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Here we present a combined theoretical and experimental study of hydrogen adsorption on Zn-functionalized MgO surface. We have systematically compared infrared spectra recorded at increasing hydrog...

The morphology, epitaxy, band-alignment and optical response of silver nanoparticles on ZnO(1010) has been analysed by microscopy, photoemission, electron diraction and dier-ential reectivity. At 700 K, clusters tend to nu-cleate and grow exclusively along [010]-oriented polar steps and corners, and not on at terraces or non-polar [001] steps. This preference leads to a specic epitaxy Ag(111)[110] ZnO[010](1010) with a large lattice mismatch along the dense row of the metal. The experiments have been rationalized through ab ini-tio simulations. A much lower adsorption energy is obtained for the O-terminated steps with a charge transfer from silver to oxygen, that corroborates the variation of ionization energy observed in photoemission and the presence of cationic silver. This anisotropy of growth reverberates in the plasmonic response of the metallic particles. The growth and epitaxy of metals on the polar (0001) and non-polar (1010) ori-entations of ZnO is discussed in terms of sur-face/step polarity compensation in the light of the present ndings.

Crystalline KOH undergoes an antiferroelectric (AFE) proton ordering phase transition at low temperatures, which results in a monoclinic bilayer structure held together by a network of weak hydrogen bonds (HBs). The Curie temperature shifts up when the compound is deuterated, an effect that classical MD is not able to catch. For deeper insights into the transition mechanism, we carry out ab initio MD simulations of KOH and KOD crystals by including quantum effects on the nuclei through Feynman path integrals. The geometric isotope effect and the evolution of the lattice parameters with temperature agree with the experimental data, while the purely classical description is not appropriate. Our results show that deuteration strengthens the HBs in the low-T AFE ordered phase. The transition is characterized by the flipping of OH/OD groups along a bending mode. Above the transition, the system is driven into a dynamical disordered paraelectric phase.

International audience; Transport of hydrogen in hydrous minerals under high pressure is a key step for the water cycle within the Earth interior. Brucite Mg(OH) 2 is one of the simplest minerals containing hydroxyl groups and is believed to decompose under the geological condition of the deep Earth's mantle. In the present study, we investigate the proton diffusion in brucite under high pressure, which results from a complex interplay between two processes: the O-H reorientations motion around the c axis and O-H covalent bond dissociations. First-principle path-integral molecular dynamics simulations reveal that the increasing pressure tends to lock the former motion, while, in contrast, it activates the latter which is mainly triggered by nuclear quantum effects. These two competing effects therefore give rise to a pressure sweet spot for proton diffusion within the mineral. In brucite Mg(OH) 2 , proton diffusion reaches a maximum for pressures close to 70GPa, while the structurally similar portlandite Ca(OH) 2 never shows proton diffusion within the pressure range and time scale that we explored. We analyze the different behavior of brucite and portlandite, which might constitute two prototypes for other minerals with same structure. Hydroxide minerals play an important role in several problems in geology, surface science or for industrial applications. Among them, brucite Mg(OH) 2 can be formed at the interface between periclase MgO and water at ambient conditions 1-3. The trigonal brucite structure consists of alternating layers along the c axis that terminate with hydroxyls (Fig. 1). This structure is common to other hydroxides of divalent metals, such as Ca(OH) 2 , Ni(OH) 2 and Cd(OH) 2. Portlandite Ca(OH) 2 is the main component of cements and concretes, which motivated a large number of investigations about its elastic properties. Because of their anisotropic structure, brucite iso-structural minerals are much more compressible along the c axis than in the other two directions, parallel to the stacks. Mg(OH) 2 can also act as a water vector in subduction zones, through complex processes that take place within the Earth interior 4,5. Therefore, the behavior of brucite and brucite-like minerals at very high pressure has been widely investigated. X-ray diffraction of Mg(OH) 2 up to 78 GPa showed that the c a / ratio decreases steadily from ambient pressure up to about 25 GPa and then stays almost constant 6. Those results suggest that the properties of brucite at very high pressure, and in particular the nature of the inter-layer bonding, could differ significantly from ambient conditions. Moreover, the hydroxyl groups that are parallel to the c axis at ambient conditions slant in three equivalent positions as the inter-layer distance shrinks even under moderate pressure 7 or when decreasing temperature 8. Besides reducing the global symmetry from P m 3 1 down to P3, the slanted OH groups can significantly alter several physical properties of brucites. Firstly, it could allow for the formation of hydrogen bonds between the layers 9 , which eventually reinforce under further compression and modify the compressibility along c 10. Secondly, when the protons form a non null θ angle with the c axis, they cannot arrange in a static ordered structure. Such proton disorder, which is closely related to proton frustration 11,12 , has also been invoked as the reason for the pressure-induced hydrogen sublattice amorphization in brucites 7,13. The existence of a quasi two-dimensional proton liquid in those extreme conditions can be conjectured, but the properties of the whole structure, if stable, have so far escaped a precise characterization. In particular, the occurrence of proton hopping is plausible, but whether this process results in a long-range diffusion is a totally open question. From the theoretical viewpoint, the previous observations call for a dynamical treatment of the proton arrangement within the brucite structure at high pressure. Moreover, in such conditions nuclear quantum effects, that is, all the properties that go beyond a purely classical description of ion dynamics 14 , such as zero-point

International audience; Methane hydrate was recently shown, both experimentally and through simulations, to be stable up to the remarkably high pressure of 150 GPa. A new methane hydrate high-pressure (MH-IV) phase, reminiscent of ice at ambient pressure, was described for pressures above approximately 40 GPa. We disentangle here the main contributions to the relative stability of the lower pressure, denoted MH-III and the high-pressure MH-IV structures. Through several simulation techniques, including metadynamics and Path Integral Molecular Dynamics for nuclear quantum effects, we analyze the phase transition mechanism, which implies hydrogen bond breaking and reforming, as well as methane reordering. The transition pathway is far from trivial and the quantum delocalization of the hydrogen nuclei plays a significant role.

Two aspects of the contribution of grazing incidence fast atom diffraction (GIFAD) to molecular beam epitaxy (MBE) are reviewed here: the ability of GIFAD to provide \emph{in-situ} a precise description of the atomic-scale surface topology, and its ability to follow larger-scale changes in surface roughness during layer-by-layer growth. Recent experimental and theoretical results obtained for the He atom beam incident along the highly corrugated $[ 1\bar{1}0 ]$ direction of the $\beta_{2}$(2$\times$4) reconstructed GaAs(001) surface are summarized and complemented by the measurements and calculations for the beam incidence along the weakly corrugated [010] direction where a periodicity twice smaller as expected is observed. The combination of the experiment, quantum scattering matrix calculations, and semiclassical analysis allows in this case to reveal structural characteristics of the surface. For the in situ measurements of GIFAD during molecular beam epitaxy of GaAs on GaAs surface we analyse the change in elastic and inelastic contributions in the scattered beam, and the variation of the diffraction pattern in polar angle scattering. This analysis outlines the robustness, the simplicity and the richness of the GIFAD as a technique to monitor the layer-by-layer epitaxial growth.

Gas hydrates consist of hydrogen-bonded water frameworks enclosing guest gas molecules and have been the focus of intense research for almost 40 y, both for their fundamental role in the understanding of hydrophobic interactions and for gas storage and energy-related applications. The stable structure of methane hydrate above 2 GPa, where CH4 molecules are located within H2O or D2O channels, is referred to as methane hydrate III (MH-III). The stability limit of MH-III and the existence of a new high-pressure phase above 40 to 50 GPa, although recently conjectured, remain unsolved to date. We report evidence for a further high-pressure, room-temperature phase of the CH4-D2O hydrate, based on Raman spectroscopy in diamond anvil cell and ab initio molecular dynamics simulations including nuclear quantum effects. Our results reveal that a methane hydrate IV (MH-IV) structure, where the D2O network is isomorphic with ice Ih, forms at similar to 40 GPa and remains stable up to 150 GPa at least. Our proposed MH-IV structure is fully consistent with previous unresolved X-ray diffraction patterns at 55 GPa [T. Tanaka et al., J. Chem. Phys. 139, 104701 (2013)]. The MH-III -> MH-IV transition mechanism, as suggested by the simulations, is complex. The MH-IV structure, where methane molecules intercalate the tetrahedral network of hexagonal ice, represents the highest-pressure gas hydrate known up to now. Repulsive interactions between methane and water dominate at the very high pressure probed here and the tetrahedral topology outperforms other possible arrangements in terms of space filling.

International audience; The configurations associated with the dissociative adsorption of water on a variety of low-coordinated sites of MgO(100) surfaces, including corners, steps, MgO vacancies and kinks on steps have been studied and assigned by combining infrared spectroscopy and ab initio calculations. Three kinds of MgO powders were examined, powders of very high specific surface area prepared by chemical vapor synthesis and well-defined cubic smoke particles obtained by combustion either in 20:80 or 60:40 O2:Ar mixtures, the latter one involving less defects and smaller particles. It appears that an imperative requirement to obtain a precise characterization of the reactive behavior of defects is to keep the samples in ultra-high vacuum conditions and to control the water partial pressure finely.

International audience; First-principle molecular dynamics simulations of methane hydrate MH-III, including the quantum properties of the hydrogen nuclei, were carried out at pressures in the 5-65 GPa range, in order to observe the H-bond symmetrization at high pressure and at room temperature. According to our simulations, the symmetrization transition takes place around 40 GPa and is little dependent on isotope substitution. We find that, consistently with the rather complex crystal structure of MH-III, the transition is much more convoluted in the hydrate than in ices VII and X. In methane hydrate, due to the presence of non equivalent O ions with distinct O-O distances, the dissimilar H bonds symmetrize in a pressure domain rather than at a single critical pressure.

Oxygen-deficient MgO(100) surfaces are studied by means of total-energy calculations, within the density-functional theory. Selected surface configurations presenting increasing densities of neutral oxygen vacancies, ranging from 12.5% to 100%, are considered, the missing oxygen being periodically distributed in the surface layer either homogeneously or in a close-packed fashion. We show that the electronic structure is characterized by occupied states in the gap which result either from the hybridization of atomiclike orbitals localized on vacancy sites, or involve surface magnesium orbitals. The latter case occurs at large vacancy concentrations, and the conditions for the apparition of metallicity in the surface layer are derived. The formation energy of oxygen vacancies is calculated as a function of the vacancy concentration and of the particular configuration. A two-body model potential is proposed to account for their interaction. The activation energy for vacancy diffusion at the surface is also computed, thus providing a complete set of parameters for a description of the thermodynamics as well as of the kinetics of oxygen vacancies on a MgO(100) surface.

The structural and electronic properties of neutral lithium suboxide clusters ${\mathrm{Li}}_{4+p}{\mathrm{O}}_{2}$ $(0l~pl~9)$ are studied by means of ab initio molecular dynamics simulations. For few excess Li atoms Li-O bonds are formed which progressively weaken as the O coordination number increases. Then Li attachment proceeds around nonstoichiometric ionic cores and results in metal segregation. The evolution of the growth mode correlates with a change of the electronic properties, some of which can be probed experimentally to obtain structural information. We predict that at small $p$ ${\mathrm{Li}}_{4+p}{\mathrm{O}}_{2}$ clusters have ionization potential much lower than pure ${\mathrm{Li}}_{p}.$ A generalization of our results to oxidized alkali-metal surfaces and bulk suboxides is proposed.

Ground-state properties of stoichiometric lithium oxide clusters ${\mathrm{Li}}_{2\mathit{n}}$${\mathrm{O}}_{\mathit{n}}$ (n\ensuremath{\le}6) are studied by means of ab initio molecular-dynamics simulations, in conjunction with a global optimization strategy. For each n value, several stable isomers are found, presenting a remarkable variety of bonding configurations. We analyze the atomic structure, the local coordination, the bond lengths, and the relative stability of these clusters. By defining a mean bond length --- averaged over all atoms having the same coordination number --- we show that it is possible to characterize the global bonding properties of a ${\mathrm{Li}}_{2\mathit{n}}$${\mathrm{O}}_{\mathit{n}}$ cluster at given n despite the structural differences among the various isomers. For some of them, energy barriers can be easily overpassed, making possible a dynamical equilibrium among a few configurations in a temperature range of some hundred K above room temperature. \textcopyright{} 1996 The American Physical Society.

International audience; A lattice-based kinetic Monte-Carlo (KMC) code has been developed to investigate the MgO\001\ crystal growth from deposition of MgO molecules, as a prototypical case of the growth of oxide thin films. The KMC approach has been designed on the basis of an extensive database including all possible diffusion mechanisms. The corresponding activation energies have been computed through first-principles calculations at zero temperature or from Arrhenius plots of the frequencies obtained by molecular dynamics simulations with empirical potentials. Crystal growth occurs layer by layer, as experimentally observed, and the diffusion of admolecules leads to a high capacity of nucleation, which is enhanced by vacancy diffusion. We have characterized the growth through surface roughness, size distribution and density of the islands, and filling ratios of the growing layers. Moreover, we have analysed the influence of each elementary mechanism on the growth. The best quality of the deposited layers is reached for temperatures larger than 700 K and for pressures smaller than 0.1 Torr. For these conditions, the simulated surface roughness is fully consistent with available experimental results. (C) 2011 Elsevier B.V. All rights reserved.

We propose a structural model of hydrogenated amorphous silicon carbide (a-SiC:H) at stoichiometric composition and low H content, based on the results of ab initio molecular-dynamics simulations. As found for a-SiC, the system has a complex structure which cannot be described as a distorted crystalline phase. Nevertheless, the degree of chemical order is larger in the hydrogenated network than in the pure network. H binds to both C and Si. H bound to C gives rise to ${\mathit{sp}}^{3}$ bonded monohydrated sites, thus favoring the formation of a tetrahedral network. H bound to Si forms diverse bonding configurations where weak and strong bonds can be identified. These results are in agreement with recent experiments. Furthermore, our findings for the evolution of Si-H and C-H bonding as a function of temperature are consistent with recent measurements of H desorption from Si-C alloys. Finally, we suggest an interpretation of x-ray and neutron-diffraction data based on analyses of several total distribution functions.

International audience; Powders of isolated and well-dispersed oxide nanocubes are promising components for photoelectronic applications that benefit from tunable optical properties, surface reactivity and the ease of realization of their controlled assembly. Here, we demonstrate that combustion of zinc and magnesium metal vapors at reduced pressures followed by subsequent vacuum annealing of the resulting nanoparticle powders yields single-crystalline Zn(x)Mg(1-x)O nanocubes of exceptional regular cubic shape and edge lengths below 25 nm. In line with ab initio calculations, which predict preferential Zn(2+) segregation into low coordinated surface elements of the MgO nanocubes, we track the occupation of edge sites by chains of Zn(2+)-O(2-) units through their spectroscopic signatures. As a method to generate composite nanostructures with controlled spatial distribution of the chemical components, the annealing induced ion segregation can be extended to other well-dispersed metastable nanoparticles. We expect that the energy of segregation mainly depends on the site coordination number, which can promote controlled demixing within the nanoparticles.

International audience; Whenever iron is grown on Zn-terminated ZnSe(001) substrates, the most stable configurations of Fe/ZnSe(001) magnetic tunnel junctions show Fe-Se bonding and include a Fe-Zn intermixed interface layer, as suggested by spectroscopic measurement and confirmed here by calculations based on the density functional theory. The presence of the intermixed Fe-Zn layer at the junction strongly reduces the spin polarization of both the interface and metal-induced gap states within the ZnSe tunnel barrier.

The electronic structure and the energetics of oxygen deficient interfaces, constituted by MgO(100) substrates that are capped by epitaxial $\mathrm{Ti}{\mathrm{O}}_{2}$ or $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ layers, are studied within first principles. Various configurations with one O vacancy per $(1\ifmmode\times\else\texttimes\fi{}1)$ surface unit cell, which is located either in the MgO or in the $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ interfacial layers, are compared. The excess electrons either fill Ti states at the bottom of the conduction band or form an $F$-center as in MgO(100), depending not only upon the side of the interface in which the vacancy is created, but also on the actual vacancy environment. In most cases that we consider, the interface has a metallic character. Our main result regards the order of stability of the various configurations; the oxygen vacancies can be formed more easily in the $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ overlayer than in the MgO substrate. Moreover, we show that the formation energies of O vacancies can be related to the nature and energy level of the excess electron state. Such a behavior is primarily determined by the interfacial electrostatic potential, which plays a prominent role as in other oxide/oxide interfaces.

The epitaxy of ultrathin SrTiO 3 films on the stoichiometric MgO(001) surface is studied through first-principles calculations. The detailed atomic structure, the elastic properties, and the bonding characteristics of various interfaces are discussed and synthesized in a coherent picture. In particular, we show that (1) only the TiO 2 /MgO contact is thermodynamically stable and makes possible the growth of thicker strontium titanate layers on magnesia; (2) many characteristics of the thick SrTiO 3 (001)/MgO(001) epitaxial interface are recovered already when a SrTiO 3 bilayer is adsorbed. The evolution of the valence band offset at the interface is also studied as a function of the nature and the thickness of the deposited films. For both epitaxial, TiO 2 terminated, adsorbed bilayer and thick SrTiO 3 (001)/MgO(001) interface, the upper edges of the valence band are almost aligned.

Surface divacancy at MgO(100) and its reactivity with water are studied by first-principles simulations. Among the most frequent point defects at MgO(100), the divacancy is estimated to be stable with respect to the formation of neutral oxygen and magnesium vacancies in moderate O environments. A single water molecule dissociates spontaneously at the divacancy. The final product of this exothermic reaction consists of a surface Mg vacancy with two adsorbed hydrogens—the (VMg,2H) complex—which shows peculiar atomic and electronic structures. In particular, the stretching frequencies of the hydroxyl groups are downshifted noticeably, which can explain recent infrared data collected on wet MgO powders. The trend towards an aggregation of the brucite-like intrusions (VMg,2H) is pointed out in relation with the complex behavior of the MgO(100) surface upon exposure to water.

Stable polar oxide surfaces must be simultaneously electrostatically compensated and in thermodynamic equilibrium with the environment. As a paradigm, the $\mathrm{MgO}(111)\mathrm{\text{\ensuremath{-}}}p(2\ifmmode\times\else\texttimes\fi{}2)$ reconstructed surface is shown to involve combinations of Mg-covered terminations with peculiar insulating electronic structure, favored in O-poor conditions, and the O-terminated octopole, stabler in more O-rich environments. Such a picture, which could not have been foreseen by either experiments or simulations separately, goes beyond the Wolf model and reconciles the theory with the experimental data taken in variable thermodynamic conditions.

The electronic and atomic structures of several $(1\ifmmode\times\else\texttimes\fi{}1)$ terminations of the (110) polar orientation of the ${\mathrm{SrTiO}}_{3}$ surface are systematically studied by first-principles calculations. The electronic structure of the two stoichiometric SrTiO and ${\mathrm{O}}_{2}$ terminations are characterized by marked differences with respect to the bulk, as a consequence of the polarity compensation. In the former, the Fermi level is located at the bottom of the conduction band, while in the latter the formation of a peroxo bond between the two surface oxygens results in a small-gap insulating surface with states in the gap of the bulk projected band structure. We also consider three nonstoichiometric terminations with TiO, Sr, and O compositions, respectively, in the outermost atomic layer, which automatically allows the surface to be free from any macroscopic polarization. They are all insulating. The cleavage and surface energies of the five terminations are computed and compared, taking into account the influence of the chemical environment as a function of the relative richness in O and Sr. From our calculations, it appears that some (110) faces can even compete with the ${\mathrm{TiO}}_{2}$ and SrO terminations of the (100) cleavage surface: in particular, the (110)-TiO termination is stable in Sr-poor conditions, the (110)-Sr one in simultaneously O- and Sr-rich environments. The available experimental data are compared to the outcomes of our calculations and discussed.

Elementary processes governing the epitaxial growth of metal oxides, like adsorption and diffusion, have been studied theoretically in the prototypical case of MgO/MgO(001). Most of these processes need to be investigated in an ab initio approach since they are usually accompanied by strong modifications of the electronic structure. This study has been carried out within the density functional theory, and complemented by molecular dynamics simulations using a rigid-ion potential. The growth of the oxide passes through several oxido-reduction reactions at the surface. The adsorption and the diffusion of Mg and O are strongly different from each other, and change considerably at steps from flat terraces. All these results point out the need of realistic models for the growth of metal oxides that take into account the chemical behaviour of the involved species as well as the influence of the defects.

We present a detailed theoretical study, based on density-functional calculations, of Na adsorption on ${\mathrm{TiO}}_{2}(110)$ for coverage equal to 1/8, 1/4, and 1/2 monolayers. Two competing threefold adsorption sites are found. In the framework of the Bader theory, we analyze the electron distribution, the interfacial electron transfer, and the screening processes as a function of the coverage and the actual adsorption geometry. At low coverage, the Na atoms bind preferentially to two bridging and one in-plane O atoms. The adsorbed sodium atoms are nearly fully ionized. The excess electron states have a nonbonding Ti character and are delocalized over a few surface and subsurface sites. Such a behavior, which is at variance with the common belief that the excess states should be essentially localized on the surface fivefold titanium, is discussed in terms of the surface electrostatic potential and the atomic relaxations. According to our calculations, the onset of covalent interactions between Na atoms occurs for coverage slightly below 1/2 of monolayer, depending on the specific adsorption configuration. It is accompanied by a reduction of the positive Bader charge of the Na atoms and by a weakening of the ionic Na-O bond strength, which results in decreasing Na adsorption energies. At 1/2 monolayer coverage, the adsorption energies are practically insensitive to the details of the Na distribution among the low-energy adsorption sites, which suggests that Na-induced surface reconstructions might be affected by the intrinsic quality of the surface and the actual deposition conditions.

The interaction of a water molecule with a neutral oxygen vacancy on the MgO(100) surface is studied in the limit of very low coverage by total-energy calculations and molecular-dynamics simulations based on the density-functional theory. Exchange and correlation effects are treated within the generalized gradient approximation. We show the existence of a local minimum corresponding to the physisorbed adsorption state, which is similar to the one found for water adsorption on the stoichiometric MgO(100) surface. At variance with the latter one, the dissociation of the water molecule is energetically favored at the vacancy site. However, the global energy minimum corresponds to the recombined configuration, in which the oxygen initially in the water molecule adsorbs in the vacancy and the two hydrogens form a ${\mathrm{H}}_{2}$ dimer. Through the evaluation of the energy barriers between the different states we discuss the kinetics of the interaction of the water molecule with the defective surface. We show that the dissociated configuration is likely only an intermediate state towards the recovery of a stoichiometric surface, thus suggesting that dissociation of an isolated water molecule at the neutral oxygen vacancy cannot be held as responsible for the existence of a stable hydroxyl group on the MgO(100) surface.

The stability of polar oxide surfaces is discussed on general grounds, and more specifically in the case of MgO(111), a prototype of standard polar surfaces, and in the case of SrTiO3(001), a prototype of ‘weakly polar’ surfaces. In both cases, on the unreconstructed stoichiometric surfaces, an electron redistribution takes place in the surface layers, which cancels the macroscopic electrostatic dipole. This process involves a metallization of the MgO(111) unreconstructed surfaces, and induces a high value of the surface energy. On SrTiO3(001), on the other hand, bond breaking at the surface suffices by itself to provide the compensating charges. A better stability of the MgO(111) surface may be achieved by the desorption of a macroscopic number of atoms in the outer layer(s). We study two surface configurations, associated with (2 × 1) and (2 ×2) unit cells, in which little electron redistribution is needed to cancel the macroscopic electrostatic dipole. We predict an even larger stability for (2n ×2n) reconstructed surfaces which present pyramid-like structures and {100} facets of increasing sizes as n grows. In addition, we analyze the formation of oxygen vacancies and their interaction on the two terminations of SrTiO3(001). In this compound, oxygen vacancies do not provide a stabilization of the surface. However, their formation induces a whole set of reconstructed structures, on which we bring some elements of discussion.

The results of a density functional calculation on gallium nitride are given. We use norm-conserving pseudopotentials with sufficiently extended sets of plane waves to investigate the ground-state properties and the electronic band structure for the zincblende phase of GaN and compare them with the corresponding results for the wurtzite structure. A comparison with the outcomes of other calculations and with the existing experimental data is also given.