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International audience; The physics of Fe-C surface interactions is of fundamental importance to phenomena such as corrosion, catalysis, synthesis of graphene, new steels, etc. To better understand this question, we perform an extensive characterization of the energy landscape for carbon diffusion from bulk to surfaces for bcc iron at low C concentration. C diffusion mechanisms over the three main Fe-surfaces - (100), (110) and (111) - are studied computationally using the kinetic activation-relaxation technique (k-ART), an off-lattice kinetic Monte Carlo algorithm. Migration and adsorption energies on surfaces as well as absorption energies into the subsurfaces are predicted and then compared to density functional theory (DFT) and experiment. The energy landscape along C-diffusion pathways from bulk to surface is constructed allowing a more extensive characterization of the diffusion pathways between surface and subsurface. In particular, effective migration energies from (100), (110) and (111) surfaces, to the bulk octahedral site are found to be around similar to 1.6 eV, similar to 1.2 eV and similar to 13 eV respectively suggesting that C insertion into the bulk cannot take place in pure crystalline Fe, irrespective of the exposed surface.

We obtain a recursive formulation for a general class of optimization problems with forward‐looking constraints which often arise in economic dynamic models, for example, in contracting problems with incentive constraints or in models of optimal policy. In this case, the solution does not satisfy the Bellman equation. Our approach consists of studying a recursive Lagrangian. Under standard general conditions, there is a recursive saddle‐point functional equation (analogous to a Bellman equation) that characterizes a recursive solution to the planner's problem. The recursive formulation is obtained after adding a co‐state variable μ t summarizing previous commitments reflected in past Lagrange multipliers. The continuation problem is obtained with μ t playing the role of weights in the objective function. Our approach is applicable to characterizing and computing solutions to a large class of dynamic contracting problems.

Rare-earth metal oxides with perovskite-type crystal structures are under consideration for use as air electrode materials for intermediate to high temperature electrochemical device applications. The surface chemistry of these materials plays a critical role in determining the kinetics of oxygen reduction and exchange reactions. Among various perovskite-structured oxides, certain members of the Ruddlesden–Popper series, e.g. La2NiO4, have been identified as significantly active for surface oxygen interactions. However, the challenge remains to be the identification of the structure and composition of active surfaces, as well as the influence of these factors on the mechanisms of surface exchange reactions. In this contribution, the changes in the electronic structure and the energetics of oxygen interactions on the surfaces of La2NiO4 are analysed using first principles calculations in the Density Functional Theory (DFT) formalism. As for the surface chemistry, LaO termination rather than NiO2 termination is presumed due to recent experimental evidence of the surfaces of various perovskite structured oxides after heat treatment in oxidizing environments being transition metal free. Our findings substantiate the fact that the LaO-terminated surface can indeed participate in the formation of surface superoxo species. Detailed charge transfer analyses revealed that it is possible for such a surface to be catalytically active owing to the enhanced electronic configurations on the neighbouring La sites to surface species. In addition, positively charged oxygen vacancies, relative to the crystal lattice, can act as active sites and catalyse the O–O bond cleavage.

The effects of hole injection in amorphous-IGZO is analyzed by means of first-principles calculations. The injection of holes in the valence band tail states leads to their capture as a polaron, with high self-trapping energies (from 0.44 to 1.15 eV). Once formed, they mediate the formation of peroxides and remain localized close to the hole injection source due to the presence of a large diffusion energy barrier (of at least 0.6eV). Their diffusion mechanism can be mediated by the presence of hydrogen. The capture of these holes is correlated with the low off-current observed for a-IGZO transistors, as well as, with the difficulty to obtain a p-type conductivity. The results further support the formation of peroxides as being the root cause of Negative bias illumination stress (NBIS). The strong self-trapping substantially reduces the injection of holes from the contact and limits the creation of peroxides from a direct hole injection. In presence of light, the concentration of holes substantially rises and mediates the creation of peroxides, responsible for NBIS., Comment: 8 pages, 8 figures, to be published in Journal of Applied Physics

The Fischer-Tropsch reaction is initiated by direct CO dissociation for Iron catalyst even though a H-assisted mechanism may be easier on other metals. In the gas phase, the CO dissociation is only favorable for Fe-clusters composed by more than 11 atoms. We show here the remarkable effect of the support TiO 2 (1 1 0), making this dissociation exothermic for Fe 4 and Fe 5 clusters. The main factor for the CO activation is the electron transfer to the reducible support. The role of the TiO 2 (1 1 0) support is to transform the neutral cluster into a positively charged one for which CO dissociation is easier.

Identification of radiation-induced radicals in relatively simple molecules is a prerequisite for the understanding of reaction pathways of the radiation chemistry of complex systems. Sucrose presents an additional practical interest as a versatile radiation dosimetric system. In this work, we present a periodic density functional theory study aimed to identify the fourth stable radical species in this carbohydrate. The proposed model is a fragment suspended in the lattice by hydrogen bonds with an unpaired electron at the original C5’ carbon of the fructose unit. It requires a double scission of the ring accompanied by substantial chemical and geometric reorganization.

Atomic layer deposition (ALD) has been recognized as a promising method to deposit conformal and uniform thin film of copper for future electronic devices. However, many aspects of the reaction mechanism and the surface chemistry of copper ALD remain unclear. In this paper, we employ plane wave density functional theory (DFT) to study the transmetalation ALD reaction of copper dimethylamino-2-propoxide [Cu(dmap)2] and diethylzinc [Et2Zn] that was realized experimentally by Lee et al. [ Angew. Chem., Int. Ed. 2009, 48, 4536−4539]. We find that the Cu(dmap)2 molecule adsorbs and dissociates through the scission of one or two Cu–O bonds into surface-bound dmap and Cu(dmap) fragments during the copper pulse. As Et2Zn adsorbs on the surface covered with Cu(dmap) and dmap fragments, butane formation and desorption was found to be facilitated by the surrounding ligands, which leads to one reaction mechanism, while the migration of ethyl groups to the surface leads to another reaction mechanism. During both reaction mechanisms, ligand diffusion and reordering are generally endothermic processes, which may result in residual ligands blocking the surface sites at the end of the Et2Zn pulse, and in residual Zn being reduced and incorporated as an impurity. We also find that the nearby ligands play a cooperative role in lowering the activation energy for formation and desorption of byproducts, which explains the advantage of using organometallic precursors and reducing agents in Cu ALD. The ALD growth rate estimated for the mechanism is consistent with the experimental value of 0.2 Å/cycle. The proposed reaction mechanisms provide insight into ALD processes for copper and other transition metals.

A new approach is reported for tailoring the pore geometry in five series of multivariate metal-organic frameworks (MOFs) based on the structure [Zn-2(bdc)(2)(dabco)] (bdc = 1,4-benzenedicarboxylate, dabco = 1,8-diazabicyclooctane), DMOF-1. A doping procedure has been adopted to form series of MOFs containing varying linker ratios. The series under investigation are [Zn-2(bdc)(2-x)(bdc-Br)(x)(dabco)]center dot nDMF 1 (bdc-Br = 2-bromo-1,4-benzenedicarboxylate), [Zn-2(bdc)(2-x)(bdc-I)(x)(dabco)]center dot nDMF 2 (bdc-I = 2-iodo-1,4-benzenedicarboxylate), [Zn-2(bdc)(2-x)(bdc-NO2)(x)(dabco)]center dot nDMF 3 (bdc-NO2 = 2-nitro-1,4-benzenedicarboxylate), [Zn-2(bdc)(2-x)(bdc-NH2)(x)(dabco)]center dot nDMF 4 (bdc-NH2 = 2-amino-1,4-benzenedicarboxylate) and [Zn-2(bdc-Br)(2-x)(bdc-I)(x)(dabco)] nDMF 5. Series 1-3 demonstrate a functionality-dependent pore geometry transition from the square, open pores of DMOF-1 to rhomboidal, narrow pores with increasing proportion of the 2-substituted bdc linker, with the rhomboidal-pore MOFs also showing a temperature-dependent phase change. In contrast, all members of series 4 and 5 have uniform pore geometries. In series 4 this is a square pore topology, whilst series 5 exhibits the rhomboidal pore form. Computational analyses reveal that the pore size and shape in systems 1 and 2 is altered through non-covalent interactions between the organic linkers within the framework, and that this can be controlled by the ligand functionality and ratio. This approach affords the potential to tailor pore geometry and shape within MOFs through judicious choice of ligand ratios.

The Atomic Simulation Environment (ASE) is a software package written in the Python programming language with the aim of setting up, steering, and analyzing atomistic simula- tions. In ASE, tasks are fully scripted in Python. The powerful syntax of Python combined with the NumPy array library make it possible to perform very complex simulation tasks. For example, a sequence of calculations may be performed with the use of a simple “for-loop” construction. Calculations of energy, forces, stresses and other quantities are performed through interfaces to many external electronic structure codes or force fields using a uniform interface. On top of this calculator interface, ASE provides modules for performing many standard simulation tasks such as structure optimization, molecular dynamics, handling of constraints and performing nudged elastic band calculations.

We obtain a recursive formulation for a general class of contracting problems involving incentive constraints. Under these constraints, the corresponding maximization (sup) problems fails to have a recursive solution. Our approach consists of studying the Lagrangian. We show that, under standard assumptions, the solution to the Lagrangian is characterized by a recursive saddle point (infsup) functional equation, analogous to Bellman's equation. Our approach applies to a large class of contractual problems. As examples, we study the optimal policy in a model with intertemporal participation constraints (which arise in models of default) and intertemporal competitive constraints (which arise in Ramsey equilibria).

We used density functional theory to characterize various crystalline phases of NiTi alloys: (i) high-temperature austenite phase B2; (ii) orthorhombic B19; (iii) the monoclinic martensite phase B19′; and (iv) a body-centered orthorhombic phase (BCO), theoretically predicted to be the ground state. We also investigated possible transition pathways between the various phases and the energetics involved. We found B19 to be metastable with a 1 meV energy barrier separating it from B19′. Interestingly, we predicted a new phase of NiTi, denoted B19′′, that is involved in the transition between B19′ and BCO. B19′′ is monoclinic and can exhibit shape memory; furthermore, its presence reduces the internal stress required to stabilize the experimentally observed B19′ structure, and it consequently plays a key role in NiTi’s properties.

Results of ab initio density-functional theory calculations of the migration energies of oxygen vacancies and interstitials in stoichiometric ${\text{UO}}_{2}$ are reported. The diffusion of oxygen vacancies in ${\text{UO}}_{2}$ is found to be highly anisotropic, and the [1 0 0] direction is energetically favored. The atomic relaxations play an important role in reducing the migration barriers. Within the generalized gradient approximation (GGA), we find that the migration energies of the preferred vacancies and interstitials paths are, respectively, 1.18 and 1.09 eV. With the inclusion of the Hubbard $U$ parameter to account for the $5f$ electron correlations in $\text{GGA}+U$, the vacancy migration energy is lowered to 1.01 eV while the interstitial migration energy increases slightly to 1.13 eV. We find, however, that the correlation effects have a drastic influence on the mechanism of interstitial migration through the stabilization of Willis-type clusters. Indeed, in contrast to GGA, in $\text{GGA}+U$ there is an inversion of the migration path with the so-called ``saddle-point'' position being lower in energy than the usual starting position. Thus while the migration barriers are nearly the same in GGA and $\text{GGA}+U$, the mechanisms are completely different. Our results clearly indicate that both vacancies and interstitials contribute almost equally to the diffusion of oxygen in ${\text{UO}}_{2}$.

We applied density functional theory based on ultrasoft pseudopotentials to study the structural properties of Ir-4 clusters both in the gas phase and adsorbed on a MgO(100) surface. To resolve the discrepancy between experimental data which suggest a tetrahedral structure for Ir-4 and theoretical results which show a strong preference for the square isomer, we investigated the effect of several adsorbates on the equilibrium atomic structure of the clusters. Calculated binding energies of a single H, C, or O atom, as well as one CO or OH molecule, to three stable Ir-4 isomers indicate that C or CO adsorption significantly influences the relative stability of Ir-4 isomers. For MgO(100)-supported Ir-4, atomic carbon is able to change the isomer preference from the square to the tetrahedral geometry.

The adsorption of molecular and atomic chlorine on perfect Ag(111) surface has been studied and characterized by means of extensive density-functional-theory calculations. For the molecular adsorption, we find that the dissociation of ${\text{Cl}}_{2}$ proceeds with an almost vanishing barrier. As for the adsorption of atomic Cl, on-surface, subsurface, and substitutional adsorptions are considered as a function of the coverage. At coverage lower than 1/2 ML, the on-surface adsorption displays the most exothermic chemisorption energies, whereas the mixed $\text{on-surface}+\text{subsurface}$ and $\text{on-surface}+\text{substitutional}$ adsorption modes become competitive with pure on-surface adsorption at about 1/2 ML of coverage and at higher coverages even preferred. The analysis of the adsorption free energy as a function of chlorine chemical potential reveals that the on-surface $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ adsorption phase is thermodynamically the most stable over a very broad range of Cl chemical potential. The mixed adsorption modes become thermodynamically more stable at high coverage for values of the Cl chemical potential that are substantially larger than those needed to form silver chloride. This finding seems to indicate that the formation of mixed adsorption phases, if they would ever occur, cannot be due to thermodynamic equilibrium but can only result from kinetic effects. We also find that the presence of open surface steps does not stabilize the subsurface Cl adsorption at low coverage. However due to the stronger Cl-surface interaction near steps, the mixed $\text{on-surface}+\text{subsurface}$ adsorption on Ag(210) at high coverage becomes thermodynamically the most stable phase at Cl chemical potential close to that needed for the formation of bulk AgCl.

We present a combined experimental and theoretical study of the CO2 interaction with the Ni(110) surface. Photoelectron spectroscopy, temperature-programmed desorption, and high-resolution electron energy loss spectroscopy measurements are performed at different coverages and for increasing surface temperature after adsorption at 90 K with the aim to study the competing processes of CO2 dissociation and desorption. Simulations are performed within the framework of density functional theory using ab initio pseudopotentials, focusing on selected chemisorption geometries, determining the energetics and the structural and vibrational properties. Both experimental and theoretical vibrational frequencies yield consistent indications about two inequivalent adsorption sites that can be simultaneously populated at low temperature: short-bridge site with the molecular plane perpendicular to the surface and hollow site with the molecular plane inclined with respect to the surface. In both sites, the molecule has pure carbon or mixed oxygen-carbon coordination with the metal and is negatively charged and bent. Predicted energy barriers for adsorption and diffusion on the surface suggest a preferential adsorption path through the short-bridge site to the hollow site, which is compatible with the experimental findings. Theoretical results qualitatively support literature data concerning the increase of the work function upon chemisorption.

We have studied the keto-enol interconversion of acetone to understand the mechanism of tautomerism relevant to numerous organic and biochemical processes. Applying the ab initio metadynamics method, we simulated the keto-enol isomerism both in the gas phase and in the presence of water. For the gas-phase intramolecular mechanism we show that no other hydrogen-transfer reactions can compete with the simple keto-enol tautomerism. We obtain an intermolecular mechanism and remarkable participation of water when acetone is solvated by neutral water. The simulations reveal that C deprotonation is the kinetic bottleneck of the keto-enol transformation, in agreement with experimental observations. The most interesting finding is the formation of short H-bonded chains of water molecules that provide the route for proton transfer from the carbon to the oxygen atom of acetone. The mechanistic picture that emerged from the present study involves proton migration and emphasizes the importance of active solvent participation in tautomeric interconversion.

The dissociation of molecular hydrogen on a Mgs0001d surface and the subsequent diffusion of atomic hydrogen into the magnesium substrate is investigated using Density Functional Theory (DFT) calculations and rate theory. The minimum energy path and corresponding transition states are located using the nudged elastic band method, and rates of the activated processes are calculated within the harmonic approximation to transition state rate theory, using both classical and quantum partition functions based atomic vibrational frequencies calculated by DFT. The dissociation/recombination of H2 is found to be rate-limiting for the ab- and desorption of hydrogen, respectively. Zero-point energy contributions are found to be substantial for the diffusion of atomic hydrogen, but classical rates are still found to be within an order of magnitude at room temperature.

We present atomistic simulations of crystal growth where realistic experimental deposition rates are reproduced, without needing any a priori information on the relevant diffusion processes. Using the temperature accelerated dynamics method, we simulate the deposition of 4 monolayers (ML) of Ag/Ag(100) at the rate of 0.075 ML/s, thus obtaining a boost of several orders of magnitude with respect to ordinary molecular dynamics. In the temperature range analyzed (0--70 K), steering and activated mechanisms compete in determining the surface roughness.

We extend the classical Neyman-Pearson theory for testing composite hypotheses versus composite alternatives, using a convex duality approach, first employed by Witting. Results of Aubin and Ekeland from non-smooth convex analysis are used, along with a theorem of Komlós, in order to establish the existence of a max-min optimal test in considerable generality, and to investigate its properties. The theory is illustrated on representative examples involving Gaussian measures on Euclidean and Wiener space.

In this paper we shall prove some results pertaining to the existence and multiplicity of normal geodesics joining two given submanifolds of an orthogonal splitting Lorentzian manifold. To this aim, we look for critical points of an unbounded suitable functional by using a Saddle-Point Theorem and the relative category theory.

The main discharge product at the cathode of non-aqueous Li-air batteries is insulating Li2O2 and its poor electronic conduction is a main limiting factor in the battery performance. Here, we apply density functional theory calculations (DFT) to investigate the potential of circumventing this passivation by controlling the morphological growth directions of Li2O2 using directed poisoning of specific nucleation sites and steps. We show SO2 to bind preferentially on steps and kinks on the (1-00) facet and to effectively lower the discharge potential by 0.4 V, yielding a more facile discharge on the (0001) surface facet. Addition of a few percent SO2 in the O-2 stream may be used to control and limit growth of Li2O2 in specific directions and increase the electronic conduction through formation of interfaces between Li2O2 and Li-2(SO2)-type inclusions, which may ultimately lead to an increased accessible battery capacity at the expense of a limited increase in the overpotentials.

We introduce a three replica potential useful to examine the structure of metastables states above the static transition temperature, in the spherical p-spin model. Studying the minima of the potential we are able to find which is the distance between the nearest equilibrium and local equilibrium states, obtaining in this way information on the dynamics of the system. Furthermore, the analysis of the potential at the dynamical transition temperature suggests that equilibrium states are not randomly distributed in the phase space., Comment: plain tex, 26 pages, 6 postscript figures

Li1+xFePO4 (0

Poor cyclic performance of electrodes in lithium-ion rechargeable cell batteries is calling for efforts to develop continuum models of diffusion under very large stresses and high solute concentrations. The present work is aimed to develop such a model based on input from atomistic simulations. We consider four fundamental features of highly nonlinear behavior associated with diffusion at high solute concentrations. First, the effect of solute-induced stresses on the activation energy of solute diffusion could be important. Second, the solute concentration may be subject to an upper limit if there exists a stoichiometric maximum concentration. Third, the strong influence of the change in local chemical environment on the interaction energy between solute and host atoms could play a significant role. Fourth, we include the effect of the solute concentration on the Young's modulus of the host material. A continuum model is developed and validated based on atomistic simulations of hydrogen diffusion in nickel. The influences of each feature above are clearly discussed through parametric studies. (C) 2010 Elsevier B.V. All rights reserved.

We present a detailed analysis of the addition-elimination reaction pathways for the gas-phase conversion of molecular hydrogen and methane on FeO(+) to water and methanol, respectively, using first-principles calculations. These two reactions represent paradigmatic, challenging test cases for electronic structure approaches to transition-metal catalysis. We compare here density-functional approaches against state-of-the-art coupled-cluster and multireference quantum chemistry approaches. The quantum chemical approaches are found to be in close agreement between themselves as well as with the available experimental evidence. For the density-functional calculations, we employ a recently introduced ab initio, self-consistent Hubbard-like correction, coupled here with a generalized-gradient approximation (GGA) for the exchange-correlation functional. We find that our formulation provides a remarkable improvement in the description of the electronic structure, hybridization, and multiplet splittings for all calculated stationary points along these reaction pathways. The Hubbard term, which is not a fitting parameter and, in principle, can augment any exchange-correlation functional, brings the density-functional theory results in close agreement with the reference calculations. In particular, thermochemical errors as large as 1.4 eV in the exit channels with the GGA functional are reduced by an order of magnitude, to less than 0.1 eV on average; additionally, close agreement with the correlated-electron reference calculations and experiments are achieved for intermediate spin splittings and structures, reaction exothermicity, and spin crossovers. The role that the Hubbard U term plays in improving both quantitative and qualitative descriptions of transition-metal chemistry is examined, and its strengths as well as possible weaknesses are discussed in detail. (C) 2008 American Institute of Physics.