The fate of chemical and radioactive wastes in the environment is related to the ability of natural phases to attenuate and immobilize contaminants through chemical sorption and precipitation processes. Our understanding of these complex processes at the atomic level is provided by a few experimental and analytical methods such as X-ray absorption and NMR spectroscopies. However, due to complexities in the structure and composition of clay and other hydrated minerals, and the inherent uncertainties of the experimental methods, it is important to apply theoretical molecular models for a fundamental atomic-level understanding, interpretation, and prediction of these phenomena. In this effort, we have developed a general force field, CLAYFF, suitable for the simulation of hydrated and multicomponent mineral systems and their interfaces with aqueous solutions. Interatomic potentials were derived from parametrizations incorporating structural and spectroscopic data for a variety of simple hydrated compounds. A...

Molecular Models of Hydroxide, Oxyhydroxide, and Clay Phases and the Development of a General Force Field

Surface chemistry of ferrihydrite: Part 1. EXAFS studies of the geometry of coprecipitated and adsorbed arsenate

A modern definition for whole powder pattern decomposition WPPD methods would be that they simultaneously have to refine the unit-cell parameters and extract the best estimations of the Bragg peak intensities from a complete diffractogram. This is done very fast nowadays, irrespective of the number of Bragg peaks present in a powder diffraction pattern, but we did not attain this comfortable situation without some past efforts. The WPPD methods’ introduction occurred slowly and progressively thanks to the increase in computer power, the improvements in graphical user interfaces, the diffractometer data digitalization, the availability of synchrotron and neutron radiation, and last but not least, the proposition of new algorithms. Innovations were not instantly accepted this being true for all the whole powder pattern fitting methods including the Rietveld and the decomposition methods or could not be applied immediately to every radiation source or diffractometer the hardware before adaptations made by an essential category of crystallographers being conceivers and developers of the software. Ancestors of the WPPD methods extracted peak intensities without the cell restraint, so that each peak position was a parameter to be refined as well as the peak intensity, the peak shape and its width. This is still useful if the aim is the search for the peak positions for indexing, though derivative methods can make that peak-position-hunting job faster. Taking advantage of the indexing see a recent review paper by Bergmann et al. 2004, new WPPD methods, applying cell restraint to the peak position, opened the door to a long list of new possibilities and applications including first indexing confirmation which are detailed in this paper. However, only some selected application references will be provided because the number of papers involved is quite high and increasing more than 2000 texts specify the use of WPPD methods. Contributions from Rennes by Louer’s group from 1987 to 1993 will be especially enlightened, not forgetting the other players during that same time, restraining generally to the structure determinations by powder diffractometry SDPD applications published in the early stages of this retrospection because the subsequent activity increased too considerably, by more than 850 SDPDs in the last ten years. If only a partial review of applications can be given, the evolution of the methods will be discussed as completely as possible.

/pdf/whole-powder-pattern-decomposition-methods-and-applications-597bm0bp60.pdf

Whole powder pattern decomposition methods and applications: A retrospection

Lithium intercalation compounds based on lithium manganese oxides are of great importance as positive electrodes for rechargeable lithium batteries. It is widely accepted that Li+ may be extracted (deintercalated) from such lithium manganese oxides accompanied by oxidation of Mn up to a maximum oxidation state of +4. However, it has been suggested recently that further Li+ removal may be possible. Among the mechanisms that have been proposed to charge balance the removal of Li+ are Mn oxidation beyond +4 or loss of O2-. To investigate this phenomenon we have selected Li2MnO3, a layered compound Li[Li1/3Mn2/3]O2 with a ready supply of mobile Li+ ions but with all Mn already in the +4 oxidation state. We show that a substantial quantity of Li (at least 1.39 Li) may be removed. At 55 °C this occurs exclusively by oxidation of the nonaqueous electrolyte, thus generating H+ which exchange one-for-one with Li+ to form Li2-xHxMnO3. The presence of H+ between the oxide layers results in a change of the layer stac...

Mechanism of Electrochemical Activity in Li2MnO3

Intrinsic proton affinity of reactive surface groups of metal(hydr)oxides: application to iron(hydr)oxides.

/pdf/deuteration-of-crystalline-hydroxides-hydrogen-bonds-of-1uhassf0yn.pdf

Deuteration of Crystalline Hydroxides. Hydrogen Bonds of gamma-AlOO(H,D) and gamma-FeOO(H,D).

Isotope effects in the bonds of α-CrOOH and α-CrOOD

/pdf/neutron-and-x-ray-crystallographic-studies-on-indium-oxide-3hkuog7r01.pdf

Neutron and X-Ray Crystallographic Studies on Indium Oxide Hydroxide.

/pdf/a-crystal-structure-determination-of-pbc2o4-from-synchrotron-2erk958upl.pdf

A Crystal Structure Determination of PbC2O4 from Synchrotron X-Ray and Neutron Powder Diffraction Data.

High resolution powder data from both neutron and X-ray (synchrotron) sources have been used to estimate the possibility of direct structure determination from powder data Two known structures were resolved by direct methods with neutron and X-ray data With synchrotron X-ray data, the measured range of data was insufficient for a structure analysis, but the R-factor calculations showed the intensities extracted from the profile data to be of acceptable quality The results were used to estimate the largest structure that might be solved using routine techniques It was found that the limit would be near twenty atoms in the asymmetric part of a centro-symmetric structure

/pdf/solving-crystal-structures-from-powder-diffraction-data-15oetwofc3.pdf

Solving crystal structures from powder diffraction data

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