W. A. Shelton

Observation of rare-earth segregation in silicon nitride ceramics at subnanometre dimensions.

Silicon nitride (Si3N4) ceramics are used in numerous applications because of their superior mechanical properties. Their intrinsically brittle nature is a critical issue, but can be overcome by introducing whisker-like microstructural features. However, the formation of such anisotropic grains is very sensitive to the type of cations used as the sintering additives. Understanding the origin of dopant effects, central to the design of high-performance Si3N4 ceramics, has been sought for many years. Here we show direct images of dopant atoms (La) within the nanometre-scale intergranular amorphous films typically found at grain boundaries, using aberration corrected Z-contrast scanning transmission electron microscopy. It is clearly shown that the La atoms preferentially segregate to the amorphous/crystal interfaces. First-principles calculations confirm the strong preference of La for the crystalline surfaces, which is essential for forming elongated grains and a toughened microstructure. Whereas principles of micrometre-scale structural design are currently used to improve the mechanical properties of ceramics, this work represents a step towards the atomic-level structural engineering required for the next generation of ceramics.

O2 reduction by lithium on Au(111) and Pt(111)

Lithium-oxygen has one of the highest specific energies among known electrochemical couples and holds the promise of substantially boosting the specific energy of portable batteries. Mechanistic information of the oxygen reduction reaction by Li (Li-ORR) is scarce, and the factors limiting the discharge and charge efficiencies of the Li-oxygen cathode are not understood. To shed light on the fundamental surface chemistry of Li-ORR, we have performed periodic density functional theory calculations in conjunction with thermodynamic modeling for two metal surfaces, Au(111) and Pt(111). On clean Au(111) initial O(2) reduction via superoxide (LiO(2)) formation has a low reversible potential of 1.51 V. On clean Pt(111), the dissociative adsorption of O(2) is facile and the reduction of atomic O has a reversible potential of 1.97 V, whereas the associative channel involving LiO(2) is limited by product stability versus O to 2.04 V. On both surfaces O(2) lithiation significantly weakens the O-O bond, so the product selectivity of the Li-ORR is monoxide (Li(x)O), not peroxide (Li(x)O(2)). Furthermore, on both surfaces Li(x)O species are energetically driven to form (Li(x)O)(n) aggregates, and the interface between (Li(x)O)(n) and the metal surfaces are active sites for forming and dissociating LiO(2). Given that bulk Li(2)O((s)) is found to be globally the most stable phase up to 2.59 V, the presence of available metal sites may allow the cathode to access the bulk Li(2)O phase across a wide range of potentials. During cycling, the discharge process (oxygen reduction) is expected to begin with the reduction of chemisorbed atomic O instead of gas-phase O(2). On Au(111) this occurs at 2.42 V, whereas the greater stability of O on Pt(111) limits the reversible potential to 1.97 V. Therefore, the intrinsic reactivity of Pt(111) renders it less effective for Li-ORR than Au(111).

Oxygen Reduction by Lithium on Model Carbon and Oxidized Carbon Structures

Li-air batteries have attracted substantial interest for their high theoretical specific energies, but the oxygen reduction reaction by Li (Li-ORR) that occurs at the carbon cathode remains poorly understood. Periodic density functional theory calculations have been performed to examine the Li-ORR on several model carbon structures, including the graphite(0001) basal plane, the (8,0) single-wall nanotube, the armchair-type edge, and a di-vacancy in the basal plane. The inertness of the basal plane limits the reversible potential of O{sub 2} reduction to 1.1 V, and slightly higher to 1.2 V on the curved nanotube. The armchair edge and di-vacancy are highly reactive and significantly oxidized at ambient conditions to various CO{sub x} groups, which are reduced by Li via redox mechanisms at 1.2-1.4 V. These CO{sub x} groups can also catalyze O{sub 2} reduction at up to 2.3 V (an overpotential of 0.4 V vs. the calculated equilibrium potential for bulk Li{sub 2}O{sub 2} formation) by chelating and stabilizing the LiO{sub 2} intermediate. The Li-ORR on graphitic carbon, if via concerted Li{sup +}/e{sup -} transfer and involving carbon, lithium, and oxygen only, is therefore expected to initiate with the smallest overpotential at under-coordinated carbon centers that are oxidized at ambient conditions.

Trends in the Catalytic Activity of Transition Metals for the Oxygen Reduction Reaction by Lithium.

Periodic density functional theory (DFT) calculations indicate that the intrinsic activity of Au, Ag, Pt, Pd, Ir, and Ru for the oxygen reduction reaction by Li (Li-ORR) forms a volcano-like trend with respect to the adsorption energy of oxygen, with Pt and Pd being the most active. The trend is based on two mechanisms: the reduction of molecular O2 on Au and Ag and of atomic O on the remaining metals. Step edges are found to be more active for catalyzing the Li-ORR than close-packed surfaces. Our findings identify important considerations in the design of catalyst-promoted air cathodes for nonaqueous Li-air batteries.

Mesoscopic Metal−Insulator Transition at Ferroelastic Domain Walls in VO2

The novel phenomena induced by symmetry breaking at homointerfaces between ferroic variants in ferroelectric and ferroelastic materials have attracted recently much attention. Using variable temperature scanning microwave microscopy, we demonstrate the mesoscopic strain-induced metal-insulator phase transitions in the vicinity of ferroelastic domain walls in the semiconductive VO(2) that nucleated at temperatures as much as 10-12 degrees C below bulk transition, resulting in the formation of conductive channels in the material. Density functional theory is used to rationalize the process low activation energy. This behavior, linked to the strain inhomogeneity inherent in ferroelastic materials, can strongly affect interpretation of phase-transition studies in VO(2) and similar materials with symmetry-lowering transitions, and can also be used to enable new generations of electronic devices though strain engineering of conductive and semiconductive regions.

A first-principles investigation of the effect of Pt cluster size on CO and NO oxidation intermediates and energetics.

As catalysis research strives toward designing structurally and functionally well-defined catalytic centers containing as few active metal atoms as possible, the importance of understanding the reactivity of small metal clusters, and in particular of systematic comparisons of reaction types and cluster sizes, has grown concomitantly. Here we report density functional theory calculations (GGA-PW91) that probe the relationship between particle size, intermediate structures, and energetics of CO and NO oxidation by molecular and atomic oxygen on Pt(x) clusters (x = 1-5 and 10). The preferred structures, charge distributions, vibrational spectra, and energetics are systematically examined for oxygen (O(2), 2O, and O), CO, CO(2), NO, and NO(2), for CO/NO co-adsorbed with O(2), 2O, and O, and for CO(2)/NO(2) co-adsorbed with O. The binding energies of oxygen, CO, NO, and of the oxidation products CO(2) and NO(2) are all markedly enhanced on Pt(x) compared to Pt(111), and they trend toward the Pt(111) levels as cluster size increases. Because of the strong interaction of both the reactants and products with the Pt(x) clusters, deep energy sinks develop on the potential energy surfaces of the respective oxidation processes, indicating worse reaction energetics than on Pt(111). Thus the smallest Pt clusters are less effective for catalyzing CO and NO oxidation in their original state than bulk Pt. Our results further suggests that oxidation by molecular O(2) is thermodynamically more favourable than by atomic O on Pt(x). Conditions and applications in which the Pt(x) clusters may be effective catalysts are discussed.

Noncollinear magnetic structure in Ni0.35Fe0.65

Magnetic structure of NicFe1−c alloys in the INVAR region has long been a matter of great scientific interest and controversy. Using the locally self-consistent multiple scattering method, which has recently been extended to treat noncollinear magnetic systems, we studied the magnetic structure of Ni0.35Fe0.65 alloys. To simulate the alloys, we constructed a large fcc based sample which contains 256 sites occupied randomly by Ni and Fe atoms. The ground state magnetic structure is found to consist of noncollinear configurations associated with Fe-rich regions. In particular, Fe sites surrounded completely by other Fe atoms have antiferromagnetic alignments, while Fe sites having less than three Ni nearest-neighbors have a variety of noncollinear arrangements.

Constrained density functional theory for first principles spin dynamics

Constrained density functional theory is used to formulate a theory of general noncollinear spin systems which makes it possible to implement first principles spin dynamics in a manner that is firmly grounded in density functional theory. At each time step, local constraining fields are calculated from a self-consistent algorithm. In addition to discussing the conceptual basis of the resulting constrained local moment model we illustrate the theory by explicit calculations for the relative rotation of the corner and body center moments of bcc iron.

Towards a constrained local moment model for first principles spin dynamics

Abstract We argue that, as originally formulated, the first principles spin dynamics of the finite-temperature and non-equilibrium properties of itinerant magnets recently proposed by Antropov et al. is not clearly defined within density functional theory. We show how constrained density functional theory can be used to provide a formal basis for describing the instantaneous non-collinear states that are being evolved according to the classical equation of motion of spin-dynamics. We propose a constrained local moment (CLM) model in which specific orientational configurations are maintained by local transverse constraining fields that are obtained self-consistently. A general algorithm for finding the constraining fields is used and the existence of a CLM state is demonstrated for specific model problems including a cell of 512 Fe atoms for which the orientations of the magnetic moments are random.

MADNESS: A Multiresolution, Adaptive Numerical Environment for Scientific Simulation

MADNESS (multiresolution adaptive numerical environment for scientific simulation) is a high-level software environment for solving integral and differential equations in many dimensions that uses adaptive and fast harmonic analysis methods with guaranteed precision that are based on multiresolution analysis and separated representations. Underpinning the numerical capabilities is a powerful petascale parallel programming environment that aims to increase both programmer productivity and code scalability. This paper describes the features and capabilities of MADNESS and briefly discusses some current applications in chemistry and several areas of physics.