scholarly journals Natural Nanomaterials: Reappraising the Elusive Structure of the Nano-Sized Mineral Ferrihydrite through X-Ray Absorption Spectroscopy at the Iron K-Edge

2012 ◽  
Vol 730-732 ◽  
pp. 931-935 ◽  
Author(s):  
Maria Ondina Figueiredo ◽  
Teresa P. Silva ◽  
Joao P. Veiga
Keyword(s):  
Distribution Functions ◽  
Mine Wastes ◽  
X Ray Diffraction ◽  
X Ray ◽  
Ferric Oxyhydroxide ◽  
Atomic Pair ◽  
Icosahedral Clusters ◽  
X Ray Absorption ◽  
Pair Distribution Functions ◽  
Tetrahedral Iron

Ferrihydrite is natural ferric oxyhydroxide occurring exclusively nanocrystalline. With ideal formula 5 Fe2 O3 . 9 H2 O, ferrihydrite is quite abundant in sediments, weathering crusts and mine wastes, being characteristic of red pre-soils formed by loose weathered rock plus mineral debris (regoliths) and commonly designated as “2-line” or “6-line” on the basis of the broadened maxima observed in the X-ray diffraction pattern. Synthetic nanocrystalline “6-line” ferrihydrite was recently studied through methods based on atomic-pair distribution functions disclosing the possible occurrence of icosahedral clusters formed by twelve octahedra centred by an inner tetrahedron, all filled by Fe 3+ ions. However, Mössbauer studies were inconclusive about the existence of 4-coordinated iron, thus suggesting that the tetrahedral cation may well be Si4+. In view of such structural uncertainty, a XANES study at the Fe K-edge was undertaken on ferrihydrite from a regolith to ascertain the occurrence of tetrahedral iron. Comparison with data collected from well crystallized iron oxide and hydroxide minerals where Fe 3+/2+ ions occur in octahedral and tetrahedral coordination is described and the results so far obtained are discussed, showing that supplementary study is needed on the elusive structure of ferrihydrite.

Atomic ordering in nanosized PtxAu1–x(x= 0, 0.51, 1) by resonant X-ray diffraction and differential atomic pair distribution functions

2012 ◽  
Vol 227 (5) ◽  
pp. 262-267 ◽  
Author(s):  
Valeri Petkov ◽  
Sarvjit Shastri ◽  
Bridgid Wanjala ◽  
Rameshiwori Loukrakpam ◽  
Jin Luo ◽  
...  
Keyword(s):  
Distribution Functions ◽  
X Ray Diffraction ◽  
Atomic Ordering ◽  
X Ray ◽  
Atomic Pair ◽  
Pair Distribution Functions

Synthesis and Characterization of Akaganeite-Like Ferric Oxyhydroxides

1996 ◽  
Vol 432 ◽  
Author(s):  
J.C. Linehan ◽  
J. G. Darab ◽  
D. W. Matson ◽  
X. Chen ◽  
J. E. Amonette
Keyword(s):  
Fine Structure ◽  
Hydrothermal Method ◽  
Chloride Ions ◽  
Synthesis And Characterization ◽  
X Ray Diffraction ◽  
X Ray ◽  
Absorption Fine ◽  
Ferric Oxyhydroxide ◽  
X Ray Absorption

AbstractAkaganeite, β-FeOOH, a natural ferric oxyhydroxide mineral, has a structure containing tunnel-like cavities in which chloride ions reside. Analogs of akaganeite in which carbonate or sulfate groups replace the chloride ions have also been synthesized. Both akaganeite and its substituted analogs are known to be precursors for active hydrocracking catalysts. We present powder X-ray diffraction (XRD) and X-ray absorption fine-structure spectroscopy (XAFS) data confirming the synthesis of new ferric oxyhydroxides having structures similar to akaganeite, but contain molybdate and tungstate oxy-anions. We also present a new hydrothermal method to prepare this family of materials.

scholarly journals Crystal structure solution from experimentally determined atomic pair distribution functions

2010 ◽  
Vol 43 (3) ◽  
pp. 623-629 ◽  
Author(s):  
P. Juhás ◽  
L. Granlund ◽  
S. R. Gujarathi ◽  
P. M. Duxbury ◽  
S. J. L. Billinge
Keyword(s):  
Crystal Structure ◽  
Distribution Functions ◽  
Unit Cell ◽  
Second Step ◽  
X Ray ◽  
Atomic Pair ◽  
Structure Solution ◽  
Pair Distribution Functions ◽  
Atomic Radii ◽  
Periodic Crystal

An extension of the Liga algorithm for structure solution from atomic pair distribution functions (PDFs), to handle periodic crystal structures with multiple elements in the unit cell, is described. The procedure is performed in three separate steps. First, pair distances are extracted from the experimental PDF. In the second step the Liga algorithm is used to find unit-cell sites consistent with these pair distances. Finally, the atom species are assigned over the cell sites by minimizing the overlap of their empirical atomic radii. The procedure has been demonstrated on synchrotron X-ray PDF data from 16 test samples. The structure solution was successful for 14 samples, including cases with enlarged supercells. The algorithm success rate and the reasons for the failed cases are discussed, together with enhancements that should improve its convergence and usability.

Local atomic structure in tetragonal pure ZrO2nanopowders

2010 ◽  
Vol 43 (2) ◽  
pp. 227-236 ◽  
Author(s):  
Leandro M. Acuña ◽  
Diego G. Lamas ◽  
Rodolfo O. Fuentes ◽  
Ismael O. Fábregas ◽  
Márcia C. A. Fantini ◽  
...  
Keyword(s):  
Tetragonal Phase ◽  
Local Structure ◽  
X Ray Diffraction ◽  
X Ray ◽  
Atomic Structures ◽  
Crystallite Sizes ◽  
Exafs Study ◽  
The Difference ◽  
X Ray Absorption ◽  
Good Agreement

The local atomic structures around the Zr atom of pure (undoped) ZrO2nanopowders with different average crystallite sizes, ranging from 7 to 40 nm, have been investigated. The nanopowders were synthesized by different wet-chemical routes, but all exhibit the high-temperature tetragonal phase stabilized at room temperature, as established by synchrotron radiation X-ray diffraction. The extended X-ray absorption fine structure (EXAFS) technique was applied to analyze the local structure around the Zr atoms. Several authors have studied this system using the EXAFS technique without obtaining a good agreement between crystallographic and EXAFS data. In this work, it is shown that the local structure of ZrO2nanopowders can be described by a model consisting of two oxygen subshells (4 + 4 atoms) with different Zr—O distances, in agreement with those independently determined by X-ray diffraction. However, the EXAFS study shows that the second oxygen subshell exhibits a Debye–Waller (DW) parameter much higher than that of the first oxygen subshell, a result that cannot be explained by the crystallographic model accepted for the tetragonal phase of zirconia-based materials. However, as proposed by other authors, the difference in the DW parameters between the two oxygen subshells around the Zr atoms can be explained by the existence of oxygen displacements perpendicular to thezdirection; these mainly affect the second oxygen subshell because of the directional character of the EXAFS DW parameter, in contradiction to the crystallographic value. It is also established that this model is similar to another model having three oxygen subshells, with a 4 + 2 + 2 distribution of atoms, with only one DW parameter for all oxygen subshells. Both models are in good agreement with the crystal structure determined by X-ray diffraction experiments.

The Relative Merits of Oxides of Hafnium, Cerium and Thorium as Surrogates for Plutonium Oxide in Calcium Phosphate Ceramics

2009 ◽  
Vol 1193 ◽  
Author(s):  
B. L. Metcalfe ◽  
S. K. Fong ◽  
L. A. Gerrard ◽  
I. W. Donald ◽  
E. S. Welch ◽  
...  
Keyword(s):  
Calcium Phosphate ◽  
Calcination Temperature ◽  
Cerium Oxide ◽  
Absorption Spectroscopy ◽  
Hafnium Oxide ◽  
Intermediate Level ◽  
X Ray Diffraction ◽  
X Ray ◽  
X Ray Absorption ◽  
Additional Phase

AbstractThe choice of surrogate for plutonium oxide for use during the initial stages of research into the immobilization of intermediate level pyrochemical wastes containing plutonium andamericium oxides in a calcium phosphate host has been investigated by powder X-ray diffraction and X-ray absorption spectroscopy. Two non-radioactive surrogates, hafnium oxide and cerium oxide, together with radioactive thorium oxide were compared. Similarities in behaviour were observed for all three surrogates when calcined at the lowest temperature, 750°C but differences became more pronounced as the calcination temperature was increased to 950°C. Although some reaction occurred between all the surrogates and the host to form a substituted whitlockite phase, increasing the temperature led to a significant increase in the cerium reaction and the formation of an additional phase, monazite. Additionally it was observed that the cerium became increasingly trivalent at higher temperatures.

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