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A kind of silicon-and-boron-modified phenolic–formaldehyde resin (SBPF) with excellent thermal stability was fabricated via a simple two-step method and used for the matrix of ablation materials. The structure and thermal stability of the modified phenolic–formaldehyde resins (PFs) were characterised. The results showed that boron and silicon elements have been incorporated into SBPF in the form of a chemical bond. Meanwhile, the cured products showed an increase in graphite structure and a decrease in disordered structure due to the incorporation of boron and silicon. Compared with the ordinary phenolic resin, the initial thermal degradation temperature and charring yield at 800°C of SBPF increased by 73°C and 15 wt%, respectively. Further, the modified PFs, nano-Al(2)O(3) powders, glass powders and vitreous silica fibres were used to obtain a novel ceramizable phenolic moulding composite. The morphology, chemical composition and ablative characteristics of the composites were explored. The results showed that a compact and homogeneous SiO(2)–Al(2)O(3) layer formed on the ablated surface and protected the carbonised matrix and fibres from further oxidation. Compared with the ordinary PF composites, the SBPF composites showed a highly decreased linear/mass ablation rates, indicating the synergistic effect of boron and silicon modification on the enhanced ablation property.

Fluorination of Al2O3 and Y2O3 surfaces was investigated by irradiating high-density, helicon-wave CF4 /O2 and SF6 /O2 plasmas. The Al2O3 surface bombarded by high-flux positive ions of the CF4 /O2 plasma was fluorinated significantly. On contrast, Y2O3 was less fluorinated than Al2O3 when they were irradiated by the same CF4 /O2 plasma. The analysis of the Al2O3 surface irradiated by the CF4 /O2 plasma suggests that the fluorination is triggered by reactions between fluorocarbon deposit and Al–O bonding with the assistance of ion bombardment. On the other hand, irradiation of the SF6 /O2 plasma induced less significant fluorination on the Al2O3 surface. This suggests a lower reaction probability between sulfur fluoride deposit and Al–O bonding. The difference in the fluorination of the Al2O3 and Y2O3 surfaces induced by the irradiations of the CF4 /O2 and SF6 /O2 plasmas is understood by comparing the bonding energies of C–O, S–O, Al–O, and Y–O.

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.

A spinel-type of lithium-rich lithium manganese oxide, Li1.1Mn1.9O4, was prepared and studied electrochemically and structurally as a positive electrode of a Li-ion battery. In cycle performance tests, capacity losses were 3 and 11 mAh/g after 75 cycles at 25 and 55°C, respectively, corresponding to 15 and 50 mAh/g for a normal lithium manganese oxide, LiMn2O4. Their losses were divided into two parts; one resulting from the deterioration of the active material itself and the other due to the poor conduction of the active material to the collector. The former and latter contributions to the losses at 55°C were 2 and 9 mAh/g, respectively for the Li-rich spinel, and 18 and 32 mAh/g, respectively for the normal spinel. The capacity loss by the poor conduction was apparently much larger for both spinels of positive electrodes. Cycle performance tests in each region of the charge states divided into five regions show that relatively larger capacity losses were observed in lower potential states. However, in every region, a capacity loss for the Li-rich spinel was much smaller than that of the normal spinel. There was a close relation found between capacity losses in the storage and those in the cycle. Electron densities spreading between manganese and oxygen atoms were larger in the Li-rich than in the normal spinels. Thus, it turned out that the structure of the Li-rich spinel was made more stable because of strong Mn-O bonds.

We present an in situ study of the formation of the interfaces between TiO2 and SiO2 evaporated thin films using spectroscopic ellipsometry and X-ray photoemission spectroscopy (XPS). The growth of TiO2 on SiO2 was studied previously, but the reverse case has not received much attention up to now. In this article, we show that a common description is valid for both interfaces, which are formed by crosslinking Ti-O-Si bonds. We show also that the growth of TiO2 on SiO2 begins with an amorphous interface layer even when growth occurs at 400 degrees C. The interface is sharp, a few angstroms, as determined by angular XPS; when SiO2 grows on TiO2, the interface is thicker, about 10 AA. Roughness and interdiffusion play roles in interface formation and their role will be discussed.

In this paper, the effect of low energy irradiation on mono-layer graphene was studied. Mono-layer graphene films were irradiated with B, N and F ions at different energy and fluence. X-ray photoelectron spectroscopy indicates that foreign ions implanted at ion energies below 35 eV could dope into the graphene lattice and form new chemical bonds with carbon atoms. The results of Raman measurement indicate that ion beam irradiation causes defects and disorder to the graphene crystal structure, and the level of defects increases with increasing of ion energy and fluence. Surface morphology images also prove that ion beam irradiation creates damages to graphene film. The experiment results suggest that low-energy irradiation with energies of about 30 eV and fluences up to 5·1014 cm−2 could realize small amount of doping, while introducing weak damage to graphene. Low energy ion beam irradiation, provides a promising approach for controlled doping of graphene. Open-Access-Publikationsfonds 2013 peerReviewed

Bimolecular chemical reaction control of gaseous CO and H2 at room temperature and atmospheric pressure, without any catalyst, using shaped femtosecond laser pulses is presented. High intensity laser radiation applied to a reaction cell facilitates non-resonant bond breakage and the formation of a range of ions, which can then react to form new products. Stable reaction products are measured after irradiation of a reaction cell, using time of flight mass spectroscopy. Bond formation of C–O, C–C, and C–H bonds is demonstrated as CO2+, C2H2+, CH+, and CH3+ were observed in the time of flight mass spectrum of the product gas, analyzed after irradiation. The formation of CO2 is shown to be dependent on laser intensity, irradiation time, and on the presence of H2 in the reaction cell. Using negatively chirped laser pulses more C–O bond formation takes place as compared to more C–C bond formation for unchirped pulses. http://dx.doi.org/1063/1.3662129

We have applied a fluids density functional theory based on that of Yu and Wu [J. Chem. Phys. 116, 7094 (2002)] to treat reversible supramolecular polymers near a hard surface. This approach combines a hard-sphere fluids density functional theory with the first-order thermodynamic perturbation theory of Wertheim. The supramolecular polymers are represented in the theory by hard-spheres with two associating sites. We explore the effects of the bonding scheme, monomer concentration, and association energy upon the equilibrium chain sizes and the depletion lengths. This study is performed on simple systems containing two-site monomers and binary mixtures of two-site monomers combined with end stopper monomers which have only a single association site. Our model has correct behavior in the dilute and overlap regimes and the bulk results can be easily connected to simpler random-flight models. We find that there is a nonmonotonic behavior of the depletion length of the polymers as a function of concentration and that this depletion length can be controlled through the concentration of end stoppers. These results are applicable to the study of colloidal dispersions in supramolecular polymer solutions.

Following recent report on antiferromagnetic ordering in EuZrO3 we performed dielectric measurements of this material as a function of temperature and magnetic field. Dielectric constant of dense EuZrO3 ceramics is 30.1 at 300 K. It gradually decreases upon cooling without any quantum paraelectric behavior; however, below TN=4.1 K it shows a pronounced drop that qualitatively resembles that observed in EuTiO3. We report that dielectric constant of EuZrO3 is magnetic field dependent. The magnitude of the magnetodielectric effect in both EuTiO3 and EuZrO3 is discussed in the light of the recently proposed coupling of the Eu–O–Eu superexchange interactions with electrons involved in partially covalent Ti(Zr)–O bond.

We calculate the equilibrium thermodynamic properties, percolation threshold, and cluster distribution functions for a model of associating colloids, which consists of hard spherical particles having on their surfaces three short-ranged attractive sites (sticky spots) of two different types, A and B. The thermodynamic properties are calculated using Wertheim's perturbation theory of associating fluids. This also allows us to find the onset of self-assembly, which can be quantified by the maxima of the specific heat at constant volume. The percolation threshold is derived, under the no-loop assumption, for the correlated bond model: In all cases it is two percolated phases that become identical at a critical point, when one exists. Finally, the cluster size distributions are calculated by mapping the model onto an effective model, characterized by a--state-dependent--functionality f and unique bonding probability p. The mapping is based on the asymptotic limit of the cluster distributions functions of the generic model and the effective parameters are defined through the requirement that the equilibrium cluster distributions of the true and effective models have the same number-averaged and weight-averaged sizes at all densities and temperatures. We also study the model numerically in the case where BB interactions are missing. In this limit, AB bonds either provide branching between A-chains (Y-junctions) if epsilon(AB)/epsilon(AA) is small, or drive the formation of a hyperbranched polymer if epsilon(AB)/epsilon(AA) is large. We find that the theoretical predictions describe quite accurately the numerical data, especially in the region where Y-junctions are present. There is fairly good agreement between theoretical and numerical results both for the thermodynamic (number of bonds and phase coexistence) and the connectivity properties of the model (cluster size distributions and percolation locus).

We report computation results obtained from extensive molecular dynamics simulations of tensile disentanglement of connector chains placed at the interface between two polymer bulks. Each polymer chain (either belonging to the bulks or being a connector) is treated as a sequence of beads interconnected by springs, using a coarse-grained representation based on the Kremer-Grest model, extended to account for stiffness along the chain backbone. Forced reptation of the connectors was observed during their disentanglement from the bulk chains. The extracted chains are clearly seen following an imaginary "tube" inside the bulks as they are pulled out. The entropic and energetic responses to the external deformation are investigated by monitoring the connector conformation tensor and the modifications of the internal parameters (bonds, bending, and torsion angles along the connectors). The work needed to separate the two bulks is computed from the tensile force induced during debonding in the connector chains. The value of the work reached at total separation is considered as the debonding energy G. The most important parameters controlling G are the length (n) of the chains placed at the interface and their areal density. Our in silico experiments are performed at relatively low areal density and are disregarded if chain scission occurs during disentanglement. As predicted by the reptation theory, for this pure pull-out regime, the power exponent from the scaling G proportional to n(a) is a approximate to 2, irrespective of chain stiffness. Small variations are found when the connectors form different number of stitches at the interface, or when their length is randomly distributed in between the two bulks. Our results show that the effects of the number of stitches and of the randomness of the block lengths have to be considered together, especially when comparing with experiments where they cannot be controlled rigorously. These results may be significant for industrial applications, such reinforcement of polymer-polymer adhesion by connector chains, when incorporated as constitutive laws at higher time/length scales in finite element calculations.

International audience; This paper deals with the analysis of nanoparticles polymerized in nitrogen/acetylene and argon/ acetylene gas mixtures in low temperature rf discharges. The polymerization process was monitored by means of mass spectroscopy. The chemical characteristics of the material were obtained in situ by infrared absorption spectroscopy and ex situ by means of near edge x-ray absorption fine structure spectroscopy. These data were supported by complimentary elemental analyses such as deuteron induced gamma emission, Rutherford backscattering, and nuclear reaction analysis. Although morphology showed no differences, further material analysis shows clearly nitrogen incorporation in the nanoparticles, mostly by multiple bonds. In comparison with the nanoparticles from argon/acetylene plasma, the amount of carbon in carbon-nitride nanoparticles remains unchanged, whereas hydrogen content strongly decreases. The results of mass spectroscopy on neutrals and ions lead to the assumption that carbon-nitride nanoparticles are formed by copolymerization of two kinds of precursors: hydrocarbon and nitrogen containing hydrocarbon species.

An efficient linear-scaling approach to the van der Waals density functional in electronic-structure calculations is demonstrated. The nonlocal correlation potential needed in self-consistent calculations is derived in a practical form. This enables also an efficient determination of the Hellmann-Feynman forces on atoms. The numerical implementation employs adaptive quadrature grids in real space resulting in a fast and an accurate evaluation of the functional and the potential. The approach is incorporated in the atomic orbital code SIESTA. The application of the method to the S22 set of noncovalently bonded molecules and comparison with the quantum chemistry data reveal an overall agreement but show that different exchange functionals should be used for different types of bonds. The van der Waals vdW interaction plays an important role in various processes in biochemistry, in surface science, and in physics of layered materials. Although the basic underlying physics of dispersion forces is understood, an accurate and efficient calculation of the vdW interaction is still challenging. The post-Hartree-Fock approaches, based on perturbation theory, are able to treat the vdW interaction. For instance, the coupled-cluster methods 1 especially coupled-cluster with single and double and perturbative triple excitations, CCSDT are very accurate and often used in benchmark calculations. Unfortunately, the high precision comes with enormous computational expenses, making the method fully applicable only for small complexes with less than a few dozen of atoms. The second-order Moller-Plesset perturbation theory MP2, although less expensive and less accurate, also contains the essential physics for describing the vdW interaction. Both of the two methods are not straightforward to apply to extended systems and the existing approaches to periodic systems, as those in Refs. 2 and 3, are fairly expensive and not in common use. Another class of methods for electronic-structure calculations is based on the density-functional theory DFT. The standard approximations to the exchange-correlation energy in DFT, such as the local-density approximation LDA and the generalized-gradient approximation GGA, are relatively cheap and easily applicable also to periodic systems. Unfortunately, these methods cannot describe the dispersion interaction properly. The problem has been addressed by Langreth, Lundqvist and co-workers, 4‐6 who constructed a nonlocal correlation functional aiming to preserve the essential mechanisms behind the vdW interaction. This approach has been successfully utilized for selected test systems and applications. 7‐10 However, most of these results are obtained either using an a posteriori energy correction scheme or a self-consistent calculation with a restricted capability of relaxing the structures. The reason for this is the computational complexity of the self-consistent approach of Ref. 11, which was employed in the earlier self-consistent calculations. In this Rapid Communication, we demonstrate our alternative highly efficient real-space approach. We derive the functional derivative of the vdW density functional vdWDF and obtain a representation, which enables a simpler evaluation of the nonlocal correlation potential and forces for self-consistent calculations with full atomic relaxation. Our implementation employs numerical quadratures on adaptive grids, which vary depending on the local properties of the electron density and are optimized to calculate the vdW-DF. We perform calculations on the widely used molecular benchmark set S22. 12 The results for its molecular complexes provide an extensive test for the reliability of the vdW-DF. Simple examples such as noble gas dimers are not suitable for these purposes as they do not provide a complete picture with respect to the variation in the nature of the bonding. Extended systems are either not discussed as a systematic discussion would require a versatile test set, which does not exist.

Atomic structures and ionic substitutions in sodium titanate $({\text{Na}}_{2\ensuremath{-}x}{\text{H}}_{x}{\text{Ti}}_{3}{\text{O}}_{7})$, having ${\text{Ti}}_{3}{\text{O}}_{7}$ sheetlike layers with ${\text{Na}}^{+}$ and ${\text{H}}^{+}$ in between, were investigated by first-principles calculations. The formation energies via ion exchange of ${\text{Na}}^{+}$ and ${\text{H}}^{+}$ were analyzed by using the supercell total energies and ionic chemical potentials determined with the aid of experimental thermodynamic data. It was found that ionic substitutions of ${\text{Na}}^{+}$ by ${\text{H}}^{+}$ take place even in alkaline solution conditions as found in previous experiments. In addition, the bonding strength between the ${\text{Ti}}_{3}{\text{O}}_{7}$ layers tends to decrease with more ${\text{Na}}^{+}$ substitutions by ${\text{H}}^{+}$, which is related to $p\text{H}$ effects on exfoliation events of ${\text{Ti}}_{3}{\text{O}}_{7}$ sheetlike layers from layered titanate prior to the nanotube formation.

A parameter set describing the Ge-H bonds in the framework of the many-body interatomic potential proposed by Tersoff and Murty has been derived using first-principles calculations. The potential was fitted to reproduce structural, energetic, and vibration properties of gas phase germanium hydrides and radicals. It demonstrates a good transferability for the description of hydrogen terminated germanium surfaces. The potential has been used to simulate the epitaxial growth of a Si layer on a Ge(100) surface using SiH4 precursor molecules. The obtained results faithfully reproduce the impact of chemisorbed hydrogen on the mechanism of Ge diffusion in the grown Si layer. ispartof: Physical Review B, Condensed Matter and Materials Physics vol:79 issue:16 status: published

Wehave performed ab initio calculations for the cubic inverse-perovskite Sc3EN(E=Al,Ga,In) systems to study their electronic band-structures and elastic properties.In this study, we used the accurate augmented plane waveplus local orbital method to find the equilibrium structural parametersand to compute the full elastic tensors. The obtained single-crystalelastic constants were used to quantify the stiffness of theSc-based ternary nitrides and to appraise their mechanical stability. Thesite-projected density of states, Fermi surfaces, and the charge-density plotshave also been used to analyze the chemical bonding betweenthe Sc6N cluster and the surrounding metallic lattice of eitherAl, Ga, or In atoms. Our calculations show that Sc3GaNhas the largest covalent Sc-N bonding-type character with the highestYoung, shear, and bulk moduli. Compared with the other twoisoelectronic systems, it also behaves as the most brittle materialwith a relatively large elastic anisotropy. Original Publication:Maurizio Mattesini, Martin Magnuson, Ferenc Tasnádi, Carina Höglund, Igor A. Abrikosov and Lars Hultman, Elastic properties and electrostructural correlations in ternary scandium-based cubic inverse perovskites: A first-principles study, 2009, Physical Review B. Condensed Matter and Materials Physics, (79), 125122.http://dx.doi.org/10.1103/PhysRevB.79.125122Copyright: American Physical Societyhttp://www.aps.org/

The local reactivity of hydrogenated platinum clusters (Pt clusters) has been studied using the regional density functional theory method. We observed that antibond orbitals constitute the preferable binding site for hydrogen molecules H(2). Those sites are characterized by lowered electronic chemical potential and strong directionality and exhibit electrophilic nature. The platinum-dihydrogen (Pt-H(2)) sigma complexes were formed only by occupation of the lowest electronic chemical potential sites associated with Pt-H antibonds (sigma(PtH) ( *)) in saturated platinum clusters. The formation of sigma complexes caused mutual stabilization with the trans Pt-H bond. Such activated H(2) molecules on Pt clusters in a sense resemble heme-oxygen (heme-O(2)) complex with interaction strength greater than physisorption or hydrogen bonding but below chemisorption strength.

International audience; We report results of a comprehensive study of liquid boron with x-ray measurements of the atomic structure and dynamics coupled with ab initio molecular dynamics simulations. There is no evidence of survival into the liquid of the icosahedral arrangements that characterize the crystal structures of boron but many atoms appear to adopt a geometry corresponding to the pentagonal pyramids of the crystalline phases. Despite similarities in the melting behavior of boron and silicon, there is little evidence of a significant structural shift with temperature that might suggest an eventual liquid-liquid phase transition. Relatively poor agreement with the observed damping of the sound excitations is obtained with the simple form of mode-coupling theory that has proved successful with other monatomic liquids, indicating that higher-order correlation functions arising from directional bonding and short-lived local structures are playing a crucial role. The large ratio of the high frequency to the isothermal sound velocity indicates a much stronger viscoelastic stiffening than in other monatomic liquids.

Experimental findings indicate that the impressively low friction and wear of diamond in humid environments are determined by the surface passivation. In this paper, we investigate the relationship between the surface chemistry and the nanotribological properties of diamond surfaces. We consider the (2x1)-C(001) surface taking into account different terminations constituted of hydrogen, oxygen, and hydroxyl groups. We analyze the adsorbate geometry and the polarization of the surface bonds. We discuss the stability of the different surface terminations in different conditions, which account for the presence in the environment of H-2, O-2, and H2O molecules in different concentrations and we present the surface phase diagram. Finally, we report the calculated adhesion energy between the passivated surfaces and analyze its variation as a function both of the surface separation and of the surface relative lateral position. In this way, we provide information on the effect of the different adsorbates on the interaction between diamond surfaces and on the magnitude and anisotropy of friction forces.

The atomistic and electronic structures of PbTiO3 nanowires with characteristic edges consisting of (100) and (010) surfaces and the crucial role of axial tensile strain on ferroelectricity have been investigated by means of ab initio density-functional theory calculations. Ferroelectricity is enhanced at the edge of the PbO-terminated nanowire because the Pb-O covalent bond that predominates ferroelectric distortions is locally strengthened. On the other hand, a considerable suppression is found in the TiO2-terminated nanowire, caused by the charge transfer from the Pb-O site to the Ti-O site. Surprisingly, the smallest PbO-terminated nanowire with a cross section of only one-unit cell can possess ferroelectricity while ferroelectricity disappears in the TiO2-terminated nanowires with a cross section smaller than four-by-four cells (diameter of about 17 Å). However, ferroelectricity is recovered by axial tension, where smaller nanowires require larger critical strains.

The ferromagnetic phase of an Ising model in d=3, with any amount of quenched antiferromagnetic bond randomness, is shown to undergo a transition to a spin-glass phase under sufficient quenched bond dilution. This result, demonstrated here with the numerically exact global renormalization-group solution of a d=3 hierarchical lattice, is expected to hold true generally, for the cubic lattice and for quenched site dilution. Conversely, in the ferromagnetic-spin-glass-antiferromagnetic phase diagram, the spin-glass phase expands under quenched dilution at the expense of the ferromagnetic and antiferromagnetic phases. In the ferromagnetic-spin-glass phase transition induced by quenched dilution, reentrance as a function of temperature is seen, as previously found in the ferromagnetic-spin-glass transition induced by increasing the antiferromagnetic bond concentration., Scientific and Technological Research Council of Turkey (TÜBİTAK); Academy of Sciences of Turkey

We investigate bond- and site-percolation models on several two-dimensional lattices numerically, by means of transfer-matrix calculations and Monte Carlo simulations. The lattices include the square, triangular, honeycomb kagome and diced lattices with nearest-neighbor bonds, and the square lattice with nearest- and next-nearest-neighbor bonds. Results are presented for the bond-percolation thresholds of the kagome and diced lattices, and the site-percolation thresholds of the square, honeycomb and diced lattices. We also include the bond- and site-percolation thresholds for the square lattice with nearest- and next-nearest-neighbor bonds. We find that corrections to scaling behave according to the second temperature dimension $X_{t2}=4$ predicted by the Coulomb gas theory and the theory of conformal invariance. In several cases there is evidence for an additional term with the same exponent, but modified by a logarithmic factor. Only for the site-percolation problem on the triangular lattice such a logarithmic term appears to be small or absent. The amplitude of the power-law correction associated with $X_{t2}=4$ is found to be dependent on the orientation of the lattice with respect to the cylindrical geometry of the finite systems., Comment: 6 pages, 1 figure, 3 tables

This paper deals with the analysis of nanoparticles polymerized in nitrogen/acetylene and argon/acetylene gas mixtures in low temperature rf discharges. The polymerization process was monitored by means of mass spectroscopy. The chemical characteristics of the material were obtained in situ by infrared absorption spectroscopy and ex situ by means of near edge x-ray absorption fine structure spectroscopy. These data were supported by complimentary elemental analyses such as deuteron induced gamma emission, Rutherford backscattering, and nuclear reaction analysis. Although morphology showed no differences, further material analysis shows clearly nitrogen incorporation in the nanoparticles, mostly by multiple bonds. In comparison with the nanoparticles from argon/acetylene plasma, the amount of carbon in carbon-nitride nanoparticles remains unchanged, whereas hydrogen content strongly decreases. The results of mass spectroscopy on neutrals and ions lead to the assumption that carbon-nitride nanoparticles are formed by copolymerization of two kinds of precursors: hydrocarbon and nitrogen containing hydrocarbon species.

Theelectronic structure of nanocrystalline (nc-) TiC/amorphous C nanocomposites has beeninvestigated by soft x-ray absorption and emission spectroscopy. The measuredspectra at the Ti 2p and C 1s thresholds of the nanocompositesare compared to those of Ti metal and amorphous C.The corresponding intensities of the electronic states for the valenceand conduction bands in the nanocomposites are shown to stronglydepend on the TiC carbide grain size. An increased chargetransfer between the Ti 3d-eg states and the C 2p states hasbeen identified as the grain size decreases, causing an increasedionicity of the TiC nanocrystallites. It is suggested that thecharge transfer occurs at the interface between the nanocrystalline-TiC andthe amorphous-C matrix and represents an interface bonding which maybe essential for the understanding of the properties of nc-TiC/amorphousC and similar nanocomposites. Original Publication: Martin Magnuson, Erik Lewin, Lars Hultman and Ulf Jansson, Electronic structure and chemical bonding of nanocrystalline-TiC/amorphous-C nanocomposites, 2009, Physical Review B. Condensed Matter and Materials Physics, (80), 235108. http://dx.doi.org/10.1103/PhysRevB.80.235108 Copyright: American Physical Society http://www.aps.org/

Extended x-ray absorption fine structure has been measured at the K edge of copper in CuScO2 and CuLaO2 to investigate the local origin of negative thermal expansion (NTE) along the c axis. A positive expansion of the Cu-O bond has been measured within the entire temperature interval to be contrasted with the negative expansion of the distance between average atomic positions measured by diffraction; the inadequacy of the riding correction to diffraction data has been evidenced. The weak temperature dependence of the parallel mean-square relative displacement (MSRD) indicates a rather stiff Cu-O bond, while the large perpendicular MSRD measures an intense relative motion of copper with respect to oxygen atoms perpendicular to the c axis. The comparison between the two compounds enlightens the relevance of the correlation of perpendicular motion to quantitatively explain the apparent NTE of the Cu-O bond.

The electronic structure and chemical bonding of the recently discovered inverse perovskite Sc3AlN, in comparison to ScN and Sc metal have been investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured Sc L, N K, Al L1, and Al L2,3 emission spectra are compared with calculated spectra using first principle density-functional theory including dipole transition matrix elements. The main Sc 3d - N 2p and Sc 3d - Al 3p chemical bond regions are identified at -4 eV and -1.4 eV below the Fermi level, respectively. A strongly modified spectral shape of 3s states in the Al L2,3 emission from Sc3AlN in comparison to pure Al metal is found, which reflects the Sc 3d - Al 3p hybridization observed in the Al L1 emission. The differences between the electronic structure of Sc3AlN, ScN, and Sc metal are discussed in relation to the change of the conductivity and elastic properties., 11 pages, 5 pictures