D. E. Ellis

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

Are you D. E. Ellis?

Claim your profile

Publications (224)490.78 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
    [Show abstract] [Hide abstract]
    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
  • Source
    S.E. Stoltz, D.E. Ellis, M.J. Bedzyk
    [Show abstract] [Hide abstract]
    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; DOI:10.1016/j.susc.2014.11.011 · 1.87 Impact Factor
  • P. Dalach, D.E. Ellis, A. van de Walle
    [Show abstract] [Hide abstract]
    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
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Density functional theory calculations and first-principles molecular dynamics (MD) simulations have been performed to examine the strain effect on the colossal oxygen ionic conductivity in selected sandwich structures of zirconia electrolytes. For the KTaO(3)/YSZ/KTaO(3) sandwich structure with 9.7% lattice mismatch, transition state calculations indicate that the strain effect changes the oxygen migration pathways from straight line into zigzag form and reduces the energy barrier by 0.2 eV. On the basis of our computational results, a possible oxygen ion diffusion highway is suggested. By finite-temperature MD simulations, an activation barrier of 0.33 eV is obtained, corresponding to an oxygen ionic conductivity which is 6.4 × 10(7) times higher than that of the unstrained bulk zirconia at 500 K. A nearly linear relationship is identified between the energy barrier and the lattice mismatch in the sandwich structures.
    Physical Chemistry Chemical Physics 01/2013; 15(8). DOI:10.1039/c2cp43350h · 4.20 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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
  • P. Dalach, D. E. Ellis, A. van de Walle
    [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    ABSTRACT: The geometric properties of the MPb10 monomers and their dimers (M = Fe, Co, Ni) are studied using the density-functional method. A lot of geometries of MPb10 have been searched. We found that the transition metal (M = Fe, Co, Ni) atom is favorable to be encapsulated into the Pb10 cage, and the structure of MPb10 with an encapsulated square antiprism is energetically favorable. Furthermore, these monomers could be assembled stable dimers and retain their structural identity. The most stable structure of the [MPb10]2 dimer is the two MPb10 monomers to be bound at the triangles facing upside down to each other. In addition, the weak interaction as well as the stability of NiPb10 cluster, suggests that NiPb10 seems better adapted for the purposes of cluster assembling. Meanwhile, the magnetic properties of these monomers and dimers are also investigated.
    Computational and Theoretical Chemistry 09/2011; 971(s 1–3):73–76. DOI:10.1016/j.comptc.2011.06.008 · 1.37 Impact Factor
  • [Show abstract] [Hide abstract]
    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 · 4.28 Impact Factor
  • 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
  • [Show abstract] [Hide abstract]
    ABSTRACT: Substitution of cadmium into bulk hydroxyapatite Ca((10-x))Cd(x)(PO(4))(6)(OH)(2) (CdHA: x = 0.12, 1.3, 2.5) is studied by combining X-ray diffraction data from synchrotron radiation, Fourier transform infra-red spectroscopy (FTIR) and density functional theory (DFT) calculations. Energetic and electronic analyses are carried out for several configurations of Cd substitution for Ca at both cationic sites. Rietveld analysis shows preferential occupation of the Ca2 site by cadmium. FTIR data suggest a non-negligible covalent character of Cd-OH. The much-discussed cation site preference for substitution is determined on the basis of relaxed-lattice energetics, and interpreted in terms of chemical concepts; theory indicates that the Ca2 site is clearly favored and this preference is related to the more covalent character of this site compared to that of site 1.
    Physical Chemistry Chemical Physics 10/2010; 12(47):15490-500. DOI:10.1039/c0cp01032d · 4.20 Impact Factor
  • Source
    P. Dalach, D. E. Ellis, A. van de Walle
    [Show abstract] [Hide abstract]
    ABSTRACT: Yttria-stabilized zirconia YSZ is modeled using a cluster expansion statistical thermodynamics method built upon a density-functional theory database. The reliability of cluster expansions in predicting atomic ordering is explored by comparing with the extensive experimental database. The cluster expansion of YSZ is utilized in lattice Monte Carlo simulations to compute the ordering of dopant and oxygen vacancies as a function of concentration. Cation dopants show a strong tendency to aggregate and vacate significantly sized domains below 9 mol % Y_2O_3, which is likely important for YSZ aging processes in ionic conductivity. Evolution of vibrational and underlying electronic properties as a function of Y doping is explored.
    Physical review. B, Condensed matter 10/2010; DOI:10.1103/PhysRevB.82.144117 · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We employed the hybrid DFT-LCAO and GGA-PW approaches as implemented in the CRYSTAL and VASP codes, respectively, for large supercell calculations of neutral O vacancies with trapped electrons (known as F centers) in the bulk and on the (001) surface of three cubic perovskite crystals (SrTiO 3 , PbTiO 3 , and PbZrO 3 ). The local lattice relaxation, charge redistribution, and positions of defect energy levels within the band gap are compared for three perovskites under study. We demonstrate how the difference in chemical composition of host materials leads to quite different defect properties.
    Ferroelectrics 09/2010; 379(2009):191-198. DOI:10.1080/00150190902852240 · 0.38 Impact Factor
  • M Matos, J Terra, D E Ellis
    [Show abstract] [Hide abstract]
    ABSTRACT: A basic understanding of Zn incorporation on bulk and hydrated (0 0 1) surfaces of hydroxyapatite (HA) is attained through electronic structure calculations which use a combined first principles density functional (DFT) and extended Hückel tight binding (EHTB) methodology. A Zn substituted hydroxyapatite relaxed structure is obtained through a periodic cell DFT geometry optimization method. Electronic structure properties are calculated by using both cluster DFT and periodic cell EHTB methods. Bond order calculations show that Zn preference for the Ca2 vacancy, near the OH channel and with greater structural flexibility, is associated with the formation of a four-fold (bulk) and nearly four-fold (surface) coordination, as in ZnO. When occupying the octahedral Ca1 vacancy, Zn remains six-fold in the bulk, but coordination decreases to five-fold in the surface. In the bulk and surface, Zn2 is found to be more covalent than Zn1, due to a decrease in bond lengths at the four-fold site, which approach the 1.99 Å ZnO value. Zn is however considerably less bound in the biomaterial than in the oxide, where calculated bond orders are twice as large as in HA. Surface phosphate groups (PO(4)) and hydroxide ions behave as compact individual units as in the bulk; no evidence is found for the presence of HPO(4). Ca-O bond orders decrease at the surface, with a consequent increase in ionicity. Comparison between DFT and EHTB results show that the latter method gives a good qualitative account of charge and bonding in these systems.
    Journal of Physics Condensed Matter 04/2010; 22(14):145502. DOI:10.1088/0953-8984/22/14/145502 · 2.22 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Using the DFT-HF hybrid LCAO approach as implemented in the CRYSTAL computer code, we have performed large supercell comparative calculations of neutral O vacancies (F centers) in the bulk and on the (001) surface of three cubic perovskite crystals (SrTiO3, PbTiO3, and PbZrO3). The local lattice relaxation, charge redistribution, and positions of defect energy levels within the band gap are compared. It is shown that the difference in the chemical composition of host materials leads to quite different defect properties.
    Solid State Communications 09/2009; 149(33-34):1359-1362. DOI:10.1016/j.ssc.2009.05.023 · 1.70 Impact Factor
  • Maria Matos, Joice Terra, D. E. Ellis
    [Show abstract] [Hide abstract]
    ABSTRACT: A systematic study is made on the electronic structure of stoichiometric calcium and lead apatites, using the tight binding extended Hückel method (eHT). The aim is to investigate the applicability of the semiempirical theory to study this family of compounds. A10(BO4)6X2 (A = Ca, Pb) apatites, differing by substitutions in the BO4 tetrahedral unit (B = P, As, and V) and X-channel ion (X = OH, Cl), are considered. The calculations show that eHT is suitable to describe basic properties especially concerning trends with atomic substitution and geometry changes. Band structure, Mulliken charge distribution, and bond orders are in good agreement with results of ab initio density functional theory (DFT) found in the literature. Large variations in the optical gap due to vanadium and lead substitutions are newly found. Changes in the anion X-channel affect the optical gap, which is in close agreement with DFT results. Analysis involving subnets are performed to determine the role of halogenic orbitals in the electronic structure of chloroapatites, showing evidence of covalent Cl bonding. It was also found that PbOH bonding in hydroxy-vanadinite Pb10(VO4)6(OH)2, recently synthesized, is weaker than that of CaOH in vanadate Ca10(VO4)6(OH)2. Arsenium is found to be more weakely bound to the O-tetrahedron than phosphorous, although CaO bond is increased with the substitution. We investigate, in addition, the electronic structure of a model system Ca10(AsO4)6(OH)2, obtained from direct As substitution in the vanadate Ca10(VO4)6(OH)2. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
    International Journal of Quantum Chemistry 03/2009; 109(4):849 - 860. DOI:10.1002/qua.21887 · 1.17 Impact Factor
  • Shuxia Yin, D.E. Ellis
    [Show abstract] [Hide abstract]
    ABSTRACT: The adsorption structures of Cr(VI) species on hydroxylated α-Fe2O3() were investigated using density functional theory within a periodic slab model. A broad structural survey of H2CrO4, and adsorbed on different surface sites was carried out. Chromate species adsorb on the hydroxylated surface via an outer-sphere mechanism through H-bonding and ion–dipole attraction. The most stable structure for H2CrO4 reveals strong H-bonding to surface and between adsorbates. For , a configuration that shows both H-bonding and Cr–O···H–O–Fe ion–dipole interaction is the most favored. In the case of , the most favored adsorption configuration shows the complex standing on the surface with a Cr–O bond aligned on OH, presenting the strongest Cr–O···H–O–Fe interaction. In these three species, displays the strongest interaction with the hydroxylated () surface with no tendency toward reduction observed, in accordance with experiment.
    Surface Science 02/2009; 603(4):736-746. DOI:10.1016/j.susc.2009.01.015 · 1.87 Impact Factor

Publication Stats

7k Citations
490.78 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 Materials Science and Engineering
      • • Department of Chemistry
      • • Center for AIDS Research
      Evanston, Illinois, United States
  • 2007
    • University of Illinois at Chicago
      • Department of Electrical and Computer Engineering
      Chicago, Illinois, United States
  • 2003
    • Northwest University
      Evanston, Illinois, United States
  • 1979–1994
    • 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