D. E. Ellis

Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, Rio de Janeiro, Brazil

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Publications (322)837.31 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The magnetic order of a low-temperature dimerized phase of Fe3O2BO3 is investigated through a density functional approach which considers full non-collinear spin–spin interactions, focusing on the 15 K crystalline structure. It is found that Fe spins in the (Fe–Fe)5+ dimer, formed during the room temperature structural change of Fe3O2BO3, are parallel and have little freedom to rotate under interaction with neighbor Fe atoms. While the Fe dimer behaves as a heavy single magnetic unit the spin magnetic moment of the third Fe3+ atom of the Fe triad has, on the contrary, much more freedom to rotate. This is responsible for a canted spin ordering, revealed by a rotation of ~80° of the trivalent Fe spin relative to the spin orientation of the dimer, due to spin–spin interaction with divalent Fe atoms outside the triad. Canting is thus seen to be responsible for the very low net magnetization, experimentally observed in this compound (T<40 K).
    Journal of Magnetism and Magnetic Materials 01/2015; 374:148–152. DOI:10.1016/j.jmmm.2014.08.025 · 2.00 Impact Factor
  • S.E. Stoltz, D.E. Ellis, M.J. Bedzyk
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    ABSTRACT: Interactions of Pd atoms, films and nanoparticles with a SrTiO3(001) substrate are studied via first principles Density Functional Theory. Effects of the substrate upon structural, electronic and chemical properties of the supported Pd are considered. By comparison of different experimentally observed particle shapes and orientation, and with atomic and planar Pd adsorbates, some detailed understanding is obtained about particle-support interactions. Adsorption of atoms (H, C, O) and small molecules (OH, CO, CH3) is used as a probe of chemical activity of different faces, edges and vertices of the particles.
    Surface Science 12/2014; 630:46–63. DOI:10.1016/j.susc.2014.06.018 · 1.87 Impact Factor
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    S.E. Stoltz, D.E. Ellis, M.J. Bedzyk
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    ABSTRACT: Density Functional Theory and X-ray standing wave atomic imaging are combined to develop models of platinum overlayers, patches and small particles on (001) surfaces of SrTiO3. The higher coverage (0.3 ML nominal Pt) experimental result matches best to a simple 1 × 1 surface model. For lower coverage (0.1 ML) a Ti deficient TiO2 double layer termination is identified as most favorable. A simple model is developed by which XSW amplitudes can be used to infer the aspect ratio of surface structures.
    Surface Science 11/2014; 633. DOI:10.1016/j.susc.2014.11.011 · 1.87 Impact Factor
  • P. Dalach, D.E. Ellis, A. van de Walle
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    ABSTRACT: An adaptive cluster expansion (ACE) methodology is presented which enables exploration of atomic ordering interactions in solids as a function of the redox environment. A previously developed cluster expansion methodology is augmented via inclusion of explicit effective charge dependence within the topological cluster basis. This augmentation produces an enhanced fit precision across a wide composition range and the ability to directly control the model’s redox state during Monte Carlo system equilibrations. The approach is validated in applications to yttria-stabilized zirconia (YSZ) and the perovskite (La0.8, Sr0.2)(Cr0.8, Ru0.2)O2.9 (LSCR), where significant variability in atomic ordering is seen across redox space. A locally adaptive lattice Monte Carlo sampling, utilizing the ACE methodology, is developed and validated in applications to determine the 0 K ground state configurations of YSZ and LSCR supercells with varying redox conditions. These equilibrations have direct relevance to solid-oxide fuel cell applications, whose components are subject to widely varying redox environments. The superior convergence of ACE results in a smaller number of numerically significant expansion terms, not only speeding the analysis but also permitting a physical interpretation of their meaning.
    Computational Materials Science 02/2014; 83:207–211. DOI:10.1016/j.commatsci.2013.10.013 · 1.88 Impact Factor
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    ABSTRACT: The atomic structure of the group-III-rich surface of III-V semiconductor compounds has been under intense debate for many years, yet none of the models agrees with the experimental data available. Here we present a model for the three-dimensional structure of the (001)-c(82) reconstruction on InSb, InAs, and GaAs surfaces based on surface x-ray diffraction data that was analyzed by direct methods and subsequent least squares refinement. Contrary to common belief the main building blocks of the structure are not dimers on the surface but subsurface dimers in the second bilayer. This essential feature of the structure is accompanied by linear arrays of atoms on nonbulklike sites at the surface which, depending on the compounds, exhibit a certain degree of disorder. A tendency to group-III-dimer formation within these chains increases when descending the periodic table. We propose that all the c(82) reconstructions of III-V semiconductor surfaces contain the same essential building blocks.
    Physical Review B 05/2013; DOI:10.1103/PhysRevB.64.075307 · 3.66 Impact Factor
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    ABSTRACT: High-temperature solid state reactions of U, M, and Q in a flux of ACl or Rb2S3 lead to the formation of Rb2Pd4U6S17 (1023 K, 96 h, 5—10 wt.% yield), Rb2Pd4U6Se17 (1223 K, 192 h, 5—10 wt.% yield), Rb2Pt4U6Se17 (1223 K, 96 h), Cs2Pd4U6S17 (1273 K, 24 h, 5—10 wt.% yield), Cs2Pd4U6Se17 (1073 K, 96 h, 70—80 wt.% yield), and Cs2Pt4U6Se17 (1073 K, 96 h, 70—80 wt.% yield).
    ChemInform 11/2012; 43(45):no-no. DOI:10.1002/chin.201245018
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    ABSTRACT: Relaxed structures and the related electronic environments of atomic monolayers and half-monolayers of tungsten with varying degrees of oxidation on the (0001) surface of hematite (α-Fe2O3) are modeled using first-principles density functional theory (DFT). This report focuses on the effect of nominally oxidizing and reducing chemical environments on surface structure and chemistry. By considering the position of W atoms relative to the substrate, calculated surface structures are compared to synchrotron X-ray standing wave (XSW) imaging results recently reported for this system. The question of W valence state, previously reported as nominally W5+ or W6+ in reducing or oxidizing surroundings, respectively, is addressed and discussed in light of X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (XAFS) results to clarify the relationship between valence state, oxygen coordination, and bond lengths.
    Surface Science 09/2012; 606(s 17–18):1367–1381. DOI:10.1016/j.susc.2012.04.023 · 1.87 Impact Factor
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    ABSTRACT: The A(2)M(4)U(6)Q(17) compounds Rb(2)Pd(4)U(6)S(17), Rb(2)Pd(4)U(6)Se(17), Rb(2)Pt(4)U(6)Se(17), Cs(2)Pd(4)U(6)S(17), Cs(2)Pd(4)U(6)Se(17), and Cs(2)Pt(4)U(6)Se(17) were synthesized by the high-temperature solid-state reactions of U, M, and Q in a flux of ACl or Rb(2)S(3). These isostructural compounds crystallize in a new structure type, with two formula units in the tetragonal space group P4/mnc. This structure consists of a network of square-planar MQ(4), monocapped trigonal-prismatic UQ(7), and square-antiprismatic UQ(8) polyhedra with A atoms in the voids. Rb(2)Pd(4)U(6)S(17) is a typical semiconductor, as deduced from electrical resistivity measurements. Magnetic susceptibility and specific heat measurements on single crystals of Rb(2)Pd(4)U(6)S(17) show a phase transition at 13 K, the result either of antiferromagnetic ordering or of a structural phase transition. Periodic spin-polarized band structure calculations were performed on Rb(2)Pd(4)U(6)S(17) with the use of the first principles DFT program VASP. Magnetic calculations included spin-orbit coupling. With U f-f correlations taken into account within the GGA+U formalism in calculating partial densities of states, the compound is predicted to be a narrow-band semiconductor with the smallest indirect and direct band gaps being 0.79 and 0.91 eV, respectively.
    Inorganic Chemistry 08/2012; 51(16):8873-81. DOI:10.1021/ic300865y · 4.79 Impact Factor
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    ABSTRACT: The compounds ThOS, ThOSe, and ThOTe have been synthesized, and their structures have been determined by means of single-crystal X-ray diffraction methods. All three compounds adopt the PbFCl structure type in the tetragonal space group D(4h)(7) - P4/nmm. More precise crystallographic data have been obtained for ThOS and ThOSe, which had previously only been known from X-ray powder diffraction data. ThOS, ThOSe, and ThOTe are yellow-, orange-, and black-colored, respectively. From single-crystal optical absorption measurements the band gaps are 2.22, 1.65, and 1.45 eV, respectively. Optical band gaps, ionic charges, and densities of states were calculated for the three compounds with the use of Density Functional methods.
    Inorganic Chemistry 07/2012; 51(15):8112-8. DOI:10.1021/ic300510x · 4.79 Impact Factor
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    ABSTRACT: Black single crystals of A(6)Cu(12)U(2)S(15) (A = K, Rb, Cs) have been synthesized by the reactive flux method. These isostructural compounds crystallize in the cubic space group Ia ̅3d at room temperature. The structure comprises a three-dimensional framework built from US(6) octahedra and CuS(3) trigonal planar units with A cations residing in the cavities. There are no S-S bonds in the structure. To elucidate the oxidation state of U in these compounds, various physical property measurements and characterization methods were carried out. Temperature-dependent electrical resistivity measurement on a single crystal of K(6)Cu(12)U(2)S(15) showed it to be a semiconductor. These three A(6)Cu(12)U(2)S(15) (A = K, Rb, Cs) compounds all exhibit small effective magnetic moments, < 0.58 μ(B)/U and band gaps of about 0.55(2) eV in their optical absorption spectra. From X-ray absorption near edge spectroscopy (XANES), the absorption edge of A(6)Cu(12)U(2)S(15) is very close to that of UO(3). Electronic band structure calculations at the density functional theory (DFT) level indicate a strong degree of covalency between U and S atoms, but theory was not conclusive about the formal oxidation state of U. All experimental data suggest that the A(6)Cu(12)U(2)S(15) family is best described as an intermediate U(5+)/U(6+) sulfide system of (A(+))(6)(Cu(+))(12)(U(5+))(2)(S(2-))(13)(S(-))(2) and (A(+))(6)(Cu(+))(12)(U(6+))(2)(S(2-))(15).
    Inorganic Chemistry 05/2012; 51(11):6153-63. DOI:10.1021/ic300172k · 4.79 Impact Factor
  • P. Dalach, D. E. Ellis, A. van de Walle
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    ABSTRACT: Tendencies toward local atomic ordering in (A,A′)(B,B′)O3−δ mixed composition perovskites are modeled to explore their influence on thermodynamic, transport, and electronic properties. In particular, dopants and defects within lanthanum chromate perovskites are studied under various simulated redox environments. (La1−x,Srx)(Cr1−y,Fey)O3−δ (LSCF) and (La1−x,Srx)(Cr1−y,Ruy)O3−δ (LSCR) are modeled using a cluster expansion statistical thermodynamics method built upon a density functional theory database of structural energies. The cluster expansions are utilized in lattice Monte Carlo simulations to compute the ordering of Sr and Fe(Ru) dopant and oxygen vacancies (Vac). Reduction processes are modeled via the introduction of oxygen vacancies, effectively forcing excess electronic charge onto remaining atoms. LSCR shows increasingly extended Ru-Vac associates and short-range Ru-Ru and Ru-Vac interactions upon reduction; LSCF shows long-range Fe-Fe and Fe-Vac interaction ordering, inhibiting mobility. First principles density functional calculations suggest that Ru-Vac associates significantly decrease the activation energy of Ru-Cr swaps in reduced LSCR. These results are discussed in view of experimentally observed extrusion of metallic Ru from LSCR nanoparticles under reducing conditions at elevated temperature.
    Physical review. B, Condensed matter 01/2012; 85(1). DOI:10.1103/PhysRevB.85.014108 · 3.66 Impact Factor
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    ABSTRACT: Hydroxyapatite (HA, Ca10(PO4)6(OH)2) is one of the most important biomaterials used in bone regeneration therapies due to their chemical properties are very similar to the inorganic phase found in bone tissues. The direct observation of the ultrastructure of HA is very important in the comprehension of their nucleation and interactions with the molecules involved in bone formation. High-resolution Transmission Electron Microscopy (HRTEM) is a currently technique used for this task. However, the interpretation of the images is not straightforward and needs the use of softwares dedicated to high-resolution images simulations. This work presents the applicability of MEGACELL software in the analysis of HRTEM images of HA nanoparticles. MEGACELL is the most newly software, developed to construct nanocrystals models for HRTEM multislice simulations. The output files generated by MEGACELL are raw data format (.xyz), containing all the atomic positions, as well as input files compatible with JEMS (Java Electron Microscopy Software) format files. High-resolution images were acquired using a JEM 3010 URP microscope, with a LaB6 thermionic electron gun operating at 300 kV, with a point-to-point resolution of 0.17 nm and a CCD Gatan 794SC multiscan digital camera, attached to the DigitalMicrographTM software for recording and image processing. Electron microscopy samples were prepared by dropping HA powder on copper TEM grids. HRTEM experimental images of HA particles, orientated along different zone axes, were interpreted applying the MEGACELL software to construct HA nanocrystal models and the multislice method to simulated them. MEGACELL improves the extraction of the ultrastructural features and facilitates a better interpretation of the phase-contrast images.
    Key Engineering Materials 10/2011; 493-494:763-767. DOI:10.4028/www.scientific.net/KEM.493-494.763 · 0.19 Impact Factor
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    ABSTRACT: High-resolution transmission electron microscopy (HRTEM) and ab initio quantum-mechanical calculations of electronic structure were combined to investigate the structure of the hydroxyapatite (HA) (010) surface, which plays an important role in HA interactions with biological media. HA was synthesized by in vitro precipitation at 37°C. HRTEM images revealed thin elongated rod nanoparticles with preferential growth along the [001] direction and terminations parallel to the (010) plane. The focal series reconstruction (FSR) technique was applied to develop an atomic-scale structural model of the high-resolution images. The HRTEM simulations identified the coexistence of two structurally distinct terminations for (010) surfaces: a rather flat Ca(II)-terminated surface and a zig-zag structure with open OH channels. Density functional theory (DFT) was applied in a periodic slab plane-wave pseudopotential approach to refine details of atomic coordination and bond lengths of Ca(I) and Ca(II) sites in hydrated HA (010) surfaces, starting from the HRTEM model.
    Colloids and surfaces B: Biointerfaces 09/2011; 89:15-22. DOI:10.1016/j.colsurfb.2011.08.016 · 4.29 Impact Factor
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    ABSTRACT: Statistically averaging surface-sensitive X-ray techniques are employed to elucidate the surface morphology of a model oxide-supported heterogeneous catalyst, tungsten oxide (WOx) on hematite (alpha-Fe2O3). Atomically flat alpha-Fe2O3 (0001) single crystals were coated with sub-monolayer WOx by atomic layer deposition (ALD). In situ X-ray standing wave (XSW) imaging with X-ray fluorescence (XRF) was used to determine W position relative to bulk-like cation lattice sites under nominally reducing and oxidizing chemical conditions. X-ray absorption fine structure (XAFS) reveals details of W coordination, bond length, and chemical state on WOx-coated hematite single crystals and nanopowders. Synchrotron characterization results are compared with morphologies predicted by density functional theory (DFT) calculations for clean WOx/alpha-Fe2O3 surfaces. Thermodynamics and atomic configurations for H2O and CO adsorption are also predicted. Excited-state self-consistent field (SCF) calculations are used to model X-ray photoelectron spectroscopy (XPS) results.
  • Yue Wu, D. E. Ellis, T. O. Mason
    MRS Online Proceeding Library 01/2011; 341. DOI:10.1557/PROC-341-201
  • MRS Online Proceeding Library 01/2011; 99. DOI:10.1557/PROC-99-177
  • MRS Online Proceeding Library 01/2011; 654. DOI:10.1557/PROC-654-AA4.5.1
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    ABSTRACT: Ab initio calculations based on density functional theory (DFT) have been used to study the energetics, fully relaxed structure, charge redistribution, and electronic density of states of adsorbed atomic and molecular oxygen on defectless unreconstructed SrO- and TiO2-terminated SrTiO3(001) surfaces. Exchange-correlation functional applied within DFT contains a “hybrid” of the non-local Hartree–Fock exchange, DFT exchange, and generalized gradient approximation correlation functionals. The calculations are performed on periodically repeated systems (two-dimensional slabs) large enough for the adsorbed species to be treated as isolated. We find substantial binding energies of up to 1.8 eV for atomic oxygen adsorption over surface oxygen and of over 2.0 eV at bridge (position between two adjacent surface atoms) sites of both SrO- and TiO2-terminated surface. A range of different adsorption sites and orientations for molecular oxygen have been studied but practically in no case does the adsorption energy exceed 0.1 eV. The stability diagram of surface structures in contact with a gaseous oxygen environment is calculated by means of atomistic thermodynamics. Adsorption of the reactants is found to depend significantly on temperature and partial pressures in the gas phase.
    MRS Online Proceeding Library 01/2011; 894. DOI:10.1557/PROC-0894-LL08-05
  • MRS Online Proceeding Library 01/2011; 1008. DOI:10.1557/PROC-1008-T10-04
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    ABSTRACT: The compounds UCuOP and NpCuOP have been synthesized and their crystal structures were determined from low-temperature single-crystal X-ray data. These isostructural compounds crystallize with two formula units in space group P4/nmm of the tetragonal system. Each An atom (An = U or Np) is coordinated to four O and four P atoms in a distorted square antiprism; each Cu atom is coordinated to four P atoms in a distorted tetrahedron. Magnetic susceptibility measurements on crushed single crystals indicate that UCuOP orders antiferromagnetically at 224(2) K. Neutron diffraction experiments at 100 and 228 K show the magnetic structure of UCuOP to be type AFI (+ - + -) where ferromagnetically aligned sheets of U atoms in the (001) plane order antiferromagnetically along [001]. The electrical conductivity of UCuOP exhibits metallic character. Its electrical resistivity measured in the ordered region with the current flowing within the tetragonal plane is governed by the scattering of the conduction electrons on antiferromagnetic spin-wave excitations. The electrical resistivity of single-crystalline NpCuOP shows semimetallic character. It is dominated by a pronounced hump at low temperatures, which likely arises owing to long-range magnetic ordering below about 90 K. Density of state analyses using the local spin-density approximation show covalent overlap between AnO and CuP layers of the structure and dominant contributions from 5f-actinide orbitals at the Fermi level. Calculations on a 2 × 2 × 2 supercell of NpCuOP show ferromagnetic ordering within the Np sheets and complex coupling between these planes. Comparisons of the physical properties of these AnCuOP compounds are made with those of the family of related tetragonal uranium phosphide compounds.
    Inorganic Chemistry 12/2010; 50(2). DOI:10.1021/ic101665g · 4.79 Impact Factor

Publication Stats

8k Citations
837.31 Total Impact Points

Institutions

  • 1987–2015
    • Centro Brasileiro de Pesquisas Físicas
      • Department of Applied Physics (APL)
      Rio de Janeiro, Rio de Janeiro, Brazil
  • 1973–2014
    • Northwestern University
      • • Department of Physics and Astronomy
      • • Department of Chemistry
      • • Department of Materials Science and Engineering
      Evanston, Illinois, United States
  • 2002–2008
    • Northwest University
      Evanston, Illinois, United States
  • 2007
    • University of Illinois at Chicago
      • Department of Electrical and Computer Engineering
      Chicago, Illinois, United States
  • 2006
    • Yantai University
      • Department of Physics
      Chifu, Shandong Sheng, China
  • 1983–1994
    • Argonne National Laboratory
      • Division of Materials Science
      Lemont, Illinois, United States
  • 1984
    • University of Science and Technology of China
      • Department of Physics
      Luchow, Anhui Sheng, China
  • 1977–1979
    • Chalmers University of Technology
      Goeteborg, Västra Götaland, Sweden
  • 1972–1977
    • VU University Amsterdam
      • Division of Theoretical Chemistry
      Amsterdamo, North Holland, Netherlands