Advances in the development of novel cobalt Fischer-Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels.

High-resolution Fe K-edge XANES spectra of a series of crystalline Fe 2+ - and Fe 3+ -bearing model compounds were measured in an effort to correlate characteristics of the pre-edge feature with oxidation state and local coordination environment of Fe atoms. The model compounds comprise 30 natural minerals and synthetic compounds, with Fe coordination environments ranging from 4 to 12 O atoms for Fe 2+ , including 5-coordinated trigonal bipyramidal Fe 2+ , and from 4 to 6 O atoms for Fe 3+ . Most pre-edge spectra show two components (due to crystal-field splitting) that are located just above the Fermi level. The most useful characteristics of the Fe-K pre-edge for determining Fe oxidation state and coordination number are the position of its centroid and its integrated intensity. The separation between the average pre-edge centroid positions for Fe 2+ and Fe 3+ is 1.4 ± 0.1 eV. Thus, the position of the pre-edge feature can be used as a measure of the average Fe-redox state, with the average pre-edge position for mixed Fe 2+ -Fe 3+ compounds occurring between positions for Fe 2+ and Fe 3+ . The lowest pre-edge normalized heights and integrated intensities are observed for the most centrosymmetric sites of Fe, in agreement with previous studies (see Waychunas et al. 1983). Examination of the pre-edge features of mechanical mixtures of phases containing different proportions of Fe 2+ and Fe 3+ suggests that the pre-edge position and intensity for these mixtures can vary quite non-linearly with the average redox state of Fe. However, distinctly different trends of pre-edge position vs. pre-edge intensity can be observed, depending on the coordination environment of Fe 2+ and Fe 3+ , with an accuracy in redox determination of ±10 mol% provided that the site geometry for each redox state is known. These methods have been used to estimate the Fe 3+ /Fe 2+ ratio in 12 minerals (magnetite, vesuvianite, franklinite, rhodonite, etc.) containing variable/unknown amounts of Fe 2+ /Fe 3+ .

http://www.minsocam.org/msa/ammin/toc/Abstracts/2001_Abstracts/MJ01_Abstracts/Wilke_p714_01.pdf

Oxidation state and coordination of Fe in minerals: An Fe K-XANES spectroscopic study

This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), en...

Compound Copper Chalcogenide Nanocrystals

Mullite (3AI2O3. 2Si02) is becoming increasingly important in electronic, optical, and high-temperature structural applications. This paper reviews the current state of mullite-related research at a fundamental level, within the framework of phase equilibria, crystal structure, synthesis, processing, and properties. Phase equilibria are discussed in terms of the problems associated with the nucleation kinetics of mullite and the large variations observed in the solid-solution range. The incongruent melting behavior of mullite is now widely accepted. Large variations in the solid solubility from 58 to 76 mot% alumina are related to the ordering/disordering of oxygen vacancies and are strongly coupled with the method of synthesis used to form mullite. Similarly, reaction sequences which lead to the formation of mullite upon heating depend on the spatial scale at which the components are mixed. Mixing at the atomic level is useful for lowtemperature (<lOOO"C) synthesis of mullite but not for low-temperature sintering. In contrast, precursors that are segregated are better suited for lowtemperature (1250" to 1500°C) densification through viscous deformation. Flexural strength and creep resistS M Wiederhorn-contributing editor

Mullite for Structural, Electronic, and Optical Applications

The distribution of bonds associated with the M2 sites in various well-ordered pyroxene minerals is determined using a topological analysis of electron density in the manner proposed by Bader (1998). Each M2 atom is bonded to 2 O1 and to 2 O2 atoms, and to zero, one, two, or four bridging O3 atoms. Each of the symmetries displayed by pyroxenes have their own bonding systematics, and each pyroxene-to-pyroxene phase transition involves a change in bonding to M2. As a function of temperature or pressure, the bonding changes appear as a well-defined sequence of steps that can be related to the degree of distortion from the ideal closest packing of anions. It is proposed that the condition at which an individual phase transition occurs is related to M2-Si repulsion through a shared edge. The bonding analysis should provide a qualitative means to interpret the behavior of all pyroxene structures over T , P , and x , and may guide the interpretation of the changes in properties observed by techniques other than X-ray diffraction, such as Raman spectroscopy.

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Topology of the pyroxenes as a function of temperature, pressure, and composition as determined from the procrystal electron density

Phase relations and superstructures of pyrrhotite, Fe1−xS

Crystal Structure of a Monoclinic Pyrrhotite (Fe7S8)

The crystal structure of a natural orthorhombic enstatite (orthoenstatite) from the Bishopville meteorite, has been refined by the least-squares method using 680 three-dimensional reflections. The space group and the cell dimensions of this orthoenstatite are Pbca and a = 18.210, b = 8.812 and c = 5.178 A. The crystal structure of orthoenstatite is compared with that of clinoenstatitc and the geometric relationship between the two polymorphs is described. The orthoenstatite structure is shown to be an exact twin of clinoenstatite on a unitcell scale. The orthorhombic cell is composed of two monoclinic cells of clinoenstatite joined on (100) through a b-glide plane. The orthoenstatite structure is also compared with the structures of hypersthene and orthoferrosilite. The corresponding interatomic distances show the following characteristics. \Vith increasing Fe replacement of Mg the interatomic distances M-O in the metal octahedra increase. The mean interatomic distances, M-O, of the octahedra can thus be used to determine the Mg:Fe

The crystal structure of orthoenstatite

Crystal Structures of Three Polymorphs of Co2SiO4

A new type of natural pyrrhotite, orthorhombic 11C type (a = 6.892, b = 11.952, c = 5.744 x 11 angstroms), and the hexagonal 6C type (a = 6.89, c = 5.76 x 6 angstroms) are described. Their compositions are Fe(10)S(11) and Fe(11)S(12), respectively. Pyrrhotites stable in nature have essentially stoichiometric composition, Fe(n)-(l)S(n) (n>/=8), with the structures of n/2C type for n even and of nC type for n odd. The solid solutions between Fe(11)S(12) and Fe(10)S(11), and between Fe(10)S(11) and Fe(9)S(10) are considered metastable in nature.

https://science.sciencemag.org/content/sci/168/3934/964.full.pdf

Nobuo Morimoto

Papers

Phase relations and superstructures of pyrrhotite, Fe1−xS

Crystal Structure of a Monoclinic Pyrrhotite (Fe7S8)

The crystal structure of orthoenstatite

Crystal Structures of Three Polymorphs of Co2SiO4

Pyrrhotites: Stoichiometric Compounds with Composition Fen--1Sn (nge8).