Results 1 - 10 of 38
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[en] A new solvothermal route for the preparation of nanocrystals of PbS and PbSe, involving the reaction of lead stearate with sulfur or selenium and tetralin (tetrahydronaphthalene) in toluene solvent is described. Tetralin in the presence of S/Se gives H2S/H2Se and gets aromatized to naphthalene. The nanocrystals have been characterized by powder X-ray diffraction and electron microscopy. Use of surfactant Triton X-100 (polyoxyethylene(10)isooctylphenyl ether) resulted in both nanorods and nanoparticles of PbSe. Capping by citric acid and malonic acid reduce the particle sizes to less than 10 nm
[en] We report the synthesis and characterization of new structural variants of the isotypic compound with the generic chemical formula, Co1-0.5xoctCoxtet(OH)2(Cl)x(H2O)n, all modifications of an α-Co(OH)2 lattice. We show that the occupancy of tetrahedrally coordinated cobalt sites and associated chloride ligands, x, is modulated by the rate of formation of the respective layered hydroxide salts from kinetically controlled aqueous hydrolysis at an air-water interface. This new level of structural control is uniquely enabled by the slow diffusion of a hydrolytic catalyst, a simple technique. Independent structural characterizations of the compounds separately describe various attributes of the materials on different length scales, revealing details hidden by the disordered average structures. The precise control over the population of distinct octahedrally and tetrahedrally coordinated cobalt ions in the lattice provides a gentle, generic method for modulating the coordination geometry of cobalt in the material without disturbing the lattice or using additional reagents. A mechanism is proposed to reconcile the observation of the kinetic control of the structure with competing interactions during the initial stages of hydrolysis and condensation.
[en] A near-UV excited, oxyfluoride phosphor solid solution Sr1.975Ce0.025Ba(AlO4F)1-x(SiO5)x has been developed for solid state white lighting applications. An examination of the host lattice, and the local structure around the Ce3+ activator ions through a combination of density functional theory, synchrotron X-ray and neutron powder diffraction and total scattering, and electron paramagnetic resonance, points to how chemical substitutions play a crucial role in tuning the optical properties of the phosphor. The maximum emission wavelength can be tuned from green (λem = 523 nm) to yellow (λem = 552 nm) by tuning the composition, x. Photoluminescent quantum yield is determined to be 70 ± 5% for some of the examples in the series. Excellent thermal properties were found for the x = 0.5 sample, with the photoluminescence intensity at 160 C only decreased to 82% of its room temperature value. Phosphor-converted LED devices fabricated using an InGaN LED (λmax = 400 nm) exhibit high color rendering white light with Ra = 70 and a correlated color temperature near 7000 K. The value of Ra could be raised to 90 by the addition of a red component, and the correlated color temperature lowered to near 4000 K.
[en] Structures of layered metal hydroxides are not well described by traditional crystallography. Total scattering from a synthesis-controlled subset of these materials, as described here, reveals that different cobalt coordination polyhedra cluster within each layer on short length scales, offering new insights and approaches for understanding the properties of these and related layered materials. Structures related to that of brucite (Mg(OH)2) are ubiquitous in the mineral world and offer a variety of useful functions ranging from catalysis and ion-exchange to sequestration and energy transduction, including applications in batteries. However, it has been difficult to resolve the atomic structure of these layered compounds because interlayer disorder disrupts the long-range periodicity necessary for diffraction-based structure determination. For this reason, traditional unit-cell-based descriptions have remained inaccurate. Here we apply, for the first time to such layered hydroxides, synchrotron X-ray total scattering methods - analyzing both the Bragg and diffuse components - to resolve the intralayer structure of three different α-cobalt hydroxides, revealing the nature and distribution of metal site coordination. The different compounds with incorporated chloride ions have been prepared with kinetic control of hydrolysis to yield different ratios of octahedrally and tetrahedrally coordinated cobalt ions within the layers, as confirmed by total scattering. Real-space analyses indicate local clustering of polyhedra within the layers, manifested in the weighted average of different ordered phases with fixed fractions of tetrahedrally coordinated cobalt sites. These results, hidden from an averaged unit-cell description, reveal new structural characteristics that are essential to understanding the origin of fundamental material properties such as color, anion exchange capacity, and magnetic behavior. Our results also provide further insights into the detailed mechanisms of aqueous hydrolysis chemistry of hydrated metal salts. We emphasize the power of the methods used here for establishing structure-property correlations in functional materials with related layered structures.
[en] The local structures of Zn1-xMgxO alloys have been studied by Raman spectroscopy and by synchrotron x-ray pair-distribution-function (PDF) analysis. Within the solid solution range (0 (le) x (le) 0.15) of Zn1-xMgxO, the wurtzite framework is maintained with Mg homogeneously distributed throughout the wurtzite lattice. The E2high Raman line of Zn1-xMgxO displays systematic changes in response to the evolution of the crystal lattice upon the Mg substitution. The redshift and broadening of the E2high mode are explained by the expansion of hexagonal ab dimensions and compositional disorder of Zn/Mg, respectively. Synchrotron x-ray PDF analyses of Zn1-xMgxO reveal that the Mg atoms have a slightly reduced wurtzite parameter u and more regular tetrahedral bond distances than the Zn atoms. For both Zn and Mg, the internal tetrahedral geometries are independent of the alloy composition.
[en] The electronic structures of four semiconductor compounds BaCu2S2, BaCu2Se2, BaAg2S2, and BaAg2Se2 are studied by density functional theory using both semi-local and hybrid functionals. The ionization energies and electron affinities were determined by aligning the electronic states with the vacuum level by calculating the electrostatic profile within a supercell slab model. The ionization energy and electron affinity of the compounds were calculated using the Heyd–Scuseria–Ernzerhof functionals and range from 4.5 eV to 5.4 eV and 3.1 eV to 3.4 eV, respectively. The replacement of Cu by Ag slightly increases the ionization energy and electron affinity, while the replacement of S by Se decreases the ionization energy but slightly increases the electron affinity. Overall, the low ionization energies and small electron affinities suggest that these compounds possess good p-type doping propensities. The band gaps are somewhat small to be ideal candidates for transparent semiconducting behavior; however, the replacement of Cu with Ag in the barium sulfide compounds can increase the band gap from 1.62 to 2.01 eV. (papers)
[en] La4LiAuO8 and La2BaPdO5, two previously known oxides, are presented as model compounds for examining the role of isolated and immobilized Au3+ and Pd2+ ions in heterogeneous catalysis. Structural characterization, stability, surface composition, and electronic structure of these compounds are presented. These are examined in studies ranging from synchrotron X-ray scattering, including pair distribution function (PDF) and maximum entropy method (MEM) analysis, to density functional calculations of the electronic structures. The exceptional stability displayed by these compounds as verified by thermogravimetric analysis can be attributed to the presence of covalent Au-O and Pd-O interactions revealed in MEM studies, which suggests a criterion for stabilizing these highly oxophobic transition metals in oxide environments. Catalytic testing of the two compounds as heterogeneous catalysts in the oxidation of CO to CO2 are presented. La2BaPdO5 appears to be an effective catalyst for CO oxidation, despite the low surface area of the oxide being used. This is the first time that a fully ordered (rather than doped) Pd2+ oxide had been used to catalyze CO oxidation. La4LiAuO8 on the other hand, is much less effective at catalyzing CO oxidation. Differences in the reactivities of the two compounds are discussed with respect to differences in their density functional electronic structures.
[en] The local structures of Zn1-xMgxO alloys have been studied by Raman spectroscopy and by synchrotron x-ray pair-distribution-function (PDF) analysis. Within the solid solution range (0≤x≤0.15) of Zn1-xMgxO, the wurtzite framework is maintained with Mg homogeneously distributed throughout the wurtzite lattice. The E2high Raman line of Zn1-xMgxO displays systematic changes in response to the evolution of the crystal lattice upon the Mg substitution. The redshift and broadening of the E2high mode are explained by the expansion of hexagonal ab dimensions and compositional disorder of Zn/Mg, respectively. Synchrotron x-ray PDF analyses of Zn1-xMgxO reveal that the Mg atoms have a slightly reduced wurtzite parameter u and more regular tetrahedral bond distances than the Zn atoms. For both Zn and Mg, the internal tetrahedral geometries are independent of the alloy composition
[en] PbPdO2, a ternary compound containing the lone pair active ion Pb2+ and the square planar d8 Pd2+ ion, has attracted recent interest because of the suggestion that its electronic structure, calculated within density functional theory using either the local density or the generalized gradient approximation, displays zero-gap behavior. In light of the potential ease of doping magnetic ions in this structure, it has been suggested that the introduction of spin, in conjunction with zero band gap, can result in unusual magnetic ground states and unusual magnetotransport. It is known that most electronic structure calculations do not properly obtain a band gap even for the simple oxide PdO, and instead obtain a metal or a zero-gap semiconductor. Here we present density functional calculations employing a screened hybrid functional which correctly obtain a band gap for the electronic structure of PdO. When employed to calculate the electronic ground state of PbPdO2, a band gap is again obtained, which is consistent with both the experimental data on this compound, as well as a consideration of valence states and of metal-oxygen connectivity in the crystal structure. We also present comparisons of the absolute positions (relative to the vacuum level) of the conduction band minima and the valence band maxima in α-PbO, PdO and PbPdO2, which suggest ease of p-type doping in PbPdO2 that has been observed even in nominally pure materials. (paper)
[en] We have prepared a series of polycrystalline samples La0.5Sr0.5CoO3-δ with 0 <δ≤0.21 and characterized their oxygen content, crystal structure, and magnetic properties. While the fully oxygenated samples are good ferromagnets, samples with larger δ values display increasingly broad magnetic transitions. The saturation magnetization at 5 K falls rapidly as δ increases. First-principles electronic structure calculations provide insights into the magnetic behaviour of the fully oxygenated compound, and the manner in which ferromagnetic ordering is affected by increasing oxygen non-stoichiometry