D. E. Ellis

Northwestern University, Evanston, Illinois, United States

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Publications (209)396.09 Total impact

  • 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. 01/2014; 83:207–211.
  • S.E. Stoltz, D.E. Ellis, M.J. Bedzyk
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    ABSTRACT: Figure optionsDownload full-size imageDownload high-quality image (63 K)Download as PowerPoint slide
    Surface Science. 01/2014; 630:46–63.
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    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; · 3.83 Impact Factor
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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. · 1.84 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). · 3.77 Impact Factor
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    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.
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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. · 3.55 Impact Factor
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    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. · 3.83 Impact Factor
  • M Matos, J Terra, D E Ellis
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    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. · 2.22 Impact Factor
  • Powder Diffraction - POWDER DIFFR. 01/2010; 25(2).
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    P. Dalach, D. E. Ellis, A. van de Walle
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    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 01/2010; · 3.77 Impact Factor
  • Lj Miljacic, D E Ellis
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    ABSTRACT: A method for improved representation of electronic charge and spin densities for molecular and solid state systems is presented, based upon extensions of least squares fits to quantum mechanical "true" densities using basis functions of limited support. Attention is given to optimization of radial degrees of fit freedom, and the design of fit functions permitting rapid analytic manipulation and calculation of properties, such as Coulomb potentials. The method is demonstrated for covalent CO and for a large metal-organic crystalline structure.
    Journal of Computational Chemistry 12/2009; 31(7):1486-94. · 3.84 Impact Factor
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    ABSTRACT: Quasi-molecular X rays observed in heavy ion collisions are interpreted within a relativistic calculation of correlation diagrams using the Dirac-Slater model. A semiquantitative description of noncharacteristic M X rays is given for the system Au-I.
    International Journal of Quantum Chemistry 06/2009; 10(S10):227 - 230. · 1.17 Impact Factor
  • Shuxia Yin, D.E. Ellis
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    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 01/2009; 603(4):736-746. · 1.84 Impact Factor
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    ABSTRACT: The adsorption of submonolayer V on an idealized model hematite (0 0 0 1) surface and subsequent oxidation under atomic O adsorption are studied by density functional theory. The preferred adsorption sites, adsorption energy and configuration changes due to V and O adsorption are investigated. It is found that in most cases V forms threefold bonds with surface O atoms, inducing a large geometry change at the hematite surface and near surface region and a bond stretch between surface Fe and O. The adsorption energy is mainly decided by interplay between adsorbed metal-surface oxygen bonding and adsorbed metal – subsurface metal interaction. The relative energy of subsequent O adsorption and geometry depends on the reformed V/hematite structure. Electronic properties such as projected densities of states and chemical state change upon V adsorption are studied through both periodic slab and embedded cluster localized orbital calculations; both strong vanadium–oxygen and vanadium–iron interactions are found. While V generally donates electrons to a hematite surface, causing nearby Fe to be partially reduced, the Fe and V oxidization state depends very much on the coverage and detailed adsorption configuration. When the V/hematite system is exposed to atomic O, V is further oxidized and surface/near surface Fe is re-oxidized. Our theoretical results are compared with X-ray surface standing wave and X-ray photoelectron spectroscopic measurements. The influence of d-electron correlation on the predicted structures is briefly discussed, making use of the DFT + U scheme.
    Surface Science 10/2008; · 1.84 Impact Factor
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    ABSTRACT: The adsorption of a monolayer of V on idealized Fe- and oxygen-terminated hematite (0 0 0 1) surfaces and subsequent oxidation under atomic O adsorption are studied by density functional theory. Theoretical results are compared with X-ray surface standing wave and X-ray photoelectron spectroscopic measurements, and interpreted in the light of data on sub-monolayer coverages. Near-surface Fe reduction under V adsorption and accompanying structural relaxation are examined. These effects and subsequent response to oxidation, are found to be highly site specific. A full monolayer of oxygen leads to a V5+ state and reoxidation of subsurface Fe to the trivalent state, seen in both theory and experiment.
    Surface Science 10/2008; · 1.84 Impact Factor
  • Maria Matos, Joice Terra, D. E. Ellis
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    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 10/2008; 109(4):849 - 860. · 1.17 Impact Factor
  • P. Dalach, H. Frost, R. Q. Snurr, D. E. Ellis
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    ABSTRACT: Metal−organic framework (MOF) materials show potential for gas separation and storage, and as hosts for highly selective catalyst molecules. Density functional theory (DFT) is applied to periodic band structures and to selected clusters representative of the cornerpost and strut environments of two MOFs to characterize the electronic environment. Binding sites and the binding energies of H2 are calculated with and without the presence of a Li dopant. It is found that Li enhances the H2 binding energies, both on the linking strut ring structures and for sites near cornerpost oxygen. MP2 correlation studies of the basic H2−Li−bipyridine interaction are carried out to explore effects of correlation beyond DFT. Contrary to previous model assumptions, we find that Li associates strongly with the cornerposts and less so with aromatic rings.
    05/2008;
  • Shuxia Yin, D. E. Ellis
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    ABSTRACT: The interaction of absorbed water with defected hematite surfaces is studied in low-coverage. Three point-defective surfaces, including an Fe-vacancy, an Fe-adatom site, and an O-vacancy were studied using periodic density functional theory in a periodic slab model. The 2-coordinated O around an Fe-adatom or vacancy shows a decreased electron occupation. As a result, the defective surfaces with Fe-adatoms and vacancies are more reactive for H2O than either clean or O-vacancy surfaces. The Fe-adatom surface is most reactive for H2O molecular adsorption and is also highly reactive for dissociative adsorption.
    Surface Science 01/2008; 602(12):2047-2054. · 1.84 Impact Factor
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    D. E. Ellis, N. M. Tubman, D. M. Wells
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    ABSTRACT: Crystalline multilayer systems with structure ABABA... offer the possibility of combining functional properties of two distinctly different materials, and of exploiting the interfaces to couple functionality of one component to the other. The multilayer environment permits the amplification of interface properties as would be important for device applications. The manipulation of ferroelectric, ferromagnetic, and/or ferroelastic properties in so-called ferroic materials through growth of thin films, multilayers, and graded composition structures has received considerable experimental and theoretical attention in recent years. We survey the current status of atomic-scale modeling of multilayer systems which could exhibit ferroic behavior; i.e., spontaneous order below a critical temperature and hysteresis in stimulus-response behavior. The roles of interfacial strain, chemical variability at the interface, and film thickness are explored, taking as a primary example the classic BaTiO3 ∣∣Fe3O4 ferroelectric∣∣ferrimagnetic interactions. First principles band structure calculations are used to determine relaxed interface structures and residual stresses, as well as the underlying electronic distributions. Embedded cluster methods are then used to extract local chemical bonding characteristics and hyperfine properties.
    Hyperfine Interactions 08/2007; 179(1):23-32. · 0.21 Impact Factor

Publication Stats

3k Citations
396.09 Total Impact Points

Institutions

  • 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
  • 2013
    • Nanjing University of Science and Technology
      Nan-ching, Jiangsu Sheng, China
  • 2010
    • Pontifícia Universidade Católica do Rio de Janeiro
      Rio de Janeiro, Rio de Janeiro, Brazil
  • 1999–2010
    • Centro Brasileiro de Pesquisas Físicas
      Rio de Janeiro, Rio de Janeiro, Brazil
  • 2007
    • California Institute of Technology
      • Division of Engineering and Applied Science
      Pasadena, CA, United States
    • University of Illinois at Chicago
      • Department of Electrical and Computer Engineering
      Chicago, Illinois, United States
  • 2003
    • Northwest University
      Evanston, Illinois, United States
  • 2001
    • Technion - Israel Institute of Technology
      • Faculty of Physics
      Haifa, Haifa District, Israel
  • 1989–1991
    • Argonne National Laboratory
      • Division of Materials Science
      Lemont, Illinois, United States
  • 1976–1979
    • Chalmers University of Technology
      • Department of Applied Physics
      Goeteborg, Västra Götaland, Sweden
  • 1976–1977
    • Judson College
      Elgin, Illinois, United States
  • 1972–1977
    • VU University Amsterdam
      Amsterdamo, North Holland, Netherlands