Structure of Feroxyhite as Determined by Simulation of X-Ray Diffraction Curves

Clay Minerals ◽  
1993 ◽  
Vol 28 (2) ◽  
pp. 209-222 ◽  
Author(s):  
V. A. Drits ◽  
B. A. Sakharov ◽  
A. Manceau
Keyword(s):  
Structural Parameters ◽  
Stacking Faults ◽  
Site Occupancy ◽  
Volume Ratio ◽  
Coherent Scattering ◽  
Close Packing ◽  
X Ray Diffraction ◽  
X Ray ◽  
Octahedral Sites ◽  
Atomic Coordinates

AbstractPowder X-ray diffraction (XRD) curves were calculated for the different structural models so far proposed for feroxyhite (δFeOOH). The influence on XRD features of different structural parameters, including site occupancy of Fe atoms, atomic coordinates, content and distribution of stacking faults, and dimension of coherent scattering domains, were considered. On the basis of agreement between experimental and simulated curves it is shown that δFeOOH is a mixture of feroxyhite proper and ultradispersed hematite in the 9 : 1 volume ratio. Feroxyhite proper consists of hexagonal close packing of anions containing 5% stacking faults. Iron atoms occupy only octahedral sites and are distributed in such a way that face-sharing filled octahedral pairs regularly alternate with vacant octahedral pairs along the c axis. This distribution of Fe atoms is quite similar to that established by Patrat et al. (1983), but in each pair, Fe atoms are displaced by the same value of 0.3 Å in opposite directions away from the centre of their octahedron. Nearest Fe-Fe distances calculated for the model proposed (2.88, 3.01, 3-39 and 3-73 Å) practically coincide with those found by EXAFS spectroscopy for the same sample (2-91, 3.04, 3.41 and 3.7-3.8 Å).

Structural Model for Ferrihydrite

Clay Minerals ◽  
1993 ◽  
Vol 28 (2) ◽  
pp. 185-207 ◽  
Author(s):  
V. A. Drits ◽  
B. A. Sakharov ◽  
A. L. Salyn ◽  
A. Manceau
Keyword(s):  
Structural Model ◽  
Structural Difference ◽  
Coherent Scattering ◽  
Close Packing ◽  
X Ray Diffraction ◽  
Actual Structure ◽  
X Ray ◽  
Cell Parameters ◽  
Super Cell ◽  
Mean Dimension

AbstractThe structure of 6-line and 2-line ferrihydrite (Fh) has been reconsidered. X-ray diffraction (XRD) curves were first simulated for the different structural models so far proposed, and it is shown that neither of these corresponds to the actual structure of ferrihydrite. On the basis of agreement between experimental and simulated XRD curves it is shown that Fh is a mixture of three components: (i) Defect-free Fh consisting of anionic ABACA . . . close packing in which Fe atoms occupy only octahedral sites with 50% probability; the hexagonal unit-cell parameters are a = 2-96 Å and c = 9-40 Å, and the space group is P1c. (ii) Defective Fh in which Ac1Bc2A and Ab1Cb2A structural fragments occur with equal probability and alternate completely at random; Fe atoms within each of these fragments have identical ordered distribution with in the hexagonal super-cell with a = 5.26 Å. (iii) Ultradispersed hematite with mean dimension of coherent scattering domains (CSD) of 10-20 Å. The main structural difference between 6-line and 2-line Fh is the size of their CSD which is extremely small for the latter structure. Nearest Fe-Fe distances calculated for this new structural model are very close to those determined by EXAFS spectroscopy on the same samples.

scholarly journals Crystal structure of Ni-sorbed synthetic vernadite: a powder X-ray diffraction study

2008 ◽  
Vol 72 (6) ◽  
pp. 1279-1291 ◽  
Author(s):  
S. Grangeon ◽  
B. Lanson ◽  
M. Lanson ◽  
A. Manceau
Keyword(s):  
Crystal Structure ◽  
Coherent Scattering ◽  
Layer Charge ◽  
Layer Plane ◽  
X Ray Diffraction ◽  
X Ray ◽  
Octahedral Sites ◽  
Ph Values ◽  
Oceanic Sediments ◽  
Ni Uptake

AbstractVernadite is a nanocrystalline turbostratic phyllomanganate containing Ni, and is widespread in surface environments and oceanic sediments. To improve our understanding of Ni uptake in this mineral, two series of analogues of vernadite (δ-MnO2) were prepared with Ni/Mn atomic ratios of 0.002—0.105 at pH4 and 0.002—0.177 at pH 7. Their structures were characterized using X-ray powder diffraction (XRD). The δ-MnO2 nano-crystals are essentially monolayers with coherent scattering domains sizes of ∼10 Å perpendicular to the layering and ∼55 Å within the layer plane. For Ni/Mn < 0.01, the layer charge deficit is apparently balanced entirely by interlayer Mn, Na and protons. At higher Ni/Mn, Ni occupies the same site as interlayer Mn above and below vacant sites within the MnO2 layer and at sites along the edges of the layer. However, the layer charge is balanced differently at the two pH values. At pH 4, Ni uptake is accompanied by a reduction in structural Na and protons, whereas interlayer Mn remains strongly bound to the layers. At pH 7, interlayer Mn is less strongly bound and is partially replaced by Ni. The results of this study also suggest that the number of vacant octahedral sites and multi-valent charge-copmpensating interlayer species are underestimated by the currently used structure models of δ-MnO2.

In-plane crystalline anisotropy of bulk β-Ga2O3

2021 ◽  
Vol 54 (4) ◽  
Author(s):  
Xiaocui Ma ◽  
Rui Xu ◽  
Jianfang Xu ◽  
Leiying Ying ◽  
Yang Mei ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Grain Boundaries ◽  
Bulk Material ◽  
Stacking Faults ◽  
Coherent Scattering ◽  
Limiting Factors ◽  
X Ray Diffraction ◽  
Limited Size ◽  
X Ray ◽  
Crystalline Anisotropy

The anisotropy of X-ray diffraction scanning of (201) β-Ga2O3 bulk material has been investigated. Symmetric rocking curves (RCs) exhibit distinctly different broadening along different azimuths, with a maximum along [102] and a minimum along a direction rotated by 30° from [010]. Williamson–Hall analysis was applied to study possible factors causing the broadening in these RCs, including instrumental factors, mosaic tilt and coherent scattering. It was found that the RC broadening is determined by both isotropic mosaic tilt and anisotropy in the length over which the crystal structure is not disrupted by limiting factors such as grain boundaries or stacking faults, which we term the `lateral limited size'. In this case, the lateral limited size is governed by {200} stacking faults along the [102] direction and grain boundaries along the [010] direction. The result presents a new anisotropy characteristic of (201) β-Ga2O3.

scholarly journals Structural and dielectric studies of Mg2+ substituted Ni–Zn ferrite

2015 ◽  
Vol 33 (4) ◽  
pp. 806-815 ◽  
Author(s):  
B.B.V.S. Vara Prasad ◽  
B. Rajesh Babu ◽  
M. Siva Ram Prasad
Keyword(s):  
Structural Properties ◽  
Cation Distribution ◽  
Structural Parameters ◽  
Site Occupancy ◽  
Dielectric Constants ◽  
Magnesium Concentration ◽  
X Ray Diffraction ◽  
X Ray ◽  
Zn Ferrite ◽  
Intensity Calculations

AbstractPolycrystalline ferrites having the chemical formula Ni0.65−xZn0.35MgxFe2O4 (0 ⩽ x ⩽ 0.2) were prepared by solid state reaction route in steps of x = 0.04. The effect of incorporation of diamagnetic divalent magnesium at expense of nickel on the structural properties of these ferrites has been studied. The proposed cation distribution was derived from theoretical X-ray diffraction intensity calculations. These intensity calculations were done by varying the concentration of magnesium ions over two sites in the lattice. For a certain amount of magnesium concentration, the calculated and observed X-ray diffraction intensities were found to be in good agreement. Site occupancy of divalent diamagnetic magnesium was established from this cation distribution. The octahedral environment facilitates magnesium to enter the B-site at about 95 % and the remaining 5 % occupy tetrahedral sites (A-sites). The movements of cations between tetrahedral and octahedral sites as a result of magnesium substitution were discussed in the view of structural parameters, such as tetrahedral and octahedral bond lengths, cation-cation and cation-anion distances, bond angles and hopping lengths, which were calculated using experimental lattice constants and oxygen parameters. All structural parameters showed slight deviations from ideal values. Among all magnesium substituted samples, the ones with x = 0.12 exhibited insignificant variation in view of structural properties. Dielectric measurements were conducted at a standard frequency of 1 kHz. Large values of the recorded dielectric constants displayed typical characteristics of bulk ferrites. Both dielectric constant and loss values showed mixed variations, attributed to the loss of zinc ions during the sintering process.

scholarly journals Probing the Deformation Mechanisms of Nanocrystalline Silver by In-Situ Tension and Synchrotron X-ray Diffraction

Metals ◽  
2020 ◽  
Vol 10 (12) ◽  
pp. 1635
Author(s):  
Baoru Sun ◽  
Tongde Shen
Keyword(s):  
Plastic Deformation ◽  
Grain Growth ◽  
Structural Parameters ◽  
Stacking Faults ◽  
Constant Strain Rate ◽  
Deformation Mechanisms ◽  
Square Root ◽  
X Ray Diffraction ◽  
X Ray ◽  
Density Of Dislocations

The mechanisms responsible for the deformation of nanocrystalline materials are not well understood although many mechanisms have been proposed. This article studies the room-temperature stress-strain relations of bulk nanocrystalline silver deformed in a tension mode at a constant strain rate. Synchrotron X-ray diffraction patterns were gathered from the deformed specimen and used to deduce such structural parameters as the grain size and the density of dislocations, twins, and stacking faults. Our quantitative results indicate that grain growth and twinning occur in the stage of elastic deformation. Detwinning and accumulation of stacking faults occur in the early stage of plastic deformation, where the strength of nanocrystalline silver correlates well with the square root of stacking faults probability. Grain shrinking and generation of statistically stored dislocations occur in the final stage of plastic deformation, where the strength of nanocrystalline silver correlates well with the square root of the density of dislocations (statistically stored and geometrically necessary). Our results suggest that multiple deformation mechanisms such as grain growth, grain shrinking, twinning, detwinning, stacking faults, and dislocations, rather than a single deformation mechanism, occur in the elastic and plastic deformation stages of nanocrystalline silver.

Calculation of Structural Parameters in Isostructural Series: the Kieserite Group

1998 ◽  
Vol 54 (5) ◽  
pp. 564-567 ◽  
Author(s):  
S. Aleksovska ◽  
V. M. Petrusevski ◽  
B. Soptrajanov
Keyword(s):  
Crystal Structures ◽  
General Formula ◽  
Structural Parameters ◽  
Unit Cell ◽  
X Ray Diffraction ◽  
Unit Cell Parameters ◽  
Ionic Radii ◽  
X Ray ◽  
Cell Parameters ◽  
Atomic Coordinates

In order to demonstrate the possibility of predicting the structural parameters of members in a sequence of isostructural compounds, the kieserite group isotypes (with the general formula M II XO4.H2O) were chosen since a number of them have accurately refined crystal structures. The unit-cell parameters and the fractional atomic coordinates were shown to vary linearly with both cation and anion size. This makes it possible to calculate the structural parameters of a particular member, taking into account only the effective ionic radii of the constituent atoms. Agreement between the calculated and experimentally refined (by X-ray diffraction) structural parameters is good. The cell constants and atomic coordinates of FeSeO4.H2O, iron selenate monohydrate, are predicted in this way.

Defect structures of Nb1+αS2 sulfidation scales

1992 ◽  
Vol 50 (1) ◽  
pp. 38-39
Author(s):  
Chuxin Zhou ◽  
L. W. Hobbs
Keyword(s):  
Stacking Faults ◽  
Rhombohedral Structure ◽  
Stacking Sequence ◽  
Defect Structures ◽  
X Ray Diffraction ◽  
X Ray ◽  
Plane Normal ◽  
Lower Case ◽  
Sulfur Layer ◽  
Two Phases

One of the major purposes in the present work is to study the high temperature sulfidation properties of Nb in severe sulfidizing environments. Kinetically, the sulfidation rate of Nb is satisfactorily slow, but the microstructures and non-stoichiometry of Nb1+αS2 challenge conventional oxidation/sulfidation theory and defect models of non-stoichiometric compounds. This challenge reflects our limited knowledge of the dependence of kinetics and atomic migration processes in solid state materials on their defect structures.Figure 1 shows a high resolution image of a platelet from the middle portion of the Nb1+αS2 scale. A thin lamellar heterogeneity (about 5nm) is observed. From X-ray diffraction results, we have shown that Nb1+αS2 scale is principally rhombohedral structure, but 2H-NbS2 can result locally due to stacking faults, because the only difference between these 2H and 3R phases is variation in the stacking sequence along the c axis. Following an ABC notation, we use capital letters A, B and C to represent the sulfur layer, and lower case letters a, b and c to refer to Nb layers. For example, the stacking sequence of 2H phase is AbACbCA, which is a ∼12Å period along the c axis; the stacking sequence of 3R phase is AbABcBCaCA to form an ∼18Å period along the c axis. Intergrowth of these two phases can take place at stacking faults or by a shear in the basal plane normal to the c axis.

scholarly journals Simultaneous Multi-Bragg Peak Coherent X-ray Diffraction Imaging

Crystals ◽  
2021 ◽  
Vol 11 (3) ◽  
pp. 312
Author(s):  
Florian Lauraux ◽  
Stéphane Labat ◽  
Sarah Yehya ◽  
Marie-Ingrid Richard ◽  
Steven J. Leake ◽  
...  
Keyword(s):  
Bragg Reflection ◽  
Volume Ratio ◽  
X Ray Diffraction ◽  
X Ray ◽  
Crystal Substrate ◽  
In Series ◽  
Diffraction Imaging ◽  
Diffraction Patterns ◽  
Lattice Planes ◽  
Oriented Parallel

The simultaneous measurement of two Bragg reflections by Bragg coherent X-ray diffraction is demonstrated on a twinned Au crystal, which was prepared by the solid-state dewetting of a 30 nm thin gold film on a sapphire substrate. The crystal was oriented on a goniometer so that two lattice planes fulfill the Bragg condition at the same time. The Au 111 and Au 200 Bragg peaks were measured simultaneously by scanning the energy of the incident X-ray beam and recording the diffraction patterns with two two-dimensional detectors. While the former Bragg reflection is not sensitive to the twin boundary, which is oriented parallel to the crystal–substrate interface, the latter reflection is only sensitive to one part of the crystal. The volume ratio between the two parts of the twinned crystal is about 1:9, which is also confirmed by Laue microdiffraction of the same crystal. The parallel measurement of multiple Bragg reflections is essential for future in situ and operando studies, which are so far limited to either a single Bragg reflection or several in series, to facilitate the precise monitoring of both the strain field and defects during the application of external stimuli.

Polyhedral characteristics of the cosalite-type crystal structures

2019 ◽  
Vol 57 (5) ◽  
pp. 647-662
Author(s):  
Sabina Kovač ◽  
Predrag Dabić ◽  
Aleksandar Kremenović ◽  
Aleksandar Pačevski ◽  
Ljiiljana Karanović ◽  
...  
Keyword(s):  
Crystal Data ◽  
Chemical Formula ◽  
X Ray Diffraction ◽  
Orthorhombic Space Group ◽  
X Ray ◽  
Area Detector ◽  
Octahedral Sites ◽  
Distorted Octahedral ◽  
Cation Substitutions ◽  
Type Crystal

Abstract The crystal structure of cosalite from the Trepča orefield was refined in the orthorhombic space group Pnma [a = 23.7878 (9), b = 4.0566 (3), c = 19.1026 (8) Å, V = 1843.35 (17) Å3, Z = 2] from single-crystal data (MoKα X-ray diffraction, CCD area detector) to the conventional R1 factor 0.031 for 1516 unique reflections with I > 2σ(I). The chemical formula (Cu0.15Ag0.24)+(Fe0.19Pb7.20)2+(Bi7.06Sb1.06)3+S20, calculated on the basis of 20 S atoms per formula unit, was determined by WDX. The unit cell contains 18 + 2 symmetrically nonequivalent atomic sites: 10 occupied by S; two by pure Pb (Pb3 and Pb4); one by pure Bi (Bi1); two by a combination of Bi and small amounts of Sb (Bi2/Sb2, Bi4/Sb3); two by Pb and Bi, and in one of these also by a small amount of Ag [Me1 = Pb2 >> Bi5 > Ag1, Me3 = Pb1 >> Bi3]; and finally one site, Me2 (Bi6 >> □), is partly occupied by Bi and partly split into an additional two adjacent trigonal planar “interstitial positions”, Cu1 and Cu2, where small amounts of Cu, Ag, and Fe can be situated. All atoms are at 4c special positions at y = 0.25 or 0.75. The structure consists of slightly to moderately distorted MeS6 octahedra sharing edges, bicapped trigonal PbS8 coordination prisms, and fairly distorted Cu1S6 and Cu2S4 polyhedra. The effects of the cation substitutions, bond valence sums, and the polyhedral characteristics are compared with other published cosalite-type structures. Among known cosalite-type structures, the largest volume contraction is shown by sample 4 (Altenberg) and involves the replacement of large cations (Bi3+ and Pb2+) by the smaller Sb3+, as well as Cu+ and Ag+. These replacements are reflected in the variations of individual Me–S bond distances, which are accompanied by variations in average Me–S distances. The degree of polyhedral distortion, Δ, progressively increases for the four Bi-hosting sites of nine cosalite-type structures: Me2 < Bi2 < Bi1 < Bi4. The Bi4 and Me3 are the most and the Me1 and Me2 are the least distorted octahedral sites of the nine cosalite-type structures.

Investigation of Residual Strains of the Second Kind in Plastically Deformed Metallic Materials

1994 ◽  
Vol 376 ◽  
Author(s):  
M. Vrána ◽  
P. Klimanek ◽  
T. Kschidock ◽  
P. Lukáš ◽  
P. Mikula
Keyword(s):  
High Resolution ◽  
Neutron Diffraction ◽  
Stacking Faults ◽  
Metallic Materials ◽  
Line Broadening ◽  
X Ray Diffraction ◽  
X Ray ◽  
Neutron Diffractometer ◽  
Residual Strains ◽  
Good Agreement

ABSTRACTInvestigation of strongly distorted crystal structures caused by dislocations, stacking-faults etc. in both plastically deformed f.c.c. and b.c.c. metallic materials was performed by the analysis of the neutron diffraction line broadening. Measurements were realized by means of the high resolution triple-axis neutron diffractometer equipped by bent Si perfect crystals as monochromator and analyzer at the NPI Řež. The substructure parameters obtained in this manner are in good agreement with the results of X-ray diffraction analysis.

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