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Hole localization accompanying the formation of a cation vacancy in bulk MgO and CaO and at the (100) MgO/CaO interface is described using the Heyd-Scuseria-Ernzerhof hybrid functionals and DFT + U method. The ground state is found to be the O-1-O-1 bipolaronic configuration both in bulk oxides and at their interfaces. The ground-state magnetic configuration is a triplet, which has an energy only about 1-2 meV lower than that of the singlet state. The one-centered O-2-O-0 bipolaron was found to be metastable with its stability being enhanced at the interfaces compared to that in bulk oxides. Possible transition between different biplaronic configurations at the interface are analyzed for chosen configurations.

In the first part [D. B. Migas et al., Phys. Chem. Chem. Phys., 2014, DOI: 10.1039/C3CP54988G] by means of ab initio calculations we have analyzed and discussed anisotropy effects on electronic properties of 001 -, 100 - and 110 - oriented anatase TiO2 nanowires. In this part we present results indicating crucial changes in morphology of anatase TiO2 nanotubes originating from TiO2 nanowires by making a hole along the wire axis. The critical wall thickness has been found to exist for the nanotubes with 001 and 110 axes: at smaller thickness their shape can be rounded, squeezed, viewed as conglomerates of nanocrystals and even represented as cylindrical and `single-walled'- like structures formed without rolling up a thin titania layer into a nanotube. In general, band dispersion near the gap region of TiO2 nanotubes is close to the one of TiO2 nanowires with the same orientation. We have also revealed that optimization of the unit cell parameter along the wire axis and consideration of quantum confinement and surface state effects are important to provide an interpretation of band-gap variation with respect to wall thickness in TiO2 nanotubes., QC 20140519

By means of ab initio calculations we have revealed the existence of sizable anisotropy in electronic properties of anatase TiO2 nanowires with respect to orientation: nanowires with 001 , 100 and 110 axes are found to be direct band-gap, indirect band-gap and degenerate semiconductor materials, respectively. The degenerate semiconducting properties of 110 oriented TiO2 nanowires are predicted to be the intrinsic features closely connected with stoichiometry. A band-gap variation with nanowire diameter is also shown to display rather complex behavior characterized by a competition between quantum confinement and surface state effects that is fully compatible with the available contradictory experimental data. Finally, we propose a model to explain the band-gap variation with size in TiO2 nanowires, nanocrystals and thin films., QC 20140613

KMCLib is a general framework for lattice kinetic Monte Carlo (KMC) simulations. The program can handle simulations of the diffusion and reaction of millions of particles in one, two, or three dimensions, and is designed to be easily extended and customized by the user to allow for the development of complex custom KMC models for specific systems without having to modify the core functionality of the program. Analysis modules and on-the-fly elementary step diffusion rate calculations can be implemented as plugins following a well-defined API. The plugin modules are loosely coupled to the core KMCLib program via the Python scripting language. KMCLib is written as a Python module with a backend C++ library. After initial compilation of the backend library KMCLib is used as a Python module; input to the program is given as a Python script executed using a standard Python interpreter. We give a detailed description of the features and implementation of the code and demonstrate its scaling behavior and parallel performance with a simple one-dimensional A-B-C lattice KMC model and a more complex three-dimensional lattice KMC model of oxygen-vacancy diffusion in a fluorite structured metal oxide. KMCLib can keep track of individual particle movements and includes tools for mean square displacement analysis, and is therefore particularly well suited for studying diffusion processes at surfaces and in solids. Program summary Program title: KMCLib Catalogue identifier: AESZ_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AESZ_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: GNU General Public License, version 3 No. of lines in distributed program, including test data, etc.: 49 064 No. of bytes in distributed program, including test data, etc.: 1 575 172 Distribution format: tar.gz Programming language: Python and C++. Computer: Any computer that can run a C++ compiler and a Python interpreter. Op

The structures of the (CdS)(42), (CdSe)(42), Cd42Se32S10, Cd42Se22S20, and Cd42Se10S32 clusters have been determined using the simulated annealing method. Factors influencing the band gap value have been analysed. We show that the gap is most significantly reduced when strongly under coordinated atoms are present on the surface of the nanoclusters. In addition, the band gap depends on the S concentration as well as on the distribution of the S and Se atoms in the clusters. We present the optical absorption spectra calculated with BSE and RPA methods based on the GW corrected quasiparticle energies. Strong electron-hole coupling is observed for all the clusters, shifting the calculated RPA onset of optical absorption to lower energies. The absorption edge is shifted to higher photon energies as S concentration increases. The calculated energy separation of the first bright exciton and first dark exciton increases with S concentration.

We have analyzed by density functional theory calculations the structural and magnetic properties of Fe-Co alloys doped by carbon. In analogy with the formation of martensite in steels we predict that such a structure also forms for Fe-Co alloys in a wide range of concentrations. These alloys are predicted to have a stable tetragonal distortion, which in turn leads to an enhanced magnetocrystalline anisotropy energy of up to 0.75 MJ/m(3) and a saturated magnetization field of 1.9 T.

We show that the formation of the wetting layer and the experimentally observed continuous shift of the H2O-OH balance toward molecular water at increasing coverage on a TiO2(110) surface can be rationalized on a molecular level. The mechanism is based on the initial formation of stable hydroxyl pairs, a repulsive interaction between these pairs, and an attractive interaction with respect to water molecules. The experimental data are obtained by synchrotron radiation photoelectron spectroscopy and interpreted with the aid of density functional theory calculations and Monte Carlo simulations.

Using a microscopic phenomenological model for longitudinal spin fluctuations (LSFs) based on density functional theory calculations, we demonstrate that under the Earth's core conditions (P approximate to 360 GPa, T approximate to 6000 K), Fe acquires substantial local magnetic moment, up to 1.3 mu(B), for different crystal structure modifications. We demonstrate that the LSFs produce a substantial effect on the magnetic and thermodynamic properties of iron, in particular, its equilibrium volume under solid Earth's core conditions.

We have investigated the bonding and adhesion of the TiN(001)/fcc-Fe(111) interface with and without defects using density functional theory. Substitutions in Fe, vacancies and impurities in TiN as well as the influence of vertical stress are studied. The performed bonding analysis shows that the main interaction between Fe and TiN originated from the Fe-N bonding, regardless of the specific set-up. For the ideal TiN(001)/fcc-Fe(111) interface, a complete separation of the interface is preferred to any transfer of iron atoms to the TiN surface. The presence of Ti and N vacancies or alloying elements at the interface further promotes this trend. Application of stress to the system lowers sliding barriers along the Tin/Fe interface.

Charge redistribution at low oxygen vacancy concentrations in ceria have been studied in the framework of the density functional theory. We propose a model to approach the dilute limit using the results of supercell calculations. It allows one to reproduce the characteristic experimentally observed behavior of composition versus oxygen pressure dependency. We show that in the dilute limit the charge redistribution is likely to be driven by a mechanism different from the one involving electron localization on cerium atoms. We demonstrate that it can involve charge localization on light element impurities.

Using density-functional theory in combination with the exact muffin-tin orbital (EMTO) method and coherent potential approximation, we investigate the alloying effect on the tetragonality of Fe-C solid solution forming the basis of steels. In order to assess the accuracy of our approach, first we perform a detailed study of the performance of the EMTO method for the Fe(16)C(1) binary system by comparing the EMTO results to those obtained using the projector augmented wave method. In the second step, we introduce different substitutional alloying elements (Al, Cr, Co, Ni) into the Fe matrix and study their impact on the structural parameters. We demonstrate that a small amount of Al, Co, and Ni enhances the tetragonal lattice ratio of Fe(16)C(1) whereas Cr leaves the ratio almost unchanged. The obtained trends are correlated with the single-crystal elastic parameters calculated for carbon-free alloys.

Stainless steel is a good example of a metal that is not easily machined. To explain such behavior an understanding of the fundamental adhesion between the workpiece and the tool is invaluable. It is a well-known fact that build-up layers form in the interface, but little attention has been given to the very first layer that adheres to the tool surface. Although this layer rapidly becomes covered by successive material transfer, this layer and its ability to stick to the tool surface control the successive material transfer and influence the cutting properties. In this work, a quick stop test is employed to interrupt the cutting of a 316L stainless steel using a TiN-coated cemented carbide cutting insert. Different analytical techniques, such as transmission electron microscopy, X-ray photoelectron spectroscopy and scanning electron microscopy, as well as theoretical atomistic modeling, were used to study the early adhesion.

We performed a systematic density functional (DF) study of the adsorption of copper, silver, and gold adatoms on pristine graphene, especially accounting for van der Waals (vdW) interactions by the vdW-DF and PBE + D2 methods. In particular, we analyze the preferred adsorption site (among top, bridge, and hollow positions) together with the corresponding distortion of the graphene sheet and identify diffusion paths. Both vdW schemes show that the coinage metal atoms do bind to the graphene sheet and that in some cases the buckling of the graphene layer can be significant. Only the results for silver are qualitatively at variance with those obtained with the generalized gradient approximation, which gives no binding in this case. However in all three cases, we observe some quantitative differences between the vdW-DF and PBE + D2 methods. For instance the adsorption energies calculated with the PBE + D2 method are systematically higher than the ones obtained with vdW-DF. Moreover, the equilibrium distances computed with PBE + D2 are shorter than those calculated with the vdW-DF method.

Recent method development and increasing computer power have made ab initio methods of computational physics a robust, complimentary tool of modern materials research. Here we discuss how these methods can be used to understand some basic mechanisms determining the ion mobility in ceria-based electrolyte materials and to predict compositions with improved ion conductivity as well as to suggest new doping strategies., QC 20150603

In order to explain the experimentally found catalytic characteristics of Au1-4 /MgO (100) we have performed a comprehensive density functional study of these systems and their ability to (co)adsorb CO and O2 molecules. Starting from the carefully determined ground-state structures we have analyzed binding mechanisms, the influence of spin-orbit coupling, and charge redistributions in Au1-4 /MgO+CO (O2). Experimentally Au1,2 /MgO were found to be inactive under a mixed atmosphere. We show that O2 strongly binds to Au1 /MgO that prevents coadsorption. Although a catalytic reaction cycle towards CO oxidation, analogous to the gas phase reaction involving Au 2-, is energetically possible for Au2 /MgO, the cluster will get blocked by a strongly bound CO. On the other hand, the catalytic activity of Au3,4 /MgO could be explained by their ability to coadsorb CO and O2, hence indicating the occurrence of a Langmuir- Hinshelwood-type reaction mechanism for these clusters.

The present density functional theory study addresses the question whether the presence of H2O influences the catalytic activity of small gold clusters, Au1-4/MgO(100), towards the oxidation of carbon monoxide. To this end, we studied the (co-)adsorption of H2O and CO/O2 on these gold clusters. The ground state structures in the presence of all three molecular species, that we found, are Au1O2/MgO and Au2-4CO/MgO with H2O adsorbed on the surface in the proximity of the clusters-molecule complex. In this configuration the catalytic activity of Au1-4/MgO is indifferent to the presence of H2O. We also found that a stable, highly activated hydroperoxyl-hydroxyl complex, O2H ·· OH, can be formed on Au1,3/MgO. For the catalytic active system Au8/MgO, it has been predicted that this complex opens an alternative catalytic reaction pathway towards CO oxidation. Our results suggest that this water mediated catalytic cycle is unlikely to occur on Au1,3/MgO. In the case of Au1/MgO the cycle is interrupted by the dissociation of the remaining (OH)2 complex after forming the first CO2 molecule. On Au3 /MgO the O2H ·· OH complex is likely to decompose upon the impact of a CO molecule, since three of its bond dissociation energies are comparable to the reaction barrier of the CO to CO2 oxidation.

In the present work we developed a theoretical method to find the noncollinear canted magnetic states of ferromagnetic film whose magnetic properties are treated within a simple phenomenological model that takes into account the discrete location of atomic layers. We apply this method to the description of stable and uniform in the film plane magnetic configurations of symmetric ferromagnetic film with equal anisotropy constants at both sides of a film. We demonstrated that besides the ground noncollinear canted state caused by the competition between the energies of surface and bulk anisotropy this film also exhibits metastable noncollinear canted states that are not necessarily caused by the competition between these energies. They represent various kinds of excited Bloch wall-like states with a wall parallel to the film plane. The generalization of the method developed to the case of explicit account of dipole interaction between the nearest-neighbor layers is presented. We considered the fcc and bcc films with (001) surface and demonstrated that novel metastable states discovered in the present work are not the artifact of a simple model of a film used. To prove this statement we demonstrate that these states exist when the dipole interaction between the nearest-neighbor layers is explicitly taken into account.

By means of first principles density functional theory, we investigate the properties of the TiN(001)/fcc Fe(111) and TiN(001)/bcc Fe(110) interfaces. We demonstrate that along certain directions Fe slides with negligible energy barriers against TiN at both interfaces, whereas sliding along other directions is involved with significant energy barriers. The interface between bcc Fe and TiN has a low energy barrier for sliding along the [110] direction of the TiN lattice, as does sliding along the [010] direction at TiN(001)/fcc Fe(111). For fcc Fe on TiN, a large energy barrier is found for sliding along the [100] direction of the TiN lattice. We show that this phenomenon and the stability of these interfaces are determined by the interplay between N–Fe bonding and Ti–Fe antibonding interactions.

We propose a procedure to accurately describe the structural parameters of an incommensurate phase using ab initio methods by approximating it with a set of analogous commensurate supercells. We apply this approach to obtain the structural parameters of the Sc-II phase, which has recently been identified as a complex incommensurate structure similar to Sr-V. The calculated incommensurate ratio gamma, lattice parameters, and Wyckoff positions of Sc-II are in excellent agreement with the available experimental data. Our results show that gamma increases with pressure up to 60 GPa approaching but never reaching the commensurate value 4/3. Hence calculations do not confirm the prediction made based on the reanalyzing of experimental data. When pressure exceeds 70 GPa, gamma shows a sharp decrease that might be considered as the precursor of a new structural phase transition.

By combining high-pressure and high-temperature Moumlssbauer spectroscopic studies of (Mg,Fe)O with results of ab initio simulations, several important properties of this material were established. Under high pressure (Mg,Fe)O shows changes in the short-range order with the tendency to form iron clusters. These changes were found to be irreversible, implying sluggish kinetics of these processes at ambient conditions. The pressure-induced spin crossover is interpreted here as a gradual noncooperative transition. The onset and width of spin crossover depends, therefore, not only on pressure, temperature, and composition, but also on short-range order in the FeO-MgO solid solution.

The relative phase stability of the C- and A-type polymorphs of rare-earth sesquioxides RE2O3 (RE = La, Ce, Pr, Nd, Pm, Sm, Gd, Er, and Lu) under pressure have been studied by ab initio methods based on density-functional theory within the local-density approximation (LDA), and generalized gradient approximation. Both functionals show that the pressure of the C- to A-phase transition increases in the oxide series, although, the LDA results appear to be in better agreement with experimental data. The equation of states and pressure dependence of bulk modulus, c/a ratio and other structural parameters are analyzed. Our results indicate that the phase stability in these oxides to a large extend is determined by the electrostatic contribution to the total energy and it is related to the relaxation pattern of the structures.

The binding and electronic properties of monoatomic nanowires, dimers and bulk structures of Cu, Ag, Au and Ni, Pd, Pt have been studied by the projector augmented-wave method (PAW) within the density functional theory (DFT) using the local density approximation (LDA) as well as generalized gradient approximation (GGA) in both Perdew-Wang (PW91) and Perdew-Burke-Ernzerhof (PBE) parameterizations. Our results show that the formation of atomic chains is not equally plausible for all the studied elements. In agreement with experimental observations Pt and An stand out as most likely elements to form monoatomic wires. Changes in the electronic structure and magnetic properties of metal chains at stretching are analyzed.

The magnetic properties of a film with competing surface and bulk anisotropies and nonuniform ferromagnetic exchange interaction between atomic layers in the surface region have been investigated. The analytic expressions for the criteria of stability of the perpendicular and in-plane states of a film of arbitrary thickness have been derived in the framework of the layer by layer approach. We find that in films characterized by an arbitrary layer-dependent ferromagnetic exchange interaction between atomic layers and layer-dependent anisotropy constants, a canted noncollinear magnetic state becomes favorable when neither the perpendicular nor in-plane state is stable. We construct the diagram of the magnetic states of such a film and analyze the behavior of the magnetic state depending on film thickness. The developed theory can be applied to describe a wide class of quasi-two-dimensional magnetic systems demonstrating phase transitions between collinear and noncollinear magnetic states. In particular, we use this theory to describe the two-step spin-reorientation transition observed in bare Co/Au films.

The properties of monoatomic chains have been studied theoretically by means of statistical mechanics methods. The applied approach can be used to evaluate the interatomic distances and lifetimes of one-dimensional (1D) and quasi-1D systems. In particular, we show that the 1D clusters of gold atoms can exist in two states with different lattice parameters (similar to 3.6 and similar to 2.8 angstrom) that can explain the whole variety of experimental observations on monoatomic gold chains without assuming any wire contamination.

The results of the experimental investigation of tunneling current-voltage characteristics of Au nanoclusters pulsed laser deposited on highly oriented pyrolytic graphite are presented. It is demonstrated that the observed size dependence of the tunneling conductance of Au nanoclusters can be caused by the change of the density of electronic states at the Fermi energy due to the interaction of conduction electrons with non-regular boundary of a cluster under investigation., Conference Information: Meeting of the European-Materials-Research-Society Strasbourg, FRANCE, MAY 28-JUN 01, 2007

The results of scanning tunneling spectroscopy of the electronic states of Au nanoclusters on the graphite surface are presented. The tunneling current is found to be different at different points of a rough-surface nano-cluster. The measured differential current-voltage curve of the clusters is nonmonotonic near the Fermi energy, and the tunneling conductance decreases by almost a factor of two as the cluster volume changes from I to 0.1 mm(3). This decrease can be associated with the change in the density of the electronic states near the Fermi energy. The observed features are qualitatively described within the framework of the mechanism of electron localization in disordered systems.

We have used density functional theory calculations within the LDA+U formulation to investigate how small amounts of dissolved SiO2, GeO2, SnO2, or PbO2 affect the redox thermodynamics of ceria (CeO2). Compared to pure ceria, reduction is facilitated and the reducibility increases in the sequence of CeO2-SnO2, CeO2-GeO2, and CeO2-SiO2, which correlates with the decrease of the ionic radii of the solutes. For low solute concentrations, there is an inverse relation between high reducibility and the solution energy of tetravalent solutes. CeO2-PbO2 is unique in the sense that the initial reduction occurs by Pb(IV)double right arrow Pb(II) instead of the usual Ce(IV)double right arrow Ce(III) reaction. Among the investigated ceria compounds, CeO2-PbO2 has the lowest reduction energy and rather low solution energy. We have studied how the solution and reduction energies depend on the concentration of Si, Ge, Sn, Pb, Ti, Zr, Hf, and Th solute ions. While the solution energy increases monotonously with concentration, the reduction energy first decreases, as compared to pure ceria (except for Th, which exhibits a small increase), and with further increase of solute concentration, it either remains almost constant (Zr, Hf, and Th) or slightly increases (Ti, Si, Ge, and Sn).

The electronic structure and thermodynamic properties of CeO2 and Ce2O3 have been studied from first principles by the all-electron projector-augmented-wave (PAW) method, as implemented in the ab initio total-energy and molecular-dynamics program VASP (Vienna ab initio simulation package). The local density approximation (LDA)+U formalism has been used to account for the strong on-site Coulomb repulsion among the localized Ce 4f electrons. We discuss how the properties of CeO2 and Ce2O3 are affected by the choice of U as well as the choice of exchange-correlation potential, i.e., the local density approximation or the generalized gradient approximation. Further, reduction of CeO2, leading to formation of Ce2O3 and CeO2-x, and its dependence on U and exchange-correlation potential have been studied in detail. Our results show that by choosing an appropriate U it is possible to consistently describe structural, thermodynamic, and electronic properties of CeO2, Ce2O3, and CeO2-x, which enables modeling of redox processes involving ceria-based materials.

The authors have used density functional theory calculations to investigate how the redox thermodynamics and kinetics of CeO2 are influenced by forming solid solutions with TiO2, ZrO2, HfO2, and ThO2. Reduction is facilitated by dissolving TiO2 (largest improvement), HfO2, or ZrO2 (least improvement), while ThO2 makes reduction slightly more difficult. The migration barrier is much lower in the neighborhood of a Ti (largest decrease), Hf, or Zr (least decrease), while the binding energy of solute ions and vacancies increases in the same sequence. They rationalize the properties of ceria solid solutions in terms of defect cluster relaxations.

The face-centered-cubic (fcc) Lennard-Jones (LJ) model can be considered as a representative model of a simple solid. We investigate the mechanism of melting at the limit of superheating in the fcc LJ solid by means of the procedure recently developed by us [Phys. Rev. B 73, 012201 (2006)]. Insight into the mechanism of melting was gained by studying diffusion and defects in the fcc LJ solid by means of molecular dynamics simulations. We found that the limit of superheating achieved by us is likely to be the highest so far. We also found that the size of the cluster which ignites the melting is very small (down to five to six atoms, depending on the size of the supercell) and closely correlates with the linear size of a supercell when the number of atoms varies between 500 and 13 500.

Electronic, bonding, and structural properties of monatomic gold wires containing impurities of atomic hydrogen and carbon have been studied by means of the projector augmented-wave method within the generalized gradient approximation. We find that light impurities cause significant charge redistribution in monatomic gold wires altering bonding and affecting the wire structural properties. Light impurities can drastically promote the parity oscillations of conductance occurring already in pure gold wires, thus demonstrating how the electronic and structural characteristics of nanowires can be tuned by appropriate doping.

The exact muffin-tin orbitals (EMTO) technique in conjunction with the coherent-potential approximation (CPA) as well as the projector-augmented-wave (PAW) method have been used to calculate the surface segregation energy of Cr at the (100) surface of Fe-rich bcc Fe-Cr alloys. We find that PAW results strongly depend on the supercell size used in the calculations. In particular, for large supercells, the surface segregation energy of Cr is positive, which means that Cr should not segregate toward the surface of diluted alloys. This is in agreement with our EMTO-CPA results as well as previous surface Green's-function calculations. However, the surface segregation energy of Cr is negative if small unit cells are used for simulations. This is in agreement with previous full-potential supercell calculations. We explain such a size dependence by a peculiar concentration dependence of interatomic interactions in ferromagnetic Fe-Cr alloys.

The conductance of monoatomic gold wires containing 3-7 gold atoms has been obtained from ab initio calculations. The transmission is found to vary significantly depending on the wire stretching and the number of incorporated atoms. Such oscillations are determined by the electronic structure of the one-dimensional (1D) part of the wire between the contacts. Our results indicate that the conductivity of 1D wires can be suppressed without breaking the contact.

It is well established that at a pressure of several megabars and low temperature Fe is stable in the hexagonal-close-packed (hcp) phase. However, there are indications that on heating a high-pressure hcp phase of Fe transforms to a less dense (open structure) phase. Two phases have been suggested as candidates for these high-temperature stable phases: namely, body-centered-cubic and body-centered-tetragonal (bct) phases. We performed first-principles molecular dynamics and phonon analysis of the bct Fe phase and demonstrated its dynamical instability. This allows us to dismiss the existence of the bct Fe phase under the high-pressure high-temperature conditions of the Earth's inner core.

The experimental investigation of the cubic -Bi2O3 phase grown on a (110) Au substrate at low temperature has disclosed a chaotic character of the conductivity at low voltage and temperature. Based on first-principles calculations, we show that the conductivity of this oxide strongly depends on the distribution of oxygen ions and that oxygen migration is able to cause a momentary switch of the conduction mechanism.

Electronic and structural properties of the (100) surfaces and bulk of MgO, CaO, SrO, and BaO have been studied using the projector augmented wave (PAW) method within the local density approximation and generalized gradient approximation. Relaxed (100) MgO shows the positive rumpling of 2.2% whereas (100) CaO, (100) SrO, and (100) BaO exhibit increasingly negative rumpling and surface contraction. (100) CaO is a border case between the two trends and the sign of the rumpling of this surface can be changed at distortion. We find that the surface structures obtained as a result of relaxation are largely determined by an increasing interaction between the cation semicore states and oxygen p states observed in the oxide series.