First-principles electronic structure of spinelLiCr2O4:A possible half-metal

We have employed first-principles electronic structure calculations to examine the hypothetical (but plausible) oxide spinel, ${\mathrm{LiCr}}_{2}{\mathrm{O}}_{4}$ with the ${d}^{2.5}$ electronic configuration. The cell (cubic) and internal (oxygen position) structural parameters have been obtained for this compound through structural relaxation in the first-principles framework. Within the one-electron band picture, we find that ${\mathrm{LiCr}}_{2}{\mathrm{O}}_{4}$ is magnetic, and a candidate half-metal. The electronic structure is substantially different from the closely related and well-known rutile half-metal ${\mathrm{CrO}}_{2}.$ In particular, we find a smaller conduction-band width…

ChemInform Abstract: Metal-Metal Bonding and Metallic Behavior in Some ABO2 Delafossites.

We present results of ab initio band structure calculations on some ABO2 delafossite oxides that have both the A and B sites occupied by transition metals. This class of materials includes insulators as well as some of the most conducting oxides. The calculations have been performed in order to understand the nature of the metallic and insulating states and the extensive metal−metal bonding displayed by these materials. The effect of polytypism on the electronic structure is examined. Among the interesting aspects of the electronic structure of these materials are the contributions from both A and B atoms to states near the Fermi energy and the highly disperse nature of bands derived from t…

Preparation and characterization of Pd2Sn nanoparticles

We report a non-aqueous solution preparation of Pd{sub 2}Sn nanoparticles with sizes near 20 nm. The intermetallic compound with the Co{sub 2}Si structure has been characterized using transmission electron microscopy, Rietveld refinement of synchrotron X-ray and neutron powder diffraction, and real-space pair distribution function analysis of high-energy synchrotron X-ray scattering. We also present a description of the electronic structure of this covalent intermetallic using density functional calculations of the electronic structure.

Covalent bonding and the nature of band gaps in some half-Heusler compounds

Half-Heusler compounds \textit{XYZ}, also called semi-Heusler compounds, crystallize in the MgAgAs structure, in the space group $F\bar43m$. We report a systematic examination of band gaps and the nature (covalent or ionic) of bonding in semiconducting 8- and 18- electron half-Heusler compounds through first-principles density functional calculations. We find the most appropriate description of these compounds from the viewpoint of electronic structures is one of a \textit{YZ} zinc blende lattice stuffed by the \textit{X} ion. Simple valence rules are obeyed for bonding in the 8-electron compound. For example, LiMgN can be written Li$^+$ + (MgN)$^-$, and (MgN)$^-$, which is isoelectronic wi…

SPS-assisted preparation of the Magnéli phase WO2.90 for thermoelectric applications

We describe the preparation and simultaneous consolidation of WO2.90 by spark plasma sintering (SPS). SPS allows for the direct manufacturing of large amounts of consolidated material. Synchrotron powder X-ray diffraction indicates that the material is single phase. Microstructure analysis indicates that the pellet is fully dense, allowing high-temperature thermoelectric properties to be reliably measured. The as-prepared samples of WO2.90 reach a ZT of 0.1 at 1100 K.

Semiconducting half-Heusler and LiGaGe structure type compounds

Compounds with LiAlSi (half-Heusler) and LiGaGe structure types have been investigated by means of band structure calculations. The LiAlSi structure type is known as the half-Heusler structure type, whereas LiGaGe is a closely related hexagonal variant. A remarkable feature of some XYZ half-Heusler compounds with 8 and 18 valence electrons is, that despite being composed of only metallic elements, they are semiconductors. More than 100 semiconducting compounds within these structure types are known. LiGaGe compounds have an additional degree of freedom, namely the degree of puckering of the layers. These compounds can become semiconducting at a certain degree of puckering. Half-metallic beh…

Rough Surfaces by Design: Gold Colloids Tethered to Gold Surfaces as Substrates for CaCO3 Crystallization

Magnetization and magnetoresistive response of LiMn2O4 near the charge ordering transition

We report magnetization and magnetoresistance studies of the geometrically frustrated spinel compound LiMn2O4 near its charge ordering temperature. The effect of a 7 T magnetic field is to very slightly shift the transition in the resistivity to lower temperatures resulting in large negative magnetoresistance with significant hysteresis. This hysteresis is not reflected in the magnetization. These observations are compared with what is found in the colossal magnetoresistance and charge ordering perovskite manganese oxides. The manner in which geometric frustration influences the coupling of charge and spin degrees of freedom is examined.

A Solvothermal Route to High-Surface-Area Nanostructured MoS2.

ChemInform Abstract: Ln2Ti2S2O5 (Ln: Nd, Pr, Sm): A Novel Series of Defective Ruddlesden-Popper Phases.

Rapid Microwave Preparation of Thermoelectric TiNiSn and TiCoSb Half-Heusler Compounds

The 18-electron ternary intermetallic systems TiNiSn and TiCoSb are promising for applications as high-temperature thermoelectrics and comprise earth-abundant, and relatively nontoxic elements. Heusler and half-Heusler compounds are usually prepared by conventional solid state methods involving arc-melting and annealing at high temperatures for an extended period of time. Here, we report an energy-saving preparation route using a domestic microwave oven, reducing the reaction time significantly from more than a week to one minute. A microwave susceptor material rapidly heats the elemental starting materials inside an evacuated quartz tube resulting in near single phase compounds. The initia…

ChemInform Abstract: Mercaptophenol-Protected Gold Colloides as Nuclei for the Crystallization of Inorganic Minerals: Templated Crystallization on Curved Surfaces.

Giant negative magnetoresistance in GdI2

Abstract GdI 2 is a layered d 1 compound which is isostructural with and nominally isoelectronic to the superconductors 2H–TaS 2 and 2H–NbSe 2 . GdI 2 orders ferromagnetically at 276(2) K and displays large negative magnetoresistance ∼70% at 7 T close to room temperature. At 10 K the saturation magnetization is 7.33(5) μ B in good agreement with the value predicted from spin polarized band structure calculations.

Templated Crystallisation of Calcium and Strontium Carbonates on Centred Rectangular Self-Assembled Monolayer Substrates

SrCO3crystals display patterns of templating when grown on tailored self-assembled monolayers (right). As SrCO3 is isostructural with aragonite, comparison of the crystallisation of SrCO3 in the aragonite/strontianite modification with the crystallisation of CaCO3 in all three modifications might yield some insights into which factors are important for crystal growth.

ChemInform Abstract: Magnetic and Electronic Structure of the CMR Chalcospinel Fe0.5Cu0.5Cr2S4

Giant Negative Magnetoresistance in GdI2: Prediction and Realization

The electronic structure of the layered d1 compound GdI2 has been examined systematically in view of its relation to other layered d1 systems including superconducting and isostructural 2H-TaS2 and 2H-NbSe2. A van Hove type instability is evident in suitable representations of the Fermi surface. The presence of the half-filled and magnetic 4f level should preclude the possibility of superconductivity. Instead GdI2 orders ferromagnetically at 290(5) K and displays large negative magnetoresistance ≈70% at 7 T close to room temperature. This finding provides support to the idea that materials can be searched rationally for interesting properties through high level electronic structure calculat…

Metall oder Nichtmetall? Das ist hier die Frage!: Festkörperphysik für Chemiker

Festkorperphysik ist viel zu spannend, um sie allein den Physikern zu uberlassen. Vor der Untersuchung der physikalischen Eigenschaften von Festkorpern steht namlich deren Synthese, und dies ist das Geschaft des Chemikers. Leider hort das Interesse (und meist auch das Verstandnis) vieler Chemiker fur die Eigenschaften einer Substanz etwa da auf, wo es fur den Physiker interessant wird. Ein wesentlicher Grund dafur ist, dass der Chemiker Grundbegriffe der Festkorperphysik nicht kennt oder bestenfalls eine vage Vorstellung von ihnen hat. Dieses Defizit konnte zwar durch die Lekture eines Festkorperphysik-Lehrbuchs behoben werden, doch ist die abstrakte mathematische Ausdrucksweise der Physike…

ChemInform Abstract: Rough Surfaces by Design: Gold Colloids Tethered to Gold Surfaces as Substrates for CaCO3 Crystallization.

Mercaptophenol-Protected Gold Colloids as Nuclei for the Crystallization of Inorganic Minerals:  Templated Crystallization on Curved Surfaces

The self-assembly of monolayers of thiols on gold(111) surfaces yields substrates that are able to template in a controlled manner, the nucleation and growth of crystals of calcium carbonate from solution. In the absence of additives, various factors such as the nature of the thiol, the temperature, and the pH are now established as influencing the nature and relative amounts of the different CaCO3 phases (calcite, vaterite, and aragonite). Recently, we have been able to extend the use of thiol/gold self-assembled monolayers as templates for the growth of inorganic crystals by utilizing protected gold colloids instead of flat gold surfaces. The thiol monolayers that protect the colloids pro…

Crystallization of SrCO3 on a self-assembled monolayer substrate: an in-situ synchrotron X-ray study

Self-assembled monolayers (SAMs) of alkanethiols on gold surfaces show great promise in controlling the nucleation and growth of inorganic minerals from solution. In doing so, they mimic the role of some biogenic macromolecules in natural biomineralisation processes. Crystallization on SAM surfaces is usually monitored ex-situ; by allowing the process to commence and to evolve for some time, removing the substrate from the mother solution, and then examining it using microscopy, diffraction etc. We present here for the first time, the use of high energy monochromatic synchrotron X-radiation in conjunction with a two dimensional detector to monitor in situ, in a time resolved fashion, the gr…

Searching for hexagonal analogues of the half-metallic half-Heusler XYZ compounds

The XYZ half-Heusler crystal structure can conveniently be described as a tetrahedral zinc blende YZ structure which is stuffed by a slightly ionic X species. This description is well suited to understand the electronic structure of semiconducting 8-electron compounds such as LiAlSi (formulated Li$^+$[AlSi]$^-$) or semiconducting 18-electron compounds such as TiCoSb (formulated Ti$^{4+}$[CoSb]$^{4-}$). The basis for this is that [AlSi]$^-$ (with the same electron count as Si$_2$) and [CoSb]$^{4-}$ (the same electron count as GaSb), are both structurally and electronically, zinc-blende semiconductors. The electronic structure of half-metallic ferromagnets in this structure type can then be d…

Metal−Metal Bonding and Metallic Behavior in Some ABO2 Delafossites

We present results of ab initio band structure calculations on some ABO2 delafossite oxides that have both the A and B sites occupied by transition metals. This class of materials includes insulators as well as some of the most conducting oxides. The calculations have been performed in order to understand the nature of the metallic and insulating states and the extensive metal−metal bonding displayed by these materials. The effect of polytypism on the electronic structure is examined. Among the interesting aspects of the electronic structure of these materials are the contributions from both A and B atoms to states near the Fermi energy and the highly disperse nature of bands derived from t…

Enhanced thermoelectric properties of the n-type Magnéli phase WO2.90: reduced thermal conductivity through microstructure engineering

The thermoelectric properties of the Magneli phase WO2.90 were investigated, with special attention to how the thermoelectric performance can be altered by changing its microstructure. Spark plasma sintering (SPS) allowed the direct preparation of large amounts of consolidated material. Adding Ta2O5 to the reaction mixture lead to the formation of solid solutions W1−xTaxO2.90via a concurrent reaction between WO3 and Ta2O5 during the SPS treatment. In addition, micron-sized inclusions containing tungsten surrounded by WOx embedded in a WO2.90 matrix were formed, which act as additional scattering centers. As a result, the thermal conductivity of the Ta-containing samples was reduced by ≈30% …

Conduction band polarization in some CMR materials

First principles electronic structure calculations reveal certain important common features in the conduction band polarization of many of the newly examined colossal magnetoresistance (CMR) materials. Most CMR compounds seem to possess a localized, magnetic band slightly below the Fermi energy. This localized band transfers polarization to a relatively broad conduction band. The nature of the two bands in different systems can be quite distinct. In the perovskite-derived manganese oxides, the magnetic band is derived from $Mn t_{2g}$ states while the conduction band is derived from Mn e states. In the chalcospinel $Fe_{^0.^5}$ $Cu_{^0.^5}$ $Cr_{2}$$ S_{4}$ , the $Crt_{2g}$ states which are…