Photo-induced low temperature structural transition in the “114” YBaFe4O7 oxide
Abstract not provided
Duffort, V.↗
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Investigating the Surface Structure of γ-Al 2 O 3 Supported WO X Catalysts by High Field 27 Al MAS NMR and Electronic Structure Calculations
The metal-support interaction in γ-Al2O3 supported WOX catalysts is investigated by a combination of high field quantitative single pulse (SP) 27Al MAS NMR spectroscopy, 2D MQMAS, 1H-27Al CP/MAS, and electronic structure calculations. NMR allows the observation of at least seven different Al sites, including a pentahedral Al site, three different tetrahedral Al sites, and three octahedral Al sites. It is found that the penta-coordinated Al (AlP) site density decreases monotonically with an increased WOX loading while the octahedral Al (AlO) site density increases concurrently. This suggests that the Alp sites are the preferred surface anchoring positions for the WOX species. Importantly, the AlP site isotropic chemical shift observed for the unsupported γ-Al2O3 at about 38 ppm migrates into the octahedral region with a new isotropic chemical shift value appearing near 7 ppm when the Alp site is anchored by WOX species. Density functional theory (DFT) computational modeling of the NMR parameters on proposed cluster models is carried out to accurately interpret the dramatic chemical shift changes from which the detailed anchoring mechanisms are obtained. It is found that tungsten dimers and monomers are the preferred supported surface species on γ-Al2O3, wherein one monomeric and several dimeric structures are identified as the most likely surface anchoring structures.
27Al MAS NMR↗
Quantifying Geometric Strain at the PbS QD-TiO 2 Anode Interface and Its Effect on Electronic Structures
Explore the source record for details and available documents.
Trejo, Orlando↗
Electric-field-induced local and mesoscale structural changes in polycrystalline dielectrics and ferroelectrics
Abstract not provided
Usher, Tedi-Marie↗
Structural properties of Sb[subscript 2]S[subscript 3] under pressure: evidence of an electronic topological transition
Abstract not provided
Efthimiopoulos, Ilias↗
Crystal structure, electronic structure, temperature-dependent optical and scintillation properties of CsCe 2 Br 7
CsCe 2 Br 7 is a self-activated inorganic scintillator that shows promising performance, but the understanding of the important structure-property relationships is lacking. In this work, we conduct a comprehensive study on CCsCe 2 Br 7 . The crystal structure of CsCe 2 Br 7 is refined using single crystal X-ray study for the first time. It crystallizes into the orthorhombic crystal system with Pmnb space group. Its electronic structure is revealed by Density Functional Theory (DFT) calculations. Two cerium emission centers are identified and the energy barriers related to the thermal quenching to 4f ground states of Ce 3+ for these two Ce centers are evaluated. CsCe 2 Br 7 single crystal has better light yield and energy resolution than CsCe 2 Cl 7 , but with an additional slow decay component of 1.7 s. The existence of a deep trap with a depth of 0.9 eV in CsCe 2 Cl 7 contributes to its higher afterglow level in comparison to that of CsCe 2 Br 7 . The most possible point defects in CsCe 2 Cl 7 and CsCe 2 Br 7 are proposed by considering the vapour pressure in the growth atmosphere upon melting point.
36 MATERIALS SCIENCE↗
Temperature dependent electronic structure of Pr{sub 0.67}Sr{sub 0.33}MnO{sub 3} film probed by X-ray absorption near edge structure
The Mn K edge X-ray absorption near edge structures (XANES) of Pr{sub 0.67}Sr{sub 0.33}MnO{sub 3} film (100 nm) on (001) LaAlO{sub 3} substrate was measured at different temperatures to probe the MnO{sub 6} octahedron distortion and corresponding electronic structure. The absorption of high temperature paramagnetic-insulator phase differed from that of the low temperature ferromagnetic-metal phase. The temperature-dependent absorption intensity of Mn K edge XANES was correlated with the relaxation of distorted MnO{sub 6} octahedron, which changed the crystal field acting on the Mn site and the related electronic structure and properties. At low temperature, the splitting of Mn majority e{sub g} orbitals decreased and the density of states above the Fermi level increased in the relaxed MnO{sub 6} octahedron, as reflected by a wider separation between two sub-peaks in the pre-edge XANES spectra.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Solid state {sup 1}H spin-lattice relaxation and isolated-molecule and cluster electronic structure calculations in organic molecular solids: The relationship between structure and methyl group and t-butyl group rotation
We report ab initio density functional theory electronic structure calculations of rotational barriers for t-butyl groups and their constituent methyl groups both in the isolated molecules and in central molecules in clusters built from the X-ray structure in four t-butyl aromatic compounds. The X-ray structures have been reported previously. We also report and interpret the temperature dependence of the solid state {sup 1}H nuclear magnetic resonance spin-lattice relaxation rate at 8.50, 22.5, and 53.0 MHz in one of the four compounds. Such experiments for the other three have been reported previously. We compare the computed barriers for methyl group and t-butyl group rotation in a central target molecule in the cluster with the activation energies determined from fitting the {sup 1}H NMR spin-lattice relaxation data. We formulate a dynamical model for the superposition of t-butyl group rotation and the rotation of the t-butyl group's constituent methyl groups. The four compounds are 2,7-di-t-butylpyrene, 1,4-di-t-butylbenzene, 2,6-di-t-butylnaphthalene, and 3-t-butylchrysene. We comment on the unusual ground state orientation of the t-butyl groups in the crystal of the pyrene and we comment on the unusually high rotational barrier of these t-butyl groups.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Ge L{sub 3}-edge x-ray absorption near-edge structure study of structural changes accompanying conductivity drift in the amorphous phase of Ge{sub 2}Sb{sub 2}Te{sub 5}
A gradual uncontrollable increase in the resistivity of the amorphous phase of phase-change alloys, such as Ge{sub 2}Sb{sub 2}Te{sub 5}, known as drift, is a serious technological issue for application of phase-change memory. While it has been proposed that drift is related to structural relaxation, no direct structural results have been reported so far. Here, we report the results of Ge L{sub 3}-edge x-ray absorption measurements that suggest that the drift in electrical conductivity is associated with the gradual conversion of tetrahedrally coordinated Ge sites into pyramidal sites, while the system still remains in the amorphous phase. Based on electronic configuration arguments, we propose that during this process, which is governed by the existence of lone-pair electrons, the concentration of free carriers in the system decreases resulting in an increase in resistance despite the structural relaxation towards the crystalline phase.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Use of density functional theory method to calculate structures of neutral carbon clusters C{sub n} (3 ≤ n ≤ 24) and study their variability of structural forms
In this work, we present modifications to the well-known basin hopping (BH) optimization algorithm [D. J. Wales and J. P. Doye, J. Phys. Chem. A 101, 5111 (1997)] by incorporating in it the unique and specific nature of interactions among valence electrons and ions in carbon atoms through calculating the cluster’s total energy by the density functional tight-binding (DFTB) theory, using it to find the lowest energy structures of carbon clusters and, from these optimized atomic and electronic structures, studying their varied forms of topological transitions, which include a linear chain, a monocyclic to a polycyclic ring, and a fullerene/cage-like geometry. In this modified BH (MBH) algorithm, we define a spatial volume within which the cluster’s lowest energy structure is to be searched, and introduce in addition a cut-and-splice genetic operator to increase the searching performance of the energy minimum than the original BH technique. The present MBH/DFTB algorithm is, therefore, characteristically distinguishable from the original BH technique commonly applied to nonmetallic and metallic clusters, technically more thorough and natural in describing the intricate couplings between valence electrons and ions in a carbon cluster, and thus theoretically sound in putting these two charged components on an equal footing. The proposed modified minimization algorithm should be more appropriate, accurate, and precise in the description of a carbon cluster. We evaluate the present algorithm, its energy-minimum searching in particular, by its optimization robustness. Specifically, we first check the MBH/DFTB technique for two representative carbon clusters of larger size, i.e., C{sub 60} and C{sub 72} against the popular cut-and-splice approach [D. M. Deaven and K. M. Ho, Phys. Rev. Lett. 75, 288 (1995)] that normally is combined with the genetic algorithm method for finding the cluster’s energy minimum, before employing it to investigate carbon clusters in the size range C{sub 3}-C{sub 24} studying their topological transitions. An effort was also made to compare our MBH/DFTB and its re-optimized results carried out by full density functional theory (DFT) calculations with some early DFT-based studies.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Structural transitions, magnetic properties, and electronic structures of Co(Fe)-doped MnNiSi compounds
The structural transitions, magnetic properties, and electronic structures of Co(Fe)-doped MnNiSi compounds are investigated by x-ray powder diffraction, differential scanning calorimetry (DSC), magnetic measurements, and first-principles calculations. Results indicate that all samples undergo a martensitic transition from the Ni{sub 2}In-type parent phase to TiNiSi-type orthorhombic phase at high temperatures. The substitution of Co(Fe) for Mn in Mn{sub 1−x}Co{sub x}NiSi (x = 0.2, 0.3, and 0.4) and Mn{sub 1−y}Fe{sub y}NiSi (y = 0.26, 0.30, 0.36, 0.46, and 0.55) samples decreases the structural transition temperature and Curie temperature of martensite. The martensite phases show a typical ferromagnetic behavior with saturation field being basically unchanged with increasing Co(Fe) content, while the saturation magnetization shows a decreasing tendency. The theoretically calculated moments are in good agreement with the experimentally measured results. The orbital hybridizations between different 3d elements are analyzed from the distribution of density of states.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Origin of structural analogies and differences between the atomic structures of GeSe{sub 4} and GeS{sub 4} glasses: A first principles study
First-principles molecular dynamics simulations based on density functional theory are employed for a comparative study of structural and bonding properties of two stoichiometrically identical chalcogenide glasses, GeSe{sub 4} and GeS{sub 4}. Two periodic cells of 120 and 480 atoms are adopted. Both glasses feature a coexistence of Ge-centered tetrahedra and Se(S) homopolar connections. Results obtained for N = 480 indicate substantial differences at the level of the Se(S) environment, since Ge–Se–Se connections are more frequent than the corresponding Ge–S–S ones. The presence of a more prominent first sharp diffraction peak in the total neutron structure factor of glassy GeS{sub 4} is rationalized in terms of a higher number of large size rings, accounting for extended Ge–Se correlations. Both the electronic density of states and appropriate electronic localization tools provide evidence of a higher ionic character of Ge–S bonds when compared to Ge–Se bonds. An interesting byproduct of these investigations is the occurrence of discernible size effects that affect structural motifs involving next nearest neighbor distances, when 120 or 480 atoms are used.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Photoelectron spectra and structure of the Mn{sub n}{sup −} anions (n = 2–16)
Photoelectron spectra of the Mn{sub n}{sup −} anion clusters (n = 2–16) are obtained by anion photoelectron spectroscopy. The electronic and geometrical structures of the anions are computed using density functional theory with generalized gradient approximation and a basis set of triple-ζ quality. The electronic and geometrical structures of the neutral Mn{sub n} clusters have also been computed to estimate the adiabatic electron affinities. The average absolute difference between the computed and experimental vertical detachment energies of an extra electron is about 0.2 eV. Beginning with n = 6, all lowest total energy states of the Mn{sub n}{sup −} anions are ferrimagnetic with the spin multiplicities which do not exceed 8. The computed ionization energies of the neutral Mn{sub n} clusters are in good agreement with previously obtained experimental data. According to the results of our computations, the binding energies of Mn atoms are nearly independent on the cluster charge for n > 6 and possess prominent peaks at Mn{sub 13} and Mn{sub 13}{sup −} in the neutral and anionic series, respectively. The density of states obtained from the results of our computations for the Mn{sub n}{sup −} anion clusters show the metallic character of the anion electronic structures.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Structural properties of Bi{sub 2−x}Mn{sub x}Se{sub 3} thin films grown via molecular beam epitaxy
The effects of Mn doping on the structural properties of the topological insulator Bi{sub 2}Se{sub 3} in thin film form were studied in samples grown via molecular beam epitaxy. Extended x-ray absorption fine structure measurements, supported by density functional theory calculations, indicate that preferential incorporation occurs substitutionally in Bi sites across the entire film volume. This finding is consistent with x-ray diffraction measurements which show that the out of plane lattice constant expands while the in plane lattice constant contracts as the Mn concentration is increased. X-ray photoelectron spectroscopy indicates that the Mn valency is 2+ and that the Mn bonding is similar to that in MnSe. The expansion along the out of plane direction is most likely due to weakening of the Van der Waals interactions between adjacent Se planes. Transport measurements are consistent with this Mn{sup 2+} substitution of Bi sites if additional structural defects induced by this substitution are taken into account.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Structural and optical properties of nano-structured CdS thin films prepared by chemical bath deposition
Cadmium sulfide (CdS) thin films have been deposited on conducting glass substrates by chemical bath deposition (CBD) technique. The effect of precursor concentration on the structural, morphological, compositional, and optical properties of the CdS films has been studied. Crystal structure of these CdS films is characterized by X-ray diffraction (XRD) and it reveals polycrystalline structure with mixture of cubic and wurtzite phases with grain size decreasing as precursor concentration is increased. Optical studies reveal that the CdS thin films have high transmittance in visible spectral region reaching 90% and the films possess direct optical band gap that decreases from 2.46 to 2.39 eV with decreasing bath concentration. Our study suggests that growth is nucleation controlled.
36 MATERIALS SCIENCE↗
Graph-based linear scaling electronic structure theory
We show how graph theory can be combined with quantum theory to calculate the electronic structure of large complex systems. The graph formalism is general and applicable to a broad range of electronic structure methods and materials, including challenging systems such as biomolecules. The methodology combines well-controlled accuracy, low computational cost, and natural low-communication parallelism. This combination addresses substantial shortcomings of linear scaling electronic structure theory, in particular with respect to quantum-based molecular dynamics simulations.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Magnetism and electronic structure of CoFeCrX (X = Si, Ge) Heusler alloys
The structural, electronic, and magnetic properties of CoFeCrX (X = Si, Ge) Heusler alloys have been investigated. Experimentally, the alloys were synthesized in the cubic L2{sub 1} structure with small disorder. The cubic phase of CoFeCrSi was found to be highly stable against heat treatment, but CoFeCrGe disintegrated into other new compounds when the temperature reached 402 °C (675 K). Although the first-principle calculation predicted the possibility of tetragonal phase in CoFeCrGe, the tetragonal phase could not be stabilized experimentally. Both CoFeCrSi and CoFeCrGe compounds showed ferrimagnetic spin order at room temperature and have Curie temperatures (T{sub C}) significantly above room temperature. The measured T{sub C} for CoFeCrSi is 790 K but that of CoFeCrGe could not be measured due to its dissociation into new compounds at 675 K. The saturation magnetizations of CoFeCrSi and CoFeCrGe are 2.82 μ{sub B}/f.u. and 2.78 μ{sub B}/f.u., respectively, which are close to the theoretically predicted value of 3 μ{sub B}/f.u. for their half-metallic phases. The calculated band gaps for CoFeCrSi and CoFeCrGe are, respectively, 1 eV and 0.5 eV. These materials have potential for spintronic device applications, as they exhibit half-metallic electronic structures with large band gaps, and Curie temperatures significantly above room temperature.
36 MATERIALS SCIENCE↗