbide reveal interesting patterns of atomic relaxations. While the stoichiometric phase constrains the Fe-atoms on the faces of the cube, a partial removal of carbon drives the Fe-clustering accompanied by the generation of large magnetic moments. Aluminum atoms, occupying the corners of the perovskite structure, are not perturbed significantly in either of the above situations. Based on the B2(CsCl)-lattice and the possible vacant site occupations of carbon atoms, a mechanism for the formation of Fe 3AlC is proposed. Further, the calculations on solute carbon in FeAl reveal a preferential occupation of octahedral sites to the tetrahedral sites. The optimal AlC bond requirements for these site occupations result in large tetragonal distortions of the host lattice.

Infrared Evidence of Dioctahedral-Trioctahedral Site Occupancy in Palygorskite

AbstractA Mg-rich palygorskite sample from phosphorite deposits of Ganntour (Morocco) with the structural formula Si8(Mg2.6Al1.19Fe0.33III□0.88)Ca0.056Na0.024K0.104O20(OH)2(OH2)4⋅4H2O, was studied by FTIR spectroscopy. In both OH-stretching and OH-bending regions, there is evidence of dioctahedral Al2□OH, AlFe□OH and trioctahedral Mg3OH features, leading to a di-trioctahedral crystallochemical model of octahedral site occupancies in ribbons of Ganntour palygorskite.This model, established through the IR spectroscopy study of a Mg-rich palygorskite, seems to be appropriate for many other palygorskites with lower Mg content in the octahedral sheet.

Infrared Spectroscopic Study of LiCoO2 Thin Films

Site disorder of Co3+ ions in sputtered films of lithium cobaltite has been examined using infrared spectroscopy. Both transmission and reflectance modes have been used to characterize the nature of IR absorption. It is found that Co3+ in the sputtered films occupy two types of octahedral sites that differ in the nature of second-neighbor environment. Li+ ions exhibit two bands, which may arise from tetrahedral and octahedral site occupancies or from the presence of disordered regions in the films.

Effects of iron oxidation states on the surface and structural properties of smectites

Abstract The oxidation state of iron (Fe) in the crystal structure of smectite clay minerals profoundly alters their physical-chemical properties. Among the properties affected are layer charge, cation exchange and fixation capacity, swelling in water, particle size, specific surface area, layer stacking order, magnetic exchange interactions, octahedral site occupancy, surface acidity, and reduction potential. Also affected is the surface chemistry of the clay, which alters clay–water and clay–organic interaction mechanisms. Rates and extents of degradation of pesticides are increased in the presence of reduced smectites compared to oxidized and reduced-reoxidized counterparts. A hypothesis regarding the mechanism for Fe reduction in clay minerals was first developed in 1963, and subsequent modifications have been proposed periodically through the present time. Recent studies clearly reveal that the process of Fe reduction involves more than the mere transfer of an electron to octahedral Fe(III) in the clay crystal. Ancillary reactions occur that produce significant structural modifications, some of which are reversible and others of which are not. Such changes in the crystal-chemical environment of structural Fe are thought to play a dominant role in altering the clay surface chemistry.

Simulation models of defects in MgAl2O4:Fe2+, Fe3+ spinels

A static computer simulation using the GULP code has been used to study dipole defects in iron doped spinels in normal and inverse structures. The occupation of tetrahedral (Mg2+) and octahedral (Al3+) sites in the normal structure was simulated and the results indicate a preferential replacement of Mg2- by an Al3+ justified by the observed stoichiometric deviation in synthetic spinels. Besides this, Al2O3 excess produces Mg2+ vacancies and favours the hole centre production. Antisite disorder of the form [Mg2+]Al 3+ and [Al3+]Mg 2+ is one of the possible mechanisms responsible for the existence of dipole defects in these materials. The obtained energy for the interchange between Al3+ and Mg2+ was 0.52eV. The calculated energies indicate that Fe2+ replaces Mg2+ and Fe3+ replaces Al3+. The presence of Fe3+ in octahedral sites takes into account the oxidation process of Fe2+ in these sites. Computer simulation has also been carried out on the inverse structure where the stability of the lattice was obtained for the Imma space group.

Structure refinement of large domain relaxors in the Pb(Mg 1/3Ta 2/3)O 3–PbZrO 3 system

The relaxor ferroelectric compound Pb(Mg 0.3Ta 0.6Zr 0.1)O 3 (PMT–PZ) was studied by X-ray, neutron and electron diffraction and transmission electron microscopy in the as-sintered and annealed states. The as-sintered sample was comprised of nanometer-sized 1:1 chemically ordered domains dispersed in a disordered matrix. After annealing at 1325°C the domain size increased to ∼30 nm and the degree of order exceeded 95% in terms of the volume fraction of the ordered domains, yet the sample retained its diffuse, frequency dependent relaxor characteristics. Refinements of the chemically ordered structure using the Rietveld analysis revealed that the octahedral (B) site occupancies were in excellent agreement with a “random site” model for the chemical ordering. In this charge-balanced model for the 1:1 ordered Pb(β′ 1/2β″ 1/2)O 3 structure the Ta cations predominantly occupy the β″ site, while the β′ site is populated by a random distribution of the Mg, Zr and remaining Ta cations. Large temperature factors for Pb and O atoms are observed in both as-sintered and annealed samples, indicating localized displacements of the Pb and O atoms. The mixed occupancy of the β′ position appears to be responsible for the relaxor characteristics in the dielectric response in spite of the growth of the chemical domains.

LIMICA: a program for estimating Li from electron-microprobe mica analyses and classifying trioctahedral micas in terms of composition and octahedral site occupancy

LIMICA: a program for estimating Li from electron-microprobe mica analyses and classifying trioctahedral micas in terms of composition and octahedral site occupancy

The Structure of the Passive Film That Forms on Iron in Aqueous Environments

In situ surface X‐ray diffraction was used to identify the detailed structure of the passive film that forms on (000)− and (110)‐oriented iron single crystals in a borate buffer solution at +0.4 V vs. mercurous sulfate reference electrode, a high passive potential. The passive film is a new phase: a spinel with a fully occupied oxygen lattice, octahedral site occupancy of 80 ± 10%, tetrahedral site occupancy of 66 ± 10%, and an octahedral interstitial site occupancy of 12 ± 4%. The passive film forms with an epitaxial relationship to the substrate iron; for growth on Fe(001), film(001)‖Fe(001) and , while for growth on Fe(110), film(111)‖Fe(110) and . The in‐plane lattice parameter for the passive film (the LAMM phase) is 8.39 ± 0.01 Å for growth on both faces, and the out‐of‐plane lattice parameter is 8.25 ± 0.1 Å [Fe(001)] and 8.42 ± 0.1 Å [Fe(110)]. The passive film forms a nanocrystalline microstructure with numerous defects. Specifically, the grain size is 50–80 Å in‐plane and about 30 Å out‐of‐plane. There is a small mosaic spread of 2.5 to 4.1° and a high density of antiphase boundaries and stacking faults. The structure of the film determined in situ was found to be identical to that found for an emersed sample, indicating that the high potential film studied here is stable on removal from the electrolyte. Some of the implications of the film structure on passivity are discussed. © 2000 The Electrochemical Society. All rights reserved.

Large zinc cation occupancy of octahedral sites in mechanically activated zinc ferrite powders

The cation site occupancy of a mechanically activated nanocrystalline zinc ferrite powder was determined as (Zn0.552+Fe0.183+)tet[Zr0.452+Fe1.823+]octO4 through analysis of extended x-ray absorption fine structure measurements, showing a large redistribution of cations between sites compared to normal zinc ferrite samples. The overpopulation of cations in the octahedral sites was attributed to the ascendance in importance of the ionic radii over the crystal energy and bonding coordination in determining which interstitial sites are occupied in this structurally disordered powder. Slight changes are observed in the local atomic environment about the zinc cations, but not the iron cations, with respect to the spinel structure. The presence of Fe3+ on both sites is consistent with the measured room temperature magnetic properties.

Neutron diffraction and mössbauer effect study of quenched 112CdFe 2O 4

Neutron diffraction analysis revealed 5% occupancy of octahedral (B) spinel lattice sites by cadmium in a 112Cd-enriched CdFe 2O 4 sample prepared by the ceramic technique and quenched from 1273 K in water. The very small magnetic contribution to intense Bragg peaks at 1.3 K hampered the magnetic structure determination. ME investigation confirmed the partial disorder of the Fe 3+ magnetic moments on B sites.