Search

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.

Nested sampling is an efficient method for calculating Bayesian evidence in data analysis and partition functions of potential energies. It is based on an exploration using a dynamical set of sampling points that evolves to higher values of the sampled function. When several maxima are present, this exploration can be a very difficult task. Different codes implement different strategies. Local maxima are generally treated separately, applying cluster recognition of the sampling points based on machine learning methods. We present here the development and implementation of different search and clustering methods on the nested_fit code. Slice sampling and the uniform search method are added in addition to the random walk already implemented. Three new cluster recognition methods are also developed. The efficiency of the different strategies, in terms of accuracy and number of likelihood calls, is compared considering a series of benchmark tests, including model comparison and a harmonic energy potential. Slice sampling proves to be the most stable and accurate search strategy. The different clustering methods present similar results but with very different computing time and scaling. Different choices of the stopping criterion of the algorithm, another critical issue of nested sampling, are also investigated with the harmonic energy potential.

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.

Electronic properties of materials are dominated by quantum effects, but nuclei, being much heavier, are usually treated as classical particles. This approximation, although tremendously convenient, is not always valid, even in close to ambient pressure and temperature conditions, especially when light nuclei such as hydrogen are involved. Zero point energy and proton tunneling can be relevant. Isotopic effects, obtained by replacing hydrogen with deuterium, are observed experimentally and are a clear indication of Nuclear Quantum Effects (NQE) since mean values obtained through classical statistical physics do not depend on mass. Introducing NQEs into simulations at an acceptable computational cost raises fundamental questions and yields subtle and unexpected results.

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.

The quantum thermal bath (QTB) has been presented as an alternative to path-integral-based methods to introduce nuclear quantum effects in molecular dynamics simulations. The method has proved to be efficient, yielding accurate results for various systems. However, the QTB method is prone to zero-point energy leakage (ZPEL) in highly anharmonic systems. This is a well-known problem in methods based on classical trajectories where part of the energy of the high-frequency modes is transferred to the low-frequency modes leading to a wrong energy distribution. In some cases, the ZPEL can have dramatic consequences on the properties of the system. Thus, we investigate the ZPEL by testing the QTB method on selected systems with increasing complexity in order to study the conditions and the parameters that influence the leakage. We also analyze the consequences of the ZPEL on the structural and vibrational properties of the system. We find that the leakage is particularly dependent on the damping coefficient and that increasing its value can reduce and, in some cases, completely remove the ZPEL. When using sufficiently high values for the damping coefficient, the expected energy distribution among the vibrational modes is ensured. In this case, the QTB method gives very encouraging results. In particular, the structural properties are well-reproduced. The dynamical properties should be regarded with caution although valuable information can still be extracted from the vibrational spectrum, even for large values of the damping term.

International audience; The adsorption of 3-ethynyl-trifluoro-toluene (ETFT) on Si(100)-2 x 1 surface in ultra high vacuum is studied in the low coverage regime, through a joint experimental and theoretical approach. The STM images of both filled and empty states revealed few distinct adsorption configurations. On the basis of Density Functional Theory (DFT) calculations the STM images were simulated and three main adsorption configurations were identified, with a predominance of di-sigma bonded species that leave the benzene ring unreacted. A discussion of the reactivity of the reconstructed silicon surface towards benzene derivatives is proposed by comparing the adsorption of ETFT close related molecules, like styrene and phenylacetylene. (C) 2010 Elsevier B.V. All rights reserved.

International audience; The dissociative adsorption of water vapor on MgO smokes is studied by infrared spectroscopy and first-principles calculations. The MgO smokes are synthesized in controlled conditions so to display a particularly high concentration of defect-free step edges (4-fold coordinated `'4C'' sites). Exposure to water vapor results in a set of twinned infrared bands at nu(A) = 3480 and nu(B) = 3710 cm(-1)-the former having never been observed before-with an intensity ratio I(nu(A))/I(nu(B)) approximate to 2. Simulated configurations include water adsorption, for various coverages, on terraces of distinct orientations, at step edges and corners. It is shown that OH stretching frequencies can be robustly correlated with OH bond lengths. The twinned (nu(A),alpha nu(B)) bands are assigned the hydrogen-bonded (O(s)(4C)-H, Mgs(4C)-OH) moiety adsorbed at fully decorated monatomic < 100 > step edges on (100) terraces. The coordination number of adsorbed OH species is shown not to be sufficient to characterize the physicochemical properties of wet MgO smokes and it is suggested that the hydrated surface can be better described in terms of (OH, MgO) complexes.

A theoretical investigation of the properties of the Si3C4, Si4C3, and Si4C4 clusters is reported. Systematic explorations of the potential energy surfaces of the three clusters are performed using a combination of ab initio molecular dynamics and local energy minimizations using density functional theory. A large number of isomers with a large variety of geometries has been found. The geometries, energies, and vibrational frequencies yielded are discussed. Furthermore, a quantitative analysis of the interatomic distances, angles, and coordination numbers observed, as well as the conclusions on the bonding properties, are presented. The cluster properties are then compared to those of solid SiC and of the smaller Si–C clusters (with size up to 6) obtained in a previous study. Analysis of our results and comparison with bulk properties show that even clusters as small as Si3C4, Si4C3, and Si4C4 exhibit properties similar to those of the amorphous bulk, in particular as for the structures and bonds formed by C atoms.

First-principles DFT calculations were performed to study molecular MgO stoichiometric flat clusters on the perfect MgO(0 0 1) surface. It is shown that a model description of their energetics could be provided within a very simple independent defect approximation with surface, step, corners and kinks. The formation energies of the first three defects are derived, respectively: E su =0.50 eV/MgO, E st =0.88 eV/MgO, E 4c =5.79 eV. The model applies even for very small cluster sizes ( n ⩾4). For large clusters kink and corner energy contributions tend to be negligible, reflecting the tendency towards a constant binding energy of a single molecule to large clusters independently of their geometry.

Various (1 x 1) terminations of the SrTiO 3 (110) polar surface are considered and their electronic and structural properties computed through first-principles calculations. The relative stability of terminations that differ by stoichiometry is compared by taking into account the influence of the chemical environment. In contrast with the metallic stoichiometric SrTiO termination, the nonstoichiometric ones are insulating and their stability comparable to that of the SrTiO 3 (100) surfaces. We thus argue that, among the (1 x 1) unreconstructed faces, some nonstoichiometric (110) orientations may be obtained in suitable O-poor environments.

We discuss several aspects of electron redistributions in low-dimensional oxides: surfaces, ultra-thin films or clusters. We rely upon a combination of first principles density functional calculations, Bader analysis and an analytical model, the bond electron transfer model, that we derive and which is valid whatever the covalent strength of the anion–cation bonding. We show that, despite the fact that nearly bulk-like charges are found on low-coordinated sites, there usually exists a strong increase of the covalent character of the oxygen–cation bonds, which is reflected in experimentally measurable quantities. On polar orientations, we discuss the efficiency of various processes yielding a cancellation of the macroscopic dipole moment.

First-principles calculations based on the density functional theory of the adsorption and migration energies of Mg atom and MgO molecule on a flat MgO (0 0 1) surface are reported and discussed. The isolated neutral Mg adatom binds quite loosely to the surface (E ads ≃0.46 eV ) and has to overcome a barrier of about 0.30 eV in between two stable O adsorption sites. The MgO molecule forms iono-covalent bonds with the surface, with an adsorption energy of 2.5±0.1 eV . At variance with the isolated Mg adatom, the surface diffusion of the MgO molecule may imply a sequence of elementary rotation step around its ends. The computed barriers for molecule, 90° rotation around its O and Mg atoms are 0.35 and 0.46 eV, respectively.

A grazing incidence fast atom diffraction (GIFAD or FAD) setup, installed on a molecular beam epitaxy chamber, has been used to characterize the ${\ensuremath{\beta}}_{2}(2\ifmmode\times\else\texttimes\fi{}4)$ reconstruction of a GaAs(001) surface at $530{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ under an ${\mathrm{As}}_{4}$ overpressure. Using a 400-eV ${}^{4}\mathrm{He}$ beam, high-resolution diffraction patterns with up to eighty well-resolved diffraction orders are observed simultaneously, providing a detailed fingerprint of the surface structure. Experimental diffraction data are in good agreement with results from quantum scattering calculations based on an ab initio projectile-surface interaction potential. Along with exact calculations, we show that a straightforward semiclassical analysis allows the features of the diffraction chart to be linked to the main characteristics of the surface reconstruction topography. Our results demonstrate that GIFAD is a technique suitable for measuring in situ the subtle details of complex surface reconstructions. We have performed measurements at very small incidence angles, where the kinetic energy of the projectile motion perpendicular to the surface can be reduced to less than 1 meV. This allowed the depth of the attractive van der Waals potential well to be estimated as $\ensuremath{-}8.7$ meV in very good agreement with results reported in literature.

International audience; The phase transition from ice VII to ice X under extreme pressures is an example where quantum proton delocalization coexists with classical thermal fluctuations. We investigate this transition, including quantum effects on the nuclear motion through adapted Langevin dynamics. This approach, which allows us to follow the semiclassical trajectories of protons, provides excellent agreement with experimental vibrational spectra indicating a transition pressure of about 65 GPa. Furthermore, we map the full dynamical problem onto a pressure-dependent, one-dimensional mean-field potential for the proton. By solving exactly the corresponding Schr¨odinger equation, we disentangle tunneling and quantum delocalization from classical thermal effects and identify the transition through the topological changes of the proton ground state and its susceptibility. The process is dominated by quantum effects even at ambient temperature and can be considered to be a paradigmatic case of a quantum-driven phase transition.

Electronic ground-state properties and quasiparticle energies of the charge-transfer insulating oxides SrO and MgO have been computed. Structural properties calculations have been performed within DFT–LDA, while for the electronic excitations an efficient DFT–GW scheme, founded on the use of a model dielectric function, has been used. The comparison of our results with available theoretical and experimental data turns out to be satisfactory either for the ground state or for the excitation spectrum.

Relying on the results obtained by a total energy, semi-empirical Hartree–Fock method, we discuss polarity effects at the (111) and (110) surfaces of SrTiO3. For these orientations, we consider some prototypical (1×1) configurations, which differ by their surface composition and the coordination number of the surface atoms. We argue that the compensation for these polar orientations is achieved through anomalous fillings of surface states, which, in principle, should be detectable by surface spectroscopies. The compensation does not imply a partial filling, so that all these surfaces keep their insulating character. The thermodynamic stability of the different configurations is also analyzed, and we conclude that, consistently with the experimental observations, these surfaces are relatively stable, especially when compared with polar faces of more ionic oxides, such as MgO(111). The efficiency of SrTiO3 for screening the strong electrostatic perturbation associated to the polar orientations is invoked to explain such a behavior.

Density functional calculations are performed on small neutral and negatively charged TinO2n clusters, with n=1–3. The relative stability of the various isomers results from a subtle competition between ionic and covalent effects in the Ti–O bonding, and is affected by the overall charge state of the cluster. Electron affinities and optical excitation gaps, calculated through total-energy differences, are discussed and compared with recent anion photoelectron data. The excess electron of the negatively charged clusters is mostly localized on titanium with the weakest electrostatic potential, which permits a simple interpretation of the computed electron affinities based on the chemical shifts of the deep Ti levels. The trends obtained for both the electron affinities and the excitation gaps as a function of n help the identification of the most stable isomers consistent with the experimental conditions.

The stability of the stoichiometric (1×1) polar (111) face of SrTiO3 is discussed using a total energy semiempirical Hartree–Fock method. A strong electron redistribution takes place in order to cancel the polarity, but the surface keeps its insulating character. The computed surface energy is close to that of the nonpolar (100) surface, at variance with polar (111) surfaces of rocksalt oxides.

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.

Electronic and structural properties of TiO2 species of various sizes, charges and stoichiometries, ranging from TinOmx clusters (n=1–3, m-n=0, 1, x=-1, 0, +1) to bulk rutile and its (110) surface, have been obtained by total energy calculation based on the density functional theory (DFT), in the local density and local spin density approximations (LSDA), and complemented by a Bader-type analysis of the total electronic density. Attention has been focused on the electron distribution to better understand how the ionocovalent character of the Ti–O bonding and the screening properties vary as a function of the size of the system, the atomic coordination and the surface orientation.

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.

The stability of the polar 111 face of MgO is discussed on the basis of the results obtained by a total-energy semi-empirical Hartree-Fock method, and on general theoretical arguments relying on a perturbative approach of covalent effects in mixed iono-covalent solids. The ideal stoichiometric (1 × 1) surface is studied and compared to two (2 × 1) and (2 × 2) reconstructed surfaces, and to a fully hydroxylated (1 × 1) surface. On the ideal surface, a strong electron redistribution takes place in order to cancel the polarity, which is associated with a metallization of the surface layers and with a high surface energy. A spontaneous symmetry breaking of the Peierls type takes place on this surface, with an opening of a gap at the Fermi level which induces a desorption of half of the surface atoms and a (2 × 1) reconstructed surface configuration. On the two reconstructed surfaces the modification of the stoichiometry in the surface layers provides the compensating charges. As a result, they are much more stable and insulating. The (2 × 2) reconstruction which involves nanopyramids with 001 facets is favored, but we predict a trend to form larger facets in (2 n × 2 n ) reconstructions.

Bonding and electronic properties of non-stoichiometric lithium oxide clusters LinO2 (n⩽10) are studied by means of abinitio simulations. We focus on the first stage of lithium enrichment of stoichiometric Li4O2, where the formation of additional Li–O bonds is favoured. The bonding configuration of the lowest-energy isomers and their stability are analysed in detail, and their structures compared to those found in bulk non-stoichiometric alkali-metal suboxides. As a function of the increasing number of excess Li atoms, coexistence of an ionic and a delocalized character of the electronic density takes place, accompanied by a progressive Li–O bond weakening. These issues, and the existence of odd–even staggering of electronic properties, are discussed, in relation to recent experiments and other anion-deficient systems such as defective oxide surfaces and alkali-metal halide clusters.

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; Atomic structure and thermodynamic stability of vicinal MgO surfaces showing monoatomic steps are studied using density functional theory. We extend the general definition of step energy to the case of ledges that are covered by adsorbates. Using this definition, we consider the effect of hydroxylation on the thermodynamic stability of steps, either polar or non-polar, on (001) or (011) terraces. Clean non-polar steps along [100] on MgO(001) are the most stable ones. Upon water adsorption, the free energy of all hydroxylated ledges, almost independently of their orientation, is significantly reduced and approaches thermal energies for increasing water pressure, favoring the formation of hydroxylated steps on MgO surfaces. Furthermore, we show that interaction between polar steps can be either repulsive or attractive. This quite unusual behavior is described in terms of electrostatic interactions between ledges.

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.

We show the application of an efficient numerical scheme to obtain the independent-particle dynamic polarizability matrix chi((0))(r,r('),omega), a key quantity in modern ab initio excited-state calculations. The method has been applied to the study of the optical response of a realistic oxidized silicon surface, including the effects of crystal local fields. The latter are shown to substantially increase the surface optical anisotropy in the energy range below the bulk band gap. Our implementation in a large-scale ab initio computational code allows us to make a quantitative study of the CPU time scaling with respect to the system size, and demonstrates the real potential of the method for the study of excited states in large systems.

Using density-functional theory, we perform an ab initio study of PbSiO3 alamosite. The lattice parameters and the internal atomic positions are found within 2% of the experimental data. We compute the electronic structure, analyze the valence bands, and identify three groups of electronic bands. The dispersion is very weak, even along the SiO4 tetrahedra chain direction. The analysis of Bader atomic charges allows us to distinguish different types of oxygen atoms. Then, using density-functional perturbation theory, we obtain the electronic dielectric tensor and the Born effective charge tensors of each atomic species. Lead and oxygen atoms are the main polarizable entities, and Born effective charge tensors of some of the oxygen atoms are quite anisotropic.