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© IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. The Madrid–Vienna cooperation has been initiated by a Marie-Curie Fellowship for J I Beltrán in the MC-Training Site ‘Atomic-scale computational materials science’ at the Universität Wien. Partial support by the Spanish Ministerio de Educación y Ciencia under contract no MAT2006-05122 is gratefully acknowledged., We present ab initio density-functional theory (DFT) calculations of the structure and stability of the monoclinic (m), tetragonal (t) and cubic (c) phases of HfO_(2) and of the stability and the structural, electronic, and magnetic properties of the polar (001) surface of t-HfO_(2) and the (100) and (111) surfaces of c-HfO_(2). We show that on all three surfaces, a termination by Hf leads to a metallic and non-magnetic surface, while surfaces covered by a full monolayer of O are predicted to be half-metallic and ferromagnetic, the magnetisms being induced by the Coulomb repulsion between p-holes in the O-2p valence band. In contrast, the partially reduced surfaces terminated by half a monolayer of oxygen are found to be insulating and non-magnetic. Ab initio statistical mechanics in combination with the DFT total-energy calculations show that the partially reduced surfaces are stable over the entire range of admissible values of the chemical potential of oxygen. Investigations of the formation of Hf vacancies on the Hf- and O-terminated surfaces of tetragonal HfO_(2) demonstrate that under oxidizing conditions, the formation of Hf subsurface vacancies is energetically favored on the partially reduced O-terminated surface. The formation of Hf vacancies causes the creation of holes in the O-2p valence band and of magnetic moments on the surrounding O atoms. That the formation of near-surface Hf vacancies on the O-terminated surface is energetically favored is in contrast to a high formation energy for neutral Hf vacancies in bulk HfO2 and suggests a cooperative mechanism between surface- and vacancy-formation. We discuss our findings in relation to recent reports on ferromagnetism in ultrathin HfO_(2) films and other models for the formation of p-wave ferromagnetism., Spanish Ministerio de Educación y Ciencia, Depto. de Estructura de la Materia, Física Térmica y Electrónica, Fac. de Ciencias Físicas, TRUE, pub

© Te Author(s) 2018. Tis work was supported by MINECO/FEDER (Projects No. MAT 2015-65274-R and MAT2016- 81720-REDC), NILS Project (008-ABELCM-2013), Notur Project No. nn4608k, and HyMatSiRen No. project272806 from the Research Council of Norway., The effects of Cr on local environment and electronic structure of rutile TiO₂ are studied combining theoretical and experimental approaches. Neutral and negatively charged substitutional Cr impurities Cr_(Ti)(0)* and Cr_(Ti)(-1)* as well as Cr-oxygen vacancy complex 2Cr_(Ti) + V₀ are studied by the density functional theory (DFT) within the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional. Experimental results based on X-Ray absorption spectroscopy (XAS) and X-Ray photoelectron spectroscopy (XPS) performed on Cr doped TiO₂ at the Synchrotron facility were compared to the theoretical results. It is shown that the electrons of the oxygen vacancy tend to be localized at the t_(2g) states of the Cr ions in order to reach the stable oxidation state of Cr(3+)*. Effects of Cr on crystal field (CF) and structural distortions in the rutile TiO₂ cell were analyzed by the DFT calculations and XAS spectra revealing that the CF and tetragonal distortions in TiO₂ are very sensitive to the concentration of Cr., Ministerio de Economía y Competitividad (MINECO)/FEDER, NILS Project, Notur Project, HyMatSiRen from the Research Council of Norway, Depto. de Física de Materiales, Fac. de Ciencias Físicas, TRUE, pub

Biomass refers to plant-based materials that are not used for food or feed. As an energy source, lignocellulosic biomass (lignin, cellulose and hemicellulose) can be converted into various forms of biofuel using thermal, chemical and biochemical methods. Chemical conversion implies the use of solid catalysts, usually oxide materials. In this context, reducible oxides are considered to be more active than non-reducible oxides. But why? Using density functional theory DFT + U calculations with the inclusion of dispersion forces, we describe the properties of anatase Ti , a reducible oxide, and tetragonal ZrO2, a non-reducible oxide, the (101) surfaces in this context. In particular, we focus on the role of surface reduction, either by direct creation of oxygen vacancies via O-2 desorption, or by treatment in hydrogen. We show that the presence of reduced centres on the surface of titania or zirconia (either Ti3+ or Zr3+ ions, or oxygen vacancies) results in lower barriers and more stable intermediates in two key reactions in biomass catalytic conversion: ketonization of acetic acid (studied on ZrO2) and deoxygenation of phenol (studied on TiO2). We discuss the role of Ru nanoparticles in these processes, and in particular in favouring H-2 dissociation and hydrogen spillover, which results in hydroxylated surfaces. We suggest that H2O desorption from the hydroxylated surfaces may be a relevant mechanism for the regeneration of oxygen vacancies, in particular on low-coordinated sites of oxide nanoparticles. Finally, we discuss the role of nanostructuring in favouring oxide reduction, by discussing the properties of ZrO2 nanoparticles of diameter of about 2 nm. This article is part of a discussion meeting issue 'Providing sustainable catalytic solutions for a rapidly changing world'

A long-standing challenge facing the combination of two-dimensional crystals into heterojunction is the unknown effect of mixing layer of different electronic properties (semiconductors, metals, insulators) on the screening features of the fabricated device platforms including their functionality. Here we use a compelling set of theoretical and experimental techniques to elucidate the intrinsic dielectric screening properties of heterostructures formed by MoS2 and graphene layers. We experimentally observed an asymmetric field screening effect relative to the polarization of the applied gate bias into the surface. Surprisingly, such behavior allows selection of the electronic states that screen external fields, and it can be enhanced with increasing of the number of layers of the semiconducting MoS2 rather than the semi-metal. This work not only provides unique insights on the screening properties of a vast amount of heterojunction fabricated so far, but also uncovers the great potential of controlling a fundamental property for device applications.

Biomass refers to plant-based materials that are not used for food or feed. As an energy source, lignocellulosic biomass (lignin, cellulose and hemicellulose) can be converted into various forms of biofuel using thermal, chemical and biochemical methods. Chemical conversion implies the use of solid catalysts, usually oxide materials. In this context, reducible oxides are considered to be more active than non-reducible oxides. But why? Using density functional theory DFT + U calculations with the inclusion of dispersion forces, we describe the properties of anatase Ti , a reducible oxide, and tetragonal ZrO2, a non-reducible oxide, the (101) surfaces in this context. In particular, we focus on the role of surface reduction, either by direct creation of oxygen vacancies via O-2 desorption, or by treatment in hydrogen. We show that the presence of reduced centres on the surface of titania or zirconia (either Ti3+ or Zr3+ ions, or oxygen vacancies) results in lower barriers and more stable intermediates in two key reactions in biomass catalytic conversion: ketonization of acetic acid (studied on ZrO2) and deoxygenation of phenol (studied on TiO2). We discuss the role of Ru nanoparticles in these processes, and in particular in favouring H-2 dissociation and hydrogen spillover, which results in hydroxylated surfaces. We suggest that H2O desorption from the hydroxylated surfaces may be a relevant mechanism for the regeneration of oxygen vacancies, in particular on low-coordinated sites of oxide nanoparticles. Finally, we discuss the role of nanostructuring in favouring oxide reduction, by discussing the properties of ZrO2 nanoparticles of diameter of about 2 nm. This article is part of a discussion meeting issue 'Providing sustainable catalytic solutions for a rapidly changing world'

Molecular metals have been essentially obtained with tetrathiafulvalene (TTF)-based precursors, either with multicomponent ionic materials or, in a few instances, with single-component systems. In that respect, gold bis- (dithiolene) complexes, in their neutral radical state, provide a prototype platform toward such single-component conductors. Herein we report the first single-component molecular metal under ambient pressure derived from such Au complexes without any TTF backbone. This complex exhibits a conductivity of 750 S·cm−1 at 300 K up to 3800 S·cm−1 at 4 K. First-principles electronic structure calculations show that the striking stability of the metallic state finds its origin in sizable internal electron transfer from the SOMO-1 to the SOMO of the complex as well as in substantial interstack and interlayer interactions.

Atomically thin boron nitride (BN) nanosheets are important two-dimensional nanomaterials with many unique properties distinct from those of graphene, but investigation into their mechanical properties remains incomplete. Here we report that high-quality single-crystalline mono- and few-layer BN nanosheets are one of the strongest electrically insulating materials. More intriguingly, few-layer BN shows mechanical behaviours quite different from those of few-layer graphene under indentation. In striking contrast to graphene, whose strength decreases by more than 30% when the number of layers increases from 1 to 8, the mechanical strength of BN nanosheets is not sensitive to increasing thickness. We attribute this difference to the distinct interlayer interactions and hence sliding tendencies in these two materials under indentation. The significantly better interlayer integrity of BN nanosheets makes them a more attractive candidate than graphene for several applications, for example, as mechanical reinforcements.

The adhesion of gold on three oxide surfaces (magnesia, titania, and ceria) is studied by means of dispersion-corrected DFT+U calculations, considering in a systematic approach an isolated Au atom, Au-20 clusters, and periodic extended interfaces to model large nanoparticles. The results show that for a Au-1 monomer the adhesion energy on the three oxides is similar: 0.83 eV (TiO2), 0.99 eV (CeO2), and 1.09 eV (MgO). The picture is more complex for nanoclusters and extended interfaces, where morphological factors largely determine the binding capability of these oxides toward Au. For Au-20, the adhesion on rutile TiO2 is smaller and dominated by long-range dispersion contributions, while the better match between Au atoms and surface oxygen anions leads to a larger binding on magnesia and ceria. Overall, a CeO2 > MgO > TiO2 trend is observed. For the extended interfaces, the trend is CeO2 approximate to MgO > TiO2. Notice that the adhesion energies of a 20-atom cluster are 2-3 times larger than those of the extended interfaces because of (a) the structural flexibility of nanoclusters and (b) the presence of several undercoordinated Au atoms at the cluster border in contact with the oxide surface. While for monomers dispersion contributions are of the order of 20% of the total adsorption energy, for clusters and nanoparticles they represent an important and sometimes dominant contribution to the adhesion energy. The picture is radically altered if the oxide supports are not perfectly stoichiometric. The presence of oxygen vacancies enhances the gold adhesion energy, an effect that may render a direct comparison of DFT results with experimental estimates of adhesion energies more complex due to the difficulty to fully control defects concentration at the oxide surfaces

The adsorption and diffusion of ruthenium adatoms on the (101) surface of tetragonal zirconia was studied by means of periodic Density Functional Theory (PBE+U) calculations. The surface termination has a decisive role in determining the diffusion capability of the adsorbed Ru atoms. On the defect-free and fully dehydroxylated surface, Ru adatoms have several stable adsorption sites with adsorption energies as large as 2.5–2.9 eV However, the kinetic diffusion barriers between adjacent adsorption sites are around 0.5–0.6 eV, indicating a rather fast diffusion process. Surface oxygen vacancies, if present, strongly bind ruthenium adatoms and act as nucleation sites. On hydroxylated surfaces, the adsorption energy of Ru atoms is comparable to the dehydroxylated case, but the kinetic barriers for diffusion are remarkably higher, thus indicating that adsorbed species are less mobile in presence of surface OH groups. The effect is more pronounced for high concentrations of OH groups, since this results in hydrogen bonded hydroxyl units that further limit the diffusion process. These results indicate a possible way to increase the life-time of Ru[sbnd]ZrO2 heterogeneous catalysts by tuning the level of surface hydroxylation, in order to slow down sintering of metal particles via Ostwald ripening process.

Compresed BiTeBr has been studied from a joint experimental and theoretical perspective. Room-temperature x-ray diffraction, Raman scattering, and transport measurements at high pressures have been performed in this layered semiconductor and interpreted with the help of ab initio calculations. A reversible first-order phase transition has been observed above 6-7 GPa, but changes in structural, vibrational, and electrical properties have also been noted near 2 GPa. Structural and vibrational changes are likely due to the hardening of interlayer forces rather than to a second-order isostructural phase transition while electrical changes are mainly attributed to changes in the electron mobility. The possibility of a pressure-induced electronic topological transition and of a pressure-induced quantum topological phase transition in BiTeBr and other bismuth tellurohalides, like BiTeI, is also discussed., BiTeBr za vysokých tlaků byl studován jak z hlediska experimentálního, tak teoretického. Experimentální výsledky rentgenové difrakce, Ramanova rozptylu a transportních vlastností při pokojové teplotě byly interpretovány v rámci ab-inicio výpočtů. Reversibilní přechod prvního řádu byl pozorován při 6-7 GPa, ale změny strukturních vibračních a elektrických vlastností byly pozorovány už při 2 GPa. Strukturní a vibrační změny jsou pravděpodobně spíš způsobeny zpevňováním vazby mezi vrstvami, než isostrukturním fázovým přechodem 2. řádu. Změny elektrických vlastností je možné vysvětlit především změnou elektronové mobility. Diskutujeme rovněž možnosti tlakem vyvolaného elektronového topologického přechodu a kvantového topologického přechodu v BiTeBr a dalších tellurohalidech bismutu.

©2016 American Physical Society. This paper has been supported by the Norway, Iceland, Liechtenstein Science and Sustainability (NILS)/EEA Grants “Sustainable oxide materials and nanostructures for energy related applications (SUSOX)” (Project No. 008-ABELCM- 2013), Norwegian Metacenter for Computational Science (Notur) Project No. nn4608k, Ministry of Economy, Industry and Competitiveness (MINECO)/European Regional Development Fund (FEDER) Project No. MAT2015-65274-R, and Consolider Ingenio Project No. CSD 2009-00013. The authors thank Dr. A. Pishtshev for practical help, Dr. J. RamírezCastellanos for kindly providing the TiO_2 nanoparticles, and the BACH beamline staff for useful advice on XPS and XAS measurements at the Elettra Synchrotron in Trieste. The work by S.Z.K. has been supported by New Indigo Project No. 237643/E20., The effects of doubly ionized oxygen vacancies [(V_O)ˆ(2+)]on the electronic structure and charge distribution in rutile TiO_2 are studied by combining first-principles calculations based on density functional theory and experimental results from x-ray photoelectron and x-ray absorption measurements carried out in synchrotron facilities on rutile TiO_2 nanoparticles. The generalized gradient approximation of the Perdew-Burke-Ernzerhof functional has demonstrated its suitability for the analysis of the [(V_O)ˆ(2+)]defects in rutile TiO_2. It has been found that the presence of empty electronic states at the conduction band shifted ̴1 eV from t_(2g) and e_(g) states can be associated with local distortions induced by [(V_O)ˆ(2+)]defects, in good agreement with Gauss-Lorentzian band deconvolution of experimental O K-edge spectra. The asymmetry of t(2g) and e(g) bands at the O-K edge has been associated with [(V_O)ˆ(2+)], which can enrich the understanding of studies where the presence of these defects plays a key role, as in the case of doped TiO_2., Unión Europea. FP7, Ministerio de Economía y Competitividad (MINECO), European Regional Development Fund (FEDER), Norway, Iceland, Liechtenstein Science and Sustainability (NILS)/EEA Grants "Sustainable oxide materials and nanostructures for energy related applications (SUSOX)", Norwegian Metacenter for Computational Science (Notur), Consolider Ingenio Project, New Indigo Project, Depto. de Física de Materiales, Fac. de Ciencias Físicas, TRUE, pub

The diffusion properties of a wide range of impurities (transition metals and Al, Si, and P) in ferritic alloys are here investigated by means of a combined ab initio-atomic diffusion theory approach. The flux-coupling mechanisms and the solute-diffusion coefficients are inferred from electronic-structure calculations of solute-defect interactions and microscopic jump frequencies. All properties except the second-nearest-neighbor binding energy are found to have a characteristic bell shape as a function of the d-band filling for the 4d and 5d series, and an M shape for the 3d row because of the out-of-trend behavior of Mn. The solute jump frequencies are governed by compressibility, which makes diffusion of large solutes faster, although this effect is partially compensated for by lower attempt frequencies and larger correlations with the vacancy. Diffusion coefficients are predicted in a wide temperature range, far below the experimentally accessible temperatures. In accordance with experiments, Co is found to be a slow diffuser in iron, and the same behavior is predicted for Re, Os, and Ir impurities. Finally, flux-coupling phenomena depend on the iron jump frequencies next to a solute atom, which are mainly controlled by similar electronic interactions to those determining the binding energies. Vacancy drag and solute enrichment at sinks systematically arise below a solute-dependent temperature threshold, directly correlated with the electronic-level interactions at the equilibrium and the saddle-point states. Early transition metals with repulsive second-nearest-neighbor interactions also diffuse via vacancy drag, although they show a lower temperature threshold than the late metals. This confirms that drag is the most common solute-vacancy coupling mechanism in iron at low temperatures, and this is likely to be confirmed as well for impurity diffusion in other transition metals., QC 20160530

We have demonstrated a nonlinear behavior for the bandgap opening of doped graphene by controlling the concentration of B and N co-dopants. X-ray absorption and emission spectra reveal that the bandgap increases from 0 to 0.6 eV as the concentration of BN dopants is increased from 0 to 6%, while the bandgap closes when the doping concentration becomes 56%. This nonlinear behavior of bandgap opening of the BN-doped graphene depending on the BN concentrations is consistent with the valenceband photoemission spectroscopic measurements. The spatially resolved B, N and C K-edge scanning transmission x-ray microscopy and their x-ray absorption near- edge structure spectra all support the scenario of the development of h-BN-like domains at high concentrations of BN. Ab initio calculation, by taking into account of the strong correlation between the bandgap and the geometry/concentration of the dopant, has been performed with various BN-dopant nano-domains embedded in the graphene monolayer to verify the unique bandgap behavior. Based on the experimental measurements and ab initio calculation, we propose the progressive formation of a phase-separated zigzag-edged BN domain from BN quantum dots with increasing BN-dopant concentration to explain the extraordinary nonlinear behavior of bandgap opening of BN-doped graphene sheets. This study reveals a new way to engineer the bandgap of low-dimensional systems., QC 20160913

Based on the ab initio calculations, we show that MoTe2, in its low-temperature orthorhombic structure characterized by an x-ray diffraction study at 100 K, realizes 4 type-II Weyl points between the Nth and (N±1)th bands, where N is the total number of valence electrons per unit cell. Other WPs and nodal lines between different other bands also appear close to the Fermi level due to a complex topological band structure. We predict a series of strain-driven topological phase transitions in this compound, opening a wide range of possible experimental realizations of different topological semimetal phases. Crucially, with no strain, the number of observable surface Fermi arcs in this material is 2 - the smallest number of arcs consistent with time-reversal symmetry. © 2016 American Physical Society.

A kind of topological material, the type-II Weyl semimetal, was proposed recently where the Weyl points emerge at the contact points of the electron and hole pockets, resulting in a highly tilted Weyl cone. In type-II Weyl semimetals, the Lorentz invariance is violated and a different type of Weyl fermion is generated that leads to intriguing physical properties. WTe2 is interesting because it is predicted to be a good candidate for realizing type-II Weyl semimetals. By utilizing laser-based angle-resolved photoemission spectroscopy with high energy and momentum resolutions, we have revealed a full picture of the electronic structure of WTe2. A clear surface state has been identified and its connection with the bulk electronic states in the momentum and energy space shows good agreement with band structure calculations. Our results provide electronic signatures that are consistent with type-II Weyl states in WTe2. They lay a foundation for further investigations on the topological nature of WTe2 and the exploration of unique phenomena and physical properties in type-II Weyl semimetals. © 2016 American Physical Society.

A kind of topological material, the type-II Weyl semimetal, was proposed recently where the Weyl points emerge at the contact points of the electron and hole pockets, resulting in a highly tilted Weyl cone. In type-II Weyl semimetals, the Lorentz invariance is violated and a different type of Weyl fermion is generated that leads to intriguing physical properties. WTe2 is interesting because it is predicted to be a good candidate for realizing type-II Weyl semimetals. By utilizing laser-based angle-resolved photoemission spectroscopy with high energy and momentum resolutions, we have revealed a full picture of the electronic structure of WTe2. A clear surface state has been identified and its connection with the bulk electronic states in the momentum and energy space shows good agreement with band structure calculations. Our results provide electronic signatures that are consistent with type-II Weyl states in WTe2. They lay a foundation for further investigations on the topological nature of WTe2 and the exploration of unique phenomena and physical properties in type-II Weyl semimetals. © 2016 American Physical Society.

Pure graphene is known as the strongest material ever discovered. However, the unavoidable defect formation in the fabrication process renders the strength of defective graphene much lower (~14%) than that of its perfect counterpart. By means of density functional theory computations, we systematically explored the effect of gas molecules (H2, N2, NH3, CO, CO2 and O2) adsorption on the mechanical strength of perfect/defective graphene. The NH3 molecule is found to play a dominant role in enhancing the strength of defective graphene by up to ~15.6%, while other gas molecules decrease the strength of graphene with varying degrees. The remarkable strength enhancement can be interpreted by the decomposition of NH3, which saturates the dangling bond and leads to charge redistribution at the defect site. The present work provides basic information for the mechanical failure of gas-adsorbed graphene and guidance for manufacturing graphene-based electromechanical devices.

Two-dimensional (2D) materials can be produced using ball milling with the help of liquid surfactants or solid exfoliation agents, as ball milling of bulk precursor materials usually produces nanosized particles because of high-energy impacts. Post-milling treatment is thus needed to purify the nanosheets. We show here that nanosheets of graphene, BN, and MoS2 can be produced by ball milling of their bulk crystals in the presence of ammonia or a hydrocarbon ethylene gas and the obtained nanosheets remain flat and maintain their single-crystalline structure with low defects density even after a long period of time; post-milling treatment is not needed. This study does not just demonstrate production of nanosheets using ball milling, but reveals surprising indestructible behaviour of 2D nanomaterials in ammonia or hydrocarbon gas under the high-energy impacts; in other milling atmospheres such as air, nitrogen or argon the same milling treatment produces nanosized particles. A systematic study reveals chemisorption of ammonia and hydrocarbon gases and chemical reactions occurring at defect sites, which heal the defects by saturating the dangling bonds. Density functional theory was used to understand the mechanism of mechanochemical reactions. Ball milling in ammonia or hydrocarbon is promising for mass-production of pure nanosheets.

A kind of topological material, the type-II Weyl semimetal, was proposed recently where the Weyl points emerge at the contact points of the electron and hole pockets, resulting in a highly tilted Weyl cone. In type-II Weyl semimetals, the Lorentz invariance is violated and a different type of Weyl fermion is generated that leads to intriguing physical properties. WTe2 is interesting because it is predicted to be a good candidate for realizing type-II Weyl semimetals. By utilizing laser-based angle-resolved photoemission spectroscopy with high energy and momentum resolutions, we have revealed a full picture of the electronic structure of WTe2. A clear surface state has been identified and its connection with the bulk electronic states in the momentum and energy space shows good agreement with band structure calculations. Our results provide electronic signatures that are consistent with type-II Weyl states in WTe2. They lay a foundation for further investigations on the topological nature of WTe2 and the exploration of unique phenomena and physical properties in type-II Weyl semimetals. © 2016 American Physical Society.

Based on the ab initio calculations, we show that MoTe2, in its low-temperature orthorhombic structure characterized by an x-ray diffraction study at 100 K, realizes 4 type-II Weyl points between the Nth and (N±1)th bands, where N is the total number of valence electrons per unit cell. Other WPs and nodal lines between different other bands also appear close to the Fermi level due to a complex topological band structure. We predict a series of strain-driven topological phase transitions in this compound, opening a wide range of possible experimental realizations of different topological semimetal phases. Crucially, with no strain, the number of observable surface Fermi arcs in this material is 2 - the smallest number of arcs consistent with time-reversal symmetry. © 2016 American Physical Society.

Strain effects on epitaxial thin films of LaNiO3 grown on different single crystalline substrates are studied by Raman scattering and first-principles simulation. New Raman modes, not present in bulk or fully-relaxed films, appear under both compressive and tensile strains, indicating symmetry reductions. Interestingly, the Raman spectra and the underlying crystal symmetry for tensile and compressively strained films are different. Extensive mapping of LaNiO3 phase stability is addressed by simulations, showing that a variety of crystalline phases are indeed stabilized under strain which may impact the electronic orbital hierarchy. The calculated Raman frequencies reproduce the principal features of the experimental spectra, supporting the validity of the multiple strain-driven structural transitions predicted by the simulations.

Removal of Mg from hydrated Ni sulfates has long been a problem in the industrial purification process of hydrated Ni sulfates. In this work, we have investigated this industrial problem using state-of-the-art molecular simulations. Periodic Density Functional Theory (DFT) and cluster DFT calculations are used to study the crystal structures and phase stability of the hexahydrated and heptahydrated Ni and Mg sulfates and their mixed phases. The calculated lattice parameters of MSO4(H2O)(n) (M=Ni, Mg; n=6, 7) crystals are in good agreement with available experimental data. The relative energy differences of the mixed phase for both hexahydrated and heptahydrated Ni/Mg sulfates obtained from both the periodic and cluster DFT calculations are generally less than kT (25.8 meV, T=300 K), indicating that a continuous solid solution is formed. We also investigated the Bader charges and electronic structures of the hexahydrated and heptahydrated Ni/Mg sulfates using the periodic DFT calculations. The energy band gaps of the hexahydrated and heptahydrated Ni and Mg sulfates were predicted by first-principles calculations. Large energy band gaps of about similar to 5.5 eV were obtained from the DFT-GGA calculations for hydrated Mg sulfates, and band gaps of about similar to 5.1 eV were obtained by the DFT-GGA+U calculations for hydrated Ni sulfates. (C) 2014 Elsevier Ltd. All rights reserved.

Whereas bulk zinc oxide (ZnO) exhibits the wurtzite crystal structure, nanoscale ZnO was recently synthesized in the rock salt structure by addition of Mg. Using first-principles methods, we investigated two stabilization routes for accessing rock salt ZnO. The first route is stabilization by Mg addition, which was investigated by considering ZnO-MgO mixed phases. The second route is through size effects, as surface energies become dominant for small nanocrystal sizes. We discovered that the surface energy of rock salt ZnO is surprisingly low at 0.63 J m-2, which is lower than those of wurtzite and zinc blende ZnO and lower than that of rock salt MgO. We predict that pure rock salt ZnO is stable for nanocrystals smaller than 1.6 nm, and that Mg additions can greatly extend the size range in which the rock salt phase is stable. Both mixed-phase and core-shell models were considered in the calculations. The present approach could be applied to predict the stabilization of many other nanocrystal phases in deviating crystal structures.

The adsorption and dissociation of molecular O-2 on copper clusters of varying size and morphology (Cur with n = 3-8, 13, and 38 atoms) has been systematically investigated using several DFT approaches. Different modes of adsorption of molecular O-2 are found for each atomicity of the copper clusters, with their relative stability depending on the cluster morphology: bridge conformations are stabilized on planar clusters, while hollow h-100 and h-111 modes are preferentially formed on 3D clusters. O-2 adsorption energies correlate with the HOMO energy of the isolated copper clusters but not with their atomicity. On the other hand, the degree of activation of the O-O bond, and therefore the activation energy barrier necessary to break it, depends on the charge transferred to the lc* MO of O-2, which in turn is determined by the mode of adsorption of O-2 on the metal. Cluster morphology appears then as the key factor determining the catalytic activity of copper, since the most activating h-111 and h-100 adsorption modes leading to lower barriers are preferentially stabilized on 3D clusters, no matter their size. All computational levels considered, including periodic and molecular calculations, lead to similar trends and conclusions.

The phase relations and thermodynamic properties of the condensed Al-Co-Cr ternary alloy system are investigated using first-principles calculations based on density functional theory (DFT) and phase-equilibria experiments that led to X-ray diffraction (XRD) and electron probe micro-analysis (EPMA) measurements. A thermodynamic description is developed by means of the calculations of phase diagrams (CALPHAD) method using experimental and computational data from the present work and the literature. Emphasis is placed on modeling the bcc-A2, B2, fcc-gamma, and tetragonal-sigma phases in the temperature range of 1173 to 1623 K. Liquid, bcc-A2 and fcc-gamma phases are modeled using substitutional solution descriptions. First-principles special quasirandom structures (SQS) calculations predict a large bcc-A2 (disordered)/B2 (ordered) miscibility gap, in agreement with experiments. A partitioning model is then used for the A2/B2 phase to effectively describe the order-disorder transitions. The critically assessed thermodynamic description describes all phase equilibria data well. A2/B2 transitions are also shown to agree well with previous experimental findings., QC 20150612

The article presents, using Bi doped ZnO, an example of a heavy ion doped oxide semiconductor, highlighting a novel p-symmetry interaction of the electronic states to stabilize ferromagnetism. The study includes both ab initio theory and experiments, which yield clear evidence for above room temperature ferromagnetism. ZnBixO1-x thin films are grown using the pulsed laser deposition technique. The room temperature ferromagnetism finds its origin in the holes introduced by the Bi doping and the p-p coupling between Bi and the host atoms. A sizeable magnetic moment is measured by means of x-ray magnetic circular dichroism at the O K-edge, probing directly the spin polarization of the O(2p) states. This result is in agreement with the theoretical predictions and inductive magnetometry measurements. Ab initio calculations of the electronic and magnetic structure of ZnBixO1-x at various doping levels allow to trace the origin of the ferromagnetic character of this material. It appears, that the spin-orbit energy of the heavy ion Bi stabilizes the ferromagnetic phase. Thus, ZnBixO1-x doped with a heavy non-ferromagnetic element, such as Bi, is a credible example of a candidate material for a new class of compounds for spintronics applications, based on the spin polarization of the p states., QC 20151216

In the present paper, the Gutzwiller density functional theory (LDA+G) has been applied to study the bilayer system of LaCoO3 grown along the (111) direction on SrTiO3. The LDA calculations show that there are two nearly flat bands located at the top and bottom of eg bands of Co atoms with the Fermi level crossing the lower one. After including both the spin-orbit coupling and the Coulomb interaction in the LDA+G method, we find that the interplay between spin-orbit coupling and Coulomb interaction stabilizes a very robust ferromagnetic insulator phase with the nonzero Chern number indicating the possibility of realizing the quantum anomalous Hall effect in this system. © 2015 American Physical Society.

The adsorption and dissociation of molecular O-2 on copper clusters of varying size and morphology (Cur with n = 3-8, 13, and 38 atoms) has been systematically investigated using several DFT approaches. Different modes of adsorption of molecular O-2 are found for each atomicity of the copper clusters, with their relative stability depending on the cluster morphology: bridge conformations are stabilized on planar clusters, while hollow h-100 and h-111 modes are preferentially formed on 3D clusters. O-2 adsorption energies correlate with the HOMO energy of the isolated copper clusters but not with their atomicity. On the other hand, the degree of activation of the O-O bond, and therefore the activation energy barrier necessary to break it, depends on the charge transferred to the lc* MO of O-2, which in turn is determined by the mode of adsorption of O-2 on the metal. Cluster morphology appears then as the key factor determining the catalytic activity of copper, since the most activating h-111 and h-100 adsorption modes leading to lower barriers are preferentially stabilized on 3D clusters, no matter their size. All computational levels considered, including periodic and molecular calculations, lead to similar trends and conclusions.

[EN] The preferential crystallization of pure silica ITQ-39 and MFI zeolites using SDAEt and SDAPr, respectively, as organic structure directing agents (SDAs) is investigated here by theoretical methods using a periodic DFT-D model. For this purpose, the templating roles of SDAEt and SDAPr have been analyzed following a systematic study of the interaction of these two SDAs with the previously described zeolites through SDA stabilization energies (balance of intermolecular SDA dispersive interactions and SDA strain). From theoretical studies, it has been found that SDAEt located inside ITQ-39 zeolite and SDAPr placed within MFI show the largest stabilizations. These theoretical results agree with previous experimental observations for the preferential crystallization of pure silica ITQ-39 and MFI using SDAEt and SDAPr molecules, respectively.

[EN] The preferential crystallization of pure silica ITQ-39 and MFI zeolites using SDAEt and SDAPr, respectively, as organic structure directing agents (SDAs) is investigated here by theoretical methods using a periodic DFT-D model. For this purpose, the templating roles of SDAEt and SDAPr have been analyzed following a systematic study of the interaction of these two SDAs with the previously described zeolites through SDA stabilization energies (balance of intermolecular SDA dispersive interactions and SDA strain). From theoretical studies, it has been found that SDAEt located inside ITQ-39 zeolite and SDAPr placed within MFI show the largest stabilizations. These theoretical results agree with previous experimental observations for the preferential crystallization of pure silica ITQ-39 and MFI using SDAEt and SDAPr molecules, respectively.

In the present paper, the Gutzwiller density functional theory (LDA+G) has been applied to study the bilayer system of LaCoO3 grown along the (111) direction on SrTiO3. The LDA calculations show that there are two nearly flat bands located at the top and bottom of eg bands of Co atoms with the Fermi level crossing the lower one. After including both the spin-orbit coupling and the Coulomb interaction in the LDA+G method, we find that the interplay between spin-orbit coupling and Coulomb interaction stabilizes a very robust ferromagnetic insulator phase with the nonzero Chern number indicating the possibility of realizing the quantum anomalous Hall effect in this system. © 2015 American Physical Society.

The electronic properties of a single Ru atom and a Ru10 cluster adsorbed on stoichiometric and reduced anatase, a-TiO2(101), and tetragonal zirconia, t-ZrO2(101), surfaces have been determined with density functional theory calculations with Hubbard corrections (DFT+U). The main purpose of the work is to better understand the role of dispersed metals on the surface of titania and zirconia catalysts in conversion of biomass to biofuels. On the stoichiometric surfaces, the metal adsorption does not imply major charge transfers. The situation is different on the reduced surfaces where electron transfer occurs from the oxide to the metal; this effect is more pronounced on zirconia than on titania. On both surfaces, the presence of the Ru nanoparticle favors the removal of O from the surface layers. This can result in the occurrence of O reverse spillover, with displacement of an O atom from a lattice position of the stoichiometric surfaces to a specific adsorption site of Ru10. This process is thermodynamically accessible for both TiO2 and ZrO2 surfaces showing that the metal deposition can result in an easier reduction of the oxide support due to this effect more than to a direct electron transfer from the metal.

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Physical Chemistry C, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://dx.doi.org/10.1021/jp512131e, We report on high-pressure angle-dispersive X-ray diffraction data up to 27 GPa for natural MgAlBO4 sinhalite mineral and ab initio total energy calculations. The experimental bulk modulus of sinhalite is B-0 = 171(3) GPa with a first-pressure derivative of B-0' = 4.2(3). A comparison with other olivine-type compounds shows that the value for B0 is 27% larger than that of Mg2SiO4 forsterite and 29% smaller than that of Al2BeO4 chrysoberyl. These differences are interpreted, on the basis of our ab initio calculations, in terms of the relative incompressibility of Al-O bonds in AlO6 octahedra (with a calculated bulk modulus of 250(1) GPa) as compared to Mg-O bonds in MgO6 octahedra (with a calculated bulk modulus of 130(1) GPa). The spatial cation distribution in the Pbnm orthorhombic unit cell and different polyhedral compressibilities entails a strong anisotropic compression comparable to that of forsterite. The axial compressibilities are 1.06(2) x 10(-3), 2.17(2) x 10(-3), and 1.30(3) x 10(-3) GPa(-1) for a, b, and c axes, respectively. The crystal chemistry of sinhalite under compression is compared to that of other olivine-like compounds. Compressibility trends and possible high-pressure phases are discussed.

In the present paper, the Gutzwiller density functional theory (LDA+G) has been applied to study the bilayer system of LaCoO3 grown along the (111) direction on SrTiO3. The LDA calculations show that there are two nearly flat bands located at the top and bottom of eg bands of Co atoms with the Fermi level crossing the lower one. After including both the spin-orbit coupling and the Coulomb interaction in the LDA+G method, we find that the interplay between spin-orbit coupling and Coulomb interaction stabilizes a very robust ferromagnetic insulator phase with the nonzero Chern number indicating the possibility of realizing the quantum anomalous Hall effect in this system. © 2015 American Physical Society.

The electronic properties of a single Ru atom and a Ru10 cluster adsorbed on stoichiometric and reduced anatase, a-TiO2(101), and tetragonal zirconia, t-ZrO2(101), surfaces have been determined with density functional theory calculations with Hubbard corrections (DFT+U). The main purpose of the work is to better understand the role of dispersed metals on the surface of titania and zirconia catalysts in conversion of biomass to biofuels. On the stoichiometric surfaces, the metal adsorption does not imply major charge transfers. The situation is different on the reduced surfaces where electron transfer occurs from the oxide to the metal; this effect is more pronounced on zirconia than on titania. On both surfaces, the presence of the Ru nanoparticle favors the removal of O from the surface layers. This can result in the occurrence of O reverse spillover, with displacement of an O atom from a lattice position of the stoichiometric surfaces to a specific adsorption site of Ru10. This process is thermodynamically accessible for both TiO2 and ZrO2 surfaces showing that the metal deposition can result in an easier reduction of the oxide support due to this effect more than to a direct electron transfer from the metal.

It is generally assumed that low-coordinated sites at extended defects of oxide surfaces like steps or edges are more reactive than the regular, fully coordinated sites at the flat terraces. In this work we have considered the properties of stepped surfaces of anatase TiO2 and tetragonal ZrO2 by means of periodic DFT+U calculations. For both oxides, the stability of oxygen vacancies located near the step edges is compared to that of the same defects at the regular terraces. The capability of the steps to induce nucleation of metal nanoparticles on the surface has been evaluated by simulating the adsorption of a single ruthenium adatom. We conclude that, for anatase, step edges have no particular role in favouring the reduction of the oxide by reducing the cost for oxygen abstraction; in the same way, there is no special role of the stepped anatase surface in stabilizing adsorbed Ru atoms. On the contrary, step edges on zirconia display some capability to stabilise oxygen vacancies and ruthenium adatoms.

We investigated the growth of titanium oxide two-dimensional nanostructures on Au(111), produced by Ti evaporation and post-deposition oxidation. Scanning tunneling microscopy and spectroscopy (STM and STS) and low-energy electron diffraction measurements characterized the morphological, structural and electronic properties of the observed structures. Five distinct TiOx phases were identified: the honeycomb and pinwheel phases appear as monolayer films wetting the gold surface, while nanocrystallites of the triangular, row and needle phases grow mainly over the honeycomb or pinwheel layers. Density Functional Theory investigation of the honeycomb structure supports a (2 × 2)structural model based on a Ti-O bilayer having Ti2O3 stoichiometry. The pinwheel phase was observed to evolve, for increasing coverage, from single triangular crystallites to a well-ordered film forming a (4√7 × 4√7)R19.1° superstructure, which can be interpreted within a moiré-like model. Structural characteristics of the other three phases were disclosed from the analysis of high-resolution STMmeasurements. STS measurements revealed a partial metallization of honeycomb and pinwheel and a semiconducting character of row and triangular phases.

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Physical Chemistry C, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://dx.doi.org/10.1021/jp512131e, We report on high-pressure angle-dispersive X-ray diffraction data up to 27 GPa for natural MgAlBO4 sinhalite mineral and ab initio total energy calculations. The experimental bulk modulus of sinhalite is B-0 = 171(3) GPa with a first-pressure derivative of B-0' = 4.2(3). A comparison with other olivine-type compounds shows that the value for B0 is 27% larger than that of Mg2SiO4 forsterite and 29% smaller than that of Al2BeO4 chrysoberyl. These differences are interpreted, on the basis of our ab initio calculations, in terms of the relative incompressibility of Al-O bonds in AlO6 octahedra (with a calculated bulk modulus of 250(1) GPa) as compared to Mg-O bonds in MgO6 octahedra (with a calculated bulk modulus of 130(1) GPa). The spatial cation distribution in the Pbnm orthorhombic unit cell and different polyhedral compressibilities entails a strong anisotropic compression comparable to that of forsterite. The axial compressibilities are 1.06(2) x 10(-3), 2.17(2) x 10(-3), and 1.30(3) x 10(-3) GPa(-1) for a, b, and c axes, respectively. The crystal chemistry of sinhalite under compression is compared to that of other olivine-like compounds. Compressibility trends and possible high-pressure phases are discussed.

We present a first-principles computational study of solid He-4 at T = 0 K and pressures up to similar to 160 GPa. Our computational strategy consists in using van der Waals density functional theory (DFT-vdW) to describe the electronic degrees of freedom in this material, and the diffusion Monte Carlo (DMC) method to solve the Schrodinger equation describing the behavior of the quantum nuclei. For this, we construct an analytical interaction function based on the pairwise Aziz potential that closely matches the volume variation of the cohesive energy calculated with DFT-vdW in dense helium. Interestingly, we find that the kinetic energy of solid He-4 does not increase appreciably with compression for P >= 85 GPa. Also, we show that the Lindemann ratio in dense solid He-4 amounts to 0.10 almost independently of pressure. The reliability of customary quasiharmonic DFT (QH DFT) approaches in describing quantum nuclear effects in solids is also studied. We find that QH DFT simulations, although provide a reasonable equation of state in agreement with experiments, are not able to reproduce correctly these critical effects in compressed He-4. In particular, we disclose huge discrepancies of at least similar to 50% in the calculated He-4 kinetic energies using both the QH DFT and present DFT-DMC methods., Postprint (published version)

Iron-oxide-filled carbon nanotubes have an intriguing charge bipolarization behaviour which allows the material to be applied in resistive memory devices. Raman analysis conducted with an electric field applied in situ shows the Kohn anomalies and a strong modification of the electronic properties related with the applied voltage intensity. As well as, the ID/IG ratio indicated the reversibility of this process. The electric characterization indicated an electronic transport governed by two main kind of charge hopping, one between the filling and nanotube and other between the nanotube shells., QC 20140211

We have used density functional theory to investigate the adsorption efficiency of a hydrogenated graphene (graphane) sheet for H2S and NH3 gases. We find that neither the pristine graphane sheet nor the sheet defected by removing a few surface H atoms have sufficient affinity for either H2S or NH3 gas molecules. However, a graphane sheet doped with Li adatoms shows a strong sensing affinity for both the mentioned gas molecules. We have calculated the absorption energies with one [referred to as half coverage] molecule and two molecules [referred to as full coverage] for both gases with the Li-doped graphane sheet. We find that for both the gases, the calculated absorption energies are adequate enough to decide that the Li-doped graphane sheet is suitable for sensing H2S and NH3 gases. The Li-doped sheet shows a higher affinity for the NH3 gas compared to the H2S gas molecules due to a stronger Li(s)-N(p) hybridization compared to that of Li(s)-S(p). However, while going from the half coverage effect to the full coverage effect, the calculated binding energies show a decreasing trend for both the gases. The calculated work function of the Li-doped graphane sheet decreases while bringing the gas molecules within its vicinity, which explains the affinity of the sheet towards both the gas molecules., QC 20140515

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., QC 20140808. Updated from manuscript to article in journal.

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., QC 20140508

We present a high-resolution angle-resolved photoemission spectroscopy study in conjunction with first-principles calculations to investigate how the interaction of electrons with phonons in graphene is modified by the presence of Yb. We find that the charges transferred from Yb to the graphene layer hybridize with the graphene pi bands, leading to a strong enhancement of the electron-phonon interaction. Specifically, the electron-phonon coupling constant is increased by as much as a factor of 10 upon the introduction of Yb with respect to as-grown graphene (<= 0.05). The observed coupling constant constitutes the highest value ever measured for graphene and suggests that the hybridization between graphene and the adatoms might be a critical parameter in realizing superconducting graphene., QC 20141021

Defect-driven diffusion of impurities is the major phenomenon leading to formation of embrittling nanoscopic precipitates in irradiated reactor pressure vessel (RPV) steels. Diffusion depends strongly on the kinetic correlations that may lead to flux coupling between solute atoms and point defects. In this work, flux coupling phenomena such as solute drag by vacancies and radiation-induced segregation at defect sinks are systematically investigated for six bcc iron-based dilute binary alloys, containing Cr, Cu, Mn, Ni, P, and Si impurities, respectively. First, solute-vacancy interactions and migration energies are obtained by means of ab initio calculations; subsequently, self-consistent mean field theory is employed in order to determine the exact Onsager matrix of the alloys. This innovative multiscale approach provides a more complete treatment of the solute-defect interaction than previous multifrequency models. Solute drag is found to be a widespread phenomenon that occurs systematically in ferritic alloys and is enhanced at low temperatures (as for instance RPV operational temperature), as long as an attractive solute-vacancy interaction is present, and that the kinetic modeling of bcc alloys requires the extension of the interaction shell to the second-nearest neighbors. Drag occurs in all alloys except Fe(Cr); the transition from dragging to nondragging regime takes place for the other alloys around (Cu, Mn, Ni) or above (P, Si) the Curie temperature. As far as only the vacancy-mediated solute migration is concerned, Cr depletion at sinks is foreseen by the model, as opposed to the other impurities which are expected to enrich up to no less than 1000 K. The results of this study confirm the current interpretation of the hardening processes in ferritic-martensitic steels under irradiation., QC 20141209

The surface electronic structure of the narrow-gap seminconductor BiTeI exhibits a large Rashba-splitting which strongly depends on the surface termination. Here we report on a detailed investigation of the surface morphology and electronic properties of cleaved BiTeI single crystals by scanning tunneling microscopy, photoelectron spectroscopy (ARPES, XPS), electron diffraction (SPA-LEED) and density functional theory calculations. Our measurements confirm a previously reported coexistence of Te- and I-terminated surface areas

The surface electronic structure of the narrow-gap seminconductor BiTeI exhibits a large Rashba-splitting which strongly depends on the surface termination. Here we report on a detailed investigation of the surface morphology and electronic properties of cleaved BiTeI single crystals by scanning tunneling microscopy, photoelectron spectroscopy (ARPES, XPS), electron diffraction (SPA-LEED) and density functional theory calculations. Our measurements confirm a previously reported coexistence of Te- and I-terminated surface areas

The structural properties of Thallium (III) oxide (Tl2O3) have been studied both experimentally and theoretically under compression at room temperature. X-ray powder diffraction measurements up to 37.7 GPa have been complemented with ab initio total-energy calculations. The equation of state of Tl2O3 has been determined and compared to related compounds. It has been found experimentally that Tl2O3 remains in its initial cubic bixbyite-type structure up to 22.0 GPa. At this pressure, the onset of amorphization is observed, being the sample fully amorphous at 25.2 GPa. The sample retains the amorphous state after pressure release. To understand the pressure-induced amorphization process, we have studied theoretically the possible high-pressure phases of Tl2O3. Although a phase transition is theoretically predicted at 5.8 GPa to the orthorhombic Rh2O3-II-type structure and at 24.2 GPa to the orthorhombic alpha-Gd2S3-type structure, neither of these phases were observed experimentally, probably due to the hindrance of the pressure-driven phase transitions at room temperature. The theoretical study of the elastic behavior of the cubic bixbyite-type structure at high-pressure shows that amorphization above 22 GPa at room temperature might be caused by the mechanical instability of the cubic bixbyite-type structure which is theoretically predicted above 23.5 GPa. (C) 2014 AIP Publishing LLC.