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The Hall symbols for describing unambiguously the generators of space groups have been extended to describe any setting of the 1651 types of magnetic space groups (Shubnikov groups). A computer program called MHall has been developed for parsing the Hall symbols, generating the full list of symmetry operators and identifying the transformation to the standard setting.

We report the preparation and electrochemical properties of Na3V(PO3)3N made by ammonolysis. Na3V(PO3)3N is reversibly oxidized to Na2V(PO3)3N at high voltage (4.0V vs. Na+/Na0 and 4.1V vs. Li+/Li0) with an unusually small difference in the insertion/extraction voltage between both alkali metal reference electrodes. In both cases, the voltage hysteresis is extremely small (~0.035V vs. sodium and ~0.065V vs. lithium), which suggests facile migration of alkali cations within the structure. Further oxidation to NaV(PO3)3N is predicted to occur beyond the voltage stability window of the electrolytes. Keywords: Li-ion batteries, Na-ion batteries, Positive electrode material, Cathode, Nitridophosphate

Solid state electrolytes could address the current safety concerns of lithium-ion batteries as well as provide higher electrochemical stability and energy density. Among solid electrolyte contenders, garnet-structured Li7La3Zr2O12 appears as a particularly promising material owing to its wide electrochemical stability window; however, its ionic conductivity remains an order of magnitude below that of ubiquitous liquid electrolytes. Here, we present an innovative dual substitution strategy developed to enhance Li-ion mobility in garnet-structured solid electrolytes. A first dopant cation, Ga3+, is introduced on the Li sites to stabilize the fast-conducting cubic phase. Simultaneously, a second cation, Sc3+, is used to partially populate the Zr sites, which consequently increases the concentration of Li ions by charge compensation. This aliovalent dual substitution strategy allows fine-tuning of the number of charge carriers in the cubic Li7La3Zr2O12 according to the resulting stoichiometry, Li7–3x+yGaxLa3Zr2–yScyO12. The coexistence of Ga and Sc cations in the garnet structure is confirmed by a set of simulation and experimental techniques: DFT calculations, XRD, ICP, SEM, STEM, EDS, solid state NMR, and EIS. This thorough characterization highlights a particular cationic distribution in Li6.65Ga0.15La3Zr1.90Sc0.10O12, with preferential Ga3+ occupation of tetrahedral Li24d sites over the distorted octahedral Li96h sites. 7Li NMR reveals a heterogeneous distribution of Li charge carriers with distinct mobilities. This unique Li local structure has a beneficial effect on the transport properties of the garnet, enhancing the ionic conductivity and lowering the activation energy, with values of 1.8 × 10–3 S cm–1 at 300 K and 0.29 eV in the temperature range of 180 to 340 K, respectively.

Progress in energy-related technologies demands new and improved materials with high ionic conductivities. Na- and Li-based compounds have high priority in this regard owing to their importance for batteries. This work presents a high-throughput exploration of the chemical space for such compounds. The results suggest that there are significantly fewer Na-based conductors with low migration energies as compared to Li-based ones. This is traced to the fact that, in contrast to Li, the low diffusion barriers hinge on unusual values of some structural properties. Crystal structures are characterized through descriptors derived from bond-valence theory, graph percolation and geometric analysis. A machine-learning analysis reveals that the ion migration energy is mainly determined by the global bottleneck for ion migration, by the coordination number of the cation and by the volume fraction of the mobile species. This workflow has been implemented in the open-source Crystallographic Fortran Modules Library (CrysFML) and the program BondStr. A ranking of Li- and Na-based ionic compounds with low migration energies is provided.

The many advantages of Na-ion batteries (NIBs) in terms of availability and cost of raw materials compared with Li-ion batteries (LIBs) are hindered by the stability of Na-based electrodes. The most promising NIB positive electrodes are Co- and Ni-free sodium manganese rich layered oxides with the general formula (y < 0.33, TM = transition metal/s). Although their stability is greatly improved when doped with electrochemically inactive species such as Mg or Ti, the rationale behind this has not been understood to date. Here, we demonstrate how a given degree of TiIV doping (z = 0.1) helps to stabilize the crystal structure of sodium manganese rich layered oxides by absorbing electrochemically induced strain; a remarkable step forward on the quest of finding the best NIB positive electrode. In this case, any Mn–Ti substitution below z = 0.1 will not be enough to absorb the strain and substitutions above this value will increase the amount of Jahn–Teller active MnIII leading to destabilization of the crystal structure with poor electrochemical performance. The possibility of controlling structural and electrochemical properties by TM substitution is the starting point towards the design of electrode materials that will ultimately lead towards competitive Na-ion batteries.

We investigate the evolution of the cross-plane thermal conductivity κ of a SiGe superlattice (SL) as it is gradually converted into an alloy via post-growth thermal treatment at temperatures varying from 650 to 1000 °C. X-ray diffraction (XRD), transmission electron microscopy (TEM), secondary-ion-mass-spectroscopy (SIMS), and photoluminescence (PL) spectroscopy measurements are used as complementary tools to gain insight into the structural properties of our SL and their evolution upon annealing. While the SL structure is preserved up to temperatures of ∼850 °C, full alloying is observed for higher temperatures. The thermal conductivity data, collected with differential 3ω method, show a monotonic increase of κ from ∼4.5 W/m K up to the values expected for a thin-film alloy. To understand the results, we compute the phonon mean-free-path (MFP) spectra using the experimentally determined composition profiles as input. The calculated thermal conductivity values are in good agreement with the experimental data and show that the increase of thermal conductivity is due to a gradual increase of MFP of low-to-mid-frequency phonons, i.e., to a weakening of interface scattering for alloyed SLs. The calculations also allow us to address finite-thickness effects on the measured thermal conductivity data. Although the used SL had a total thickness of only ∼250 nm, its thermal conductivity can be assumed to coincide with that of a “bulk” (infinitely thick) SL prior to annealing. As interdiffusion increases, boundary scattering at the film/substrate interface becomes relevant and leads to a slow increase of κ. Besides its fundamental relevance, this work shows that the thermal stability of superlattices is limited, and that sizeable structural changes (including defect formation) occur already at operation temperatures of ∼650 °C.

Alkali-metal superoxides are gaining increasing interest as 2p magnetic materials for information and energy storage. Despite significant research efforts on bulk materials, gaps in our knowledge of the electronic and magnetic properties at the nanoscale still remain. Here, we focused on the role that structural details play in determining stability, electronic structure, and magnetic couplings of (MO2)n (M = Li, Na, and K, with n = 2–8) clusters. Using first-principles density functional theory based on the Perdew-Burke-Ernzerhof and Heyd-Scuseria-Ernzerhof functionals, we examined the effect of atomic structure on the relative stability of different polymorphs within each investigated cluster size. We found that small clusters prefer to form planar-ring structures, whereas non-planar geometries become more stable when increasing the cluster size. However, the crossover point depends on the nature of the alkali metal. Our analysis revealed that electrostatic interactions govern the highly ionic M–O2 bond...

Using a first principles theoretical approach, we show that vacancies give anomalously strong suppression of the lattice thermal conductivity $\ensuremath{\kappa}$ of cubic Boron arsenide (BAs), which has recently been predicted to have an exceptionally high $\ensuremath{\kappa}$. This effect is tied to the unusually large phonon lifetimes in BAs and results in a stronger reduction in the BAs $\ensuremath{\kappa}$ than occurs in diamond. The large changes in bonding around vacancies cannot be accurately captured using standard perturbative methods and are instead treated here using an ab initio Green function approach. As and B vacancies are found to have similar effects on $\ensuremath{\kappa}$. In contrast, we show that commonly used mass disorder models for vacancies fail for large mass ratio compounds such as BAs, incorrectly predicting much stronger (weaker) phonon scattering when the vacancy is on the heavy (light) atom site. The quantitative treatment given here contributes to fundamental understanding of the effect of point defects on thermal transport in solids and provides guidance to synthesis efforts to grow high quality BAs.

The aim of this paper is to illustrate the use of Multi-Wavelength Anomalous Diffraction (MAD) and Diffraction Anomalous Fine Structure (DAFS) spectroscopy for the study of structural properties of semiconductor nanostructures. We give a brief introduction on the basic principles of these techniques providing a detailed bibliography. Then we focus on the data reduction and analysis and we give specific examples of their application on three different kinds of semiconductor nanostructures: Ge/Si nanoislands, AlN capped GaN/AlN Quantum Dots and AlGaN/AlN Nanowires. We show that the combination of MAD and DAFS is a very powerful tool to solve the structural problem of these materials of high technological impact. In particular, the effects of composition and strain on diffraction are disentangled and composition can be determined in a reliable way, even at the interface between nanostructure and substrate. We show the great possibilities of this method and give the reader the basic tools to undertake its use.

During the past two decades, the demand for the Electrical Energy Storage systems has increased for both portable and stationary applications, predominantly in the form of batteries [1]. In this sense, lithium-ion batteries have dominated the market as they offer the highest gravimetric capacity. However, owing to concerns over lithium cost and sustainability of resources, there is a growing interest in sodium-ion batteries since sodium chemistry is considered as an attractive alternative to LIBs in terms of cost, safety, and sustainability [2]. Current research into cathode materials for sodium ion batteries encompasses a wide range of different chemistries. Amongst them, polyanionic compounds exhibit a very stable framework that enables facile low-energy alkali migration pathways leading to long-term structural stability. In this sense, theoretical calculations have recently suggested that N-based polyanionic compounds might be the next frontier in electrode design for Li-ion and Na-ion batteries [3-4]. Partial substitution of O by N lowers the voltage at which the transition metal is oxidized, which could permit to operate more than one redox couple per transition metal. In addition, nitrogen could also participate to the alkali de-intercalation through a reversible oxidation of the NIII- anions. This kind of compounds could thus open the path to new attractive multi-electron materials, combining high energy with the safety and cost advantages of the polyanionic frameworks. In this context, our group is exploring the synthesis and the electrochemical performances of a series of new lithium- and sodium-based N-substituted polyanionic compounds. Among them, we will present the attractive reversible redox activity at medium and high voltage showed by some members of the family of CUBICON compounds NaxMy(PO3)3N. [5-7] Acknowledgements This work is financially supported by the Spanish Ministero de Economía y Competitividad (MINECO) through the projects ENE2013-44330-R, ENE2016-81020-R and ENE2016-75242-R and the post-doctoral grants FPDI-2013-17329 and FJCI-2014-19990. The authors would also like to thank the Basque Government for financial support through ELKARTEK project CICE17. References [1] J. M. Tarascon and M. Armand, Nature 414 (2001) 359-367 [2] V. Palomares, P. Serras, I. Villaluenga, K. B. Hueso, J. Carretero-González and T. Rojo, Energy Environ. Sci. 5 (2012) 5884-5901 [3] M. Armand and M. E. Arroyo y de Dompablo. J. Mater. Chem. 21 (2011) 10026-10034. [4] B. Rousseau, V. Timoshevskii, N. Mousseau, M. Côté and K. Zaghib. Mater. Sc. Eng. B. 211 (2016) 185-190. [5] J. Liu, X. Yu, E. Hu, K.W. Nam, X.Q Yang and P. Khalifah. Chem. Mater. 25 (2013) 3929-3931. [6] J. Liu, D. Chang, P. Whitfield, Y. Janssen, X. Yu, Y. Zhou, J. Bai, J. Ko, K-W. Nam, L. Wu, Y. Zhu, M. Feygenson, G. Amatucci, A. Van der Ven, X.Q. Yang and P. Khalifah. Chem. Mater. 26 (2014) 3695-3305. [7] M. Reynaud, A. Wizner, M. Galceran, L. Loaiza, T. Rojo, M. Armand and M. Casas-Cabanas, 84 (2017) 14-18.

The preparation, characterization and comparison of nanostructured carbons derived by direct chlorination of Cr 3 C 2 and Cr(C 5 H 7 O 2 ) 3 are reported in this work. Cr 3 C 2 precursor was treated at 400 and 900 °C with a reaction time of 1 h. The nanostructure of the products has been characterized in some detail by means of transmission electron microscopy and associated techniques, such as electron energy-loss and X-ray energy dispersive spectroscopies and high-angle annular dark field imaging. Remains of Cr 3 C 2 encapsulated in an amorphous carbon shell were observed at 400 °C, whereas carbon with higher ordering degree was produced at 900 °C. In the latter case, the sample can be described as a continuous variation from poorly-stacked graphene-like carbon to graphitic agglomerates. Remains of the reaction by-product, CrCl 3 , are detected in the carbon particles, forming monolayers intercalated inside the graphitic agglomerates and amorphous nanoparticles. As a comparison, carbon samples derived from Cr(C 5 H 7 O 2 ) 3 were prepared at 300 and 900 °C. They mainly consist of highly disordered carbon, with local graphite-like stacking in the sample prepared at 900 °C.

6 pags., 9 figs., 1 tab., The chlorination of ferrocene at 900 °C yields very interesting carbon hollow nanospheres (CHNSs) with diameters of ∼50-150 nm and ∼12-25 nm thick walls. X-ray energy-dispersive spectroscopy shows no traces of chlorine or iron and the π*/σ* ratio of the carbon bonding was quantified by electron energy-loss spectroscopy with 80% sp2 (100% sp2 for pure graphite). Energy-filtered transmission electron microscopy has been used to establish the hollow nature of the CHNSs by thickness mapping. Electron energy loss spectroscopy, high-resolution TEM, and Raman microspectroscopy techniques have stated that the CHNS carbon walls are composed of disordered and independent curved graphene nanoflakes ∼3-4 nm long that tend to graphitize with longer reaction times. © 2007 American Chemical Society., The authors are thankful for financial support from the UCM-Santander project with reference PR 27/05-13982.

New carbon materials have been synthesized by chlorinating niobium carbide at different temperatures. During the reaction, the volatile niobium chloride, mainly NbCl5, is formed and eliminated leaving the carbon in the shape of spherical particles. TEM examination of the remaining particles revealed that at 400 °C and 700 °C they are composed of a NbC core with an amorphous carbon shell outside. The NbC cores are very small (∼20 nm) at 700 °C and they are not observed at 900 °C. Carbon particles without the NbC core present a more ordered structure composed of concentric wavy graphene layers. This structure seems intermediate between carbon onions and carbon blacks. The 900 °C sample presents a very high BET surface area (1292 m2 g−1) and the lower temperature samples exhibit a hysteresis phenomenon that can be attributed to the existence of large pores in the interspace between the NbC core and the carbon shell.

We show that impurities and vacancies affect the thermal conductivity much more strongly than what is predicted by widely accepted models. When local distortions around point defects are strong, standard perturbative approaches fail, and phonon scattering can only be accounted for by an exact Green's function calculation. We apply the theory to the study, from first-principles, of nitrogen and vacancy defects in diamond. The thermal conductivity is computed by solving the linearized Boltzmann transport equation. The Born approximation underestimates the phonon scattering cross sections of nitrogen and vacancies by factors of 3 and 10, respectively. Thermal conductivity calculations are in good agreement with experiment.

Under the terms of the Creative Commons Attribution 3.0 Unported License., In this work, we investigate the local atomic structure of defect-free homogeneous and self-organized core-shell structure nanowires by means of X-ray Absorption Fine Structure (XAFS) Spectroscopy at the In LIII and K edges and Multiwavelength Anomalous Diffraction. The results are interpreted by comparison of the experimental data with X-ray absorption calculations carried out with ab initio structural models. Extended-XAFS data analysis at In K-edge shows an anisotropic In distribution in the second nearest neighbors pointing out to a deviation from randomness in In distribution for the core-shell sample. © 2014 AIP Publishing LLC., M.G.P. acknowledges the support of project MAT2009-03074 of Spanish Ministry of Science and Innovation and is grateful to the Diputación General de Aragón for granting support in the frame of the Researchers Mobility program (Grant No. 224-183). C.L., G.T., B.D., and H.R. acknowledge the French ANR for support (Fidel project ANR 2011 NANO 02902).

In this paper we review simulation and experimental studies of thermal capillary wave fluctuations as an ideal means for probing the underlying disjoining pressure and surface tensions, and more generally, fine details of the Interfacial Hamiltonian Model. We discuss recent simulation results that reveal a film-height-dependent surface tension not accounted for in the classical Interfacial Hamiltonian Model. We show how this observation may be explained bottom-up from sound principles of statistical thermodynamics and discuss some of its implications., Comment: File is accepted version with 70 pages and 13 figures. Submitted 23/08/2013; Accepted 06/11/2013; Online 17/11/2013

It has been highly debated whether the thermal conductivity κ of a disordered SiGe alloy can be lowered by redistributing its constituent species so as to form an ordered superlattice. By ab initio calculations backed by systematic experiments, we show that Ge segregation occurring during epitaxial growth can lead to κ values not only lower than the alloy's, but also lower than the perfect superlattice values. Thus we theoretically demonstrate that κ does not monotonically decrease as the Si- and Ge-rich regions become more sharply defined. Instead, an intermediate concentration profile is able to lower κ below both the alloy limit (total intermixing) and also the abrupt interface limit (zero intermixing). This unexpected result is attributed to the peculiar behavior of the phonon mean free path in realistic Si/Ge superlattices, which shows a crossover from abrupt-interface- to alloylike values at intermediate phonon frequencies of ∼3 THz. Our calculated κ's quantitatively agree with the measurements when the realistic, partially intermixed profiles produced by segregation are considered.

Our understanding of both structure and dynamics of adsorbed liquids heavily relies on the capillary wave Hamiltonian, but a thorough test of this model is still lacking. Here we study the capillary wave fluctuations of a liquid film with short range forces adsorbed on a solid exhibiting van der Waals interactions. We show for the first time that the measured capillary wave spectrum right above the first order wetting transition provides an interface potential consistent with independent calculations from thermodynamic integration. However, the surface tension exhibits an oscillatory film thick dependence which reveals a hitherto unnoticed capillary wave broadening mechanism beyond mere interfacial displacements., 5 pages (+ 1 supplemental), 3 figures; Published in Phys Rev Lett

Transition metal layered oxides are promising cathode materials for sodium-ion batteries. Phase transitions involving different stacking sequences of the oxide layers often plague the electrochemistry of these materials during cycling, which strongly impacts in their electrochemical performance. However, the underlying mechanisms of these processes remain elusive. Interestingly, P2- and O3-Na2/3Fe2/3Mn1/3O2 phases are the first transition metal layered oxide polymorphs that have been synthesized with exactly the same composition. This offers unprecedented access to the study of bistability in these systems as well as isolates the effect of local structure on Na ion mobility. Here, first-principles calculations and experiments are combined to unveil the physical origin of such bistability and identify important differences in Na ion diffusion between these two phases. It has been found that electrostatic interactions between oxide layers control the bistable nature of P2 and O3 phases. It is also put forward that the interlayer distance between oxide layers may be a useful descriptor to rationalize the relative stability of other P and O phases in general. Furthermore, this study tracks down to the molecular level the differences regarding Na ion mobility in P2- and O3-Na2/3Fe2/3Mn1/3O2 by computing activation energies and estimating diffusion coefficients.

Usingab initiocalculations we have investigated the thermal conductivity (κ) of diamond nanowires, unveiling unusual features unique to this system. In sharp contrast with Si, κ(T) of diamond nanowires as thick as 400 nm still increase monotonically with temperature up to 300 K, and room-temperature size effects are stronger than for Si. A marked dependence of κ on the crystallographic orientation is predicted, which is apparent even at room temperature. [001] growth direction always possesses the largest κ in diamond nanowires. The predicted features point to a potential use of diamond nanowires for the precise control of thermal flow in nanoscale devices.

The lattice thermal conductivity of (Bi${}_{1\ensuremath{-}x}$Sb${}_{x}$)${}_{2}$Te${}_{3}$ alloys, with and without spherical nanoparticle inclusions, has been computed using the Boltzmann transport equation with classical interatomic potentials within the relaxation time approximation. The experimental thermal conductivity of the pure alloys is fairly well reproduced as a function of concentration. We establish upper and lower limits for the thermal conductivities that could be obtained via seamlessly embedded spherical nanoparticles as a function of their size, density, and volume fraction. Large reductions in thermal conductivities (40--50$%$) can only be achieved with small nanoparticles with diameters below 10 nm.

We propose a method to account for the long tail corrections of dispersive forces in inhomogeneous systems. This method deals separately with the two interfaces that are usually present in a simulation setup, effectively establishing semi-infinite boundary conditions that are appropriate for the study of the interface between two infinite bulk phases. Using the wandering interface method, we calculate surface free energies of vapor–liquid, wall–liquid, and wall–vapor interfaces for a model of Lennard– Jones argon adsorbed on solid carbon dioxide. The results are employed as input to Young’s equation, and the wetting temperature located. This estimate is compared with predictions from the method of effective interface potentials and good agreement is found. Our results show that truncating Ar–Ar interactions at two and a half molecular diameters results in a dramatic decrease of the wetting temperature of about 40%., We would like to thank Marcus Müller for suggesting us to describe the cutoff dependence of wetting properties by means of the sharp-kink approximation (cf., Sec. V). We also benefitted from helpful discussions with P. Bryk, A. Archer, and E. de Miguel. Generous financial support of Ministerio de Educacion y Ciencia through Project Nos. FIS2010- 22047-C05-05 and FIS2010-14866; Comunidad Autónoma de Madrid through Project No. MODELICO-P2009/ESP- 1691; and Junta de Andalucía through Project No. P07- FQM02884 is gratefully acknowledged.

The potential to control the composition and crystal phase at the nanometer scale enable the production of nanocrystalline materials with enhanced functionalities and new applications. In the present work, we detail a novel colloidal synthesis route to prepare nanoparticles of the ternary semiconductor Cu2GeSe3 (CGSe) with nanometer-scale control over their crystal phases. We also demonstrate the structural effect on the thermoelectric properties of bottom-upprepared CGSe nanomaterials. By careful adjustment of the nucleation and growth temperatures, pure orthorhombic CGSe nanoparticles with cationic order or polytypic CGSe nanoparticles with disordered cation positions can be produced. In this second type of nanoparticle, a high density of twins can be created to periodically change the atomic plane stacking, forming a hexagonal wurtzite CGSe phase. The high yield of the synthetic routes reported here allows the production of single-phase and multiphase CGSe nanoparticles in the gram scale, which permits characterization of the thermoelectric properties of these materials. Reduced thermal conductivities and a related 2.5-fold increase of the thermoelectric figure of merit for multiphase nanomaterials compared to pure-phase CGSe are systematically obtained. These results are discussed in terms of the density and efficiency of phonon scattering centers in both types of materials.

8 pags, 6 figs, 2 tabs, In the present paper, we aim to show the interest of combining Multiwavelength Anomalous Diffraction (MAD) and Diffraction Anomalous Fine Structure (DAFS) spectroscopy, in grazing incidence, to obtain structural properties (composition, strain and atomic ordering) of semiconductor heterostructures and nanostructures. As an example we report on preliminary results obtained on a series of Ge/Si(001) nano-island samples: pyramides and domes on nominal and prepatterned surfaces. For free standing domes, it is shown that the Ge content strongly depends on the growth condition with a tendency to increase from the bottom to the top of the nano-islands. There is also some indication of atomic ordering in the upper part of the islands. For small, capped pyramids, we show that the Diffraction Anomalous Fine Structure spectroscopy is the unique non destructive method that allows to recover the actual Ge content, the in-plane and out-of-plane strain and to detect atomic ordering. © EDP Sciences and Springer 2009.

The preparation of nanocarbons and the study of their promising applications have attracted considerable attention in the last years [1,2]. Carbide-derived carbons (CDC) are a well-known group of nanostructured carbon materials, prepared by direct chlorination of the metal carbides according to the following reaction: MCx(s) + y/2 Cl2(g) → x C(s) + MCly(g). Previously, we have reported the preparation of CDC from NbC [3] as well as the results of the direct chlorination of metallocenes: ferrocene [4] and cobaltocene [5]. In this work we are presenting the characterization of nanostructured carbon samples obtained from Cr3C2 and from Cr(acac)3 - acac: acetylacetonate, (C5H7O2)− -, mainly by means of TEM and associated techniques (EDX, EELS), employing a CM 200FEG microscope.

With the concourse of a variety of experimental techniques (neutron diffraction, x-ray photoelectron spectroscopy, $^{13}\text{C}$ nuclear magnetic resonance, electron microscopy, and Raman spectroscopy) and a combination of reverse Monte Carlo, molecular dynamics, and Monte Carlo simulations, we propose a model for the microscopic structure of a sample of carbon nanospheres obtained from chlorination of cobaltocene. The sample, which exhibits a high porosity, is shown to be formed by a series of interconnected sheets of graphene. Despite the large degree $(\ensuremath{\approx}80%)$ of $s{p}^{2}$ hybridization shown by carbon atoms, there is a non-negligible amount of $s{p}^{3}$-bonded carbons, some of them acting as links between graphene sheets. The transmission electron microscopy images simulated from the microscopic structure, which is extracted from the neutron diffraction data by a mixed reverse Monte Carlo--molecular dynamics/Monte Carlo procedure, agree remarkably well with the experimental results.

9 pages, 9 figures, 3 tables., In this work we investigate the microscopic structure and dynamics of the molten equimolar alloy, Se50Te50 using a combination of neutron and x-ray diffraction experiments, reverse Monte Carlo analysis, and first principles molecular dynamics. The range of temperatures studied covers the semiconductor/metal transition. From our results it can be seen that the latter is associated with an increase in coordination numbers and a reduced tendency to heterocoordination. In agreement with previous inelastic neutron scattering experiments, our molecular dynamics calculation predict a certain widening of the stretching vibrational modes band in connection with the increase of coordination and the presence of longer bonds in the metallic phase., This research project has been supported by the European Commission under the sixth Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures Contract No. RII3-CT- 2003-505925. Financial support of the Dirección General de Investigación Científica y Técnica under Grant No. FIS2004- 02954-C03-01 and the Dirección General de Universidades e Investigación de la Comunidad de Madrid under Grant No. S0505/ESP/0299, program MOSSNOHO-CM, is gratefully acknowledged. Generous computer time allocation at the Mare Nostrum supercomputer facility of the Barcelona Supercomputing Center (Centro Nacional de Supercomputación) is also acknowledged. One of us (P.Z.) gratefully acknowledges the Spanish Ministry of Education and Science for support of his sabbatical stay at the IQFR under Grant No. SAB2005-0042. Use of the Advanced Photon Souce was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Science, under Contract No. DE-AC02-CH11357. Dr. Peter L. Lee is gratefully acknowledged for the use of his heading device in the APS measurements.

In this work we report some new well-defined carbon nanostructures produced by direct chlorination of metallocenes (ferrocene and cobaltocene) and NbC, at temperatures from 100 to 900 8C. Thus, amorphous carbon nanotubes with variable dimensions depending on reaction temperature were produced from ferrocene. When cobaltocene is the carbon precursor the main product are solid amorphous nanospheres. The high refractory metal carbide NbC as carbon source favours the growth of nanospherical cabbage-like particles with a higher degree of graphene sheets order. Besides, NbC crystallites encapsulated in an amorphous carbon shell were also found at lower temperatures (T � 700 8C). # 2006 Elsevier Ltd. All rights reserved.

In this work we present an investigation on the composition dependence of the local structure in Sex Te1-x crystalline alloys analyzing their experimental energy-loss spectra with the aid of a real-space multiple-scattering modeling approach and first-principles molecular dynamics. The concourse of this latter technique is essential for a proper modeling of the alloy spectra. From our results, it can be inferred that Sex Te1-x alloys exhibit a high degree of substitutional disorder ruling out the existence of fully ordered alternating copolymer chains of Se and Te atoms. © 2006 The American Physical Society.

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

We have studied the effect of nanocrystalline microstructure on the thermal conductivity of SiGe alloys using molecular dynamics simulations. Nanograins are modeled using both the coincidence site lattice and the Voronoi tessellation methods, and the thermal conductivity is computed using the Green-Kubo formalism. We analyze the dependence of the thermal conductivity with temperature, grain size L, and misorientation angle. We find a power dependence of L1/4 of the thermal conductivity with the grain size, instead of the linear dependence shown by non-alloyed nanograined systems. This dependence can be derived analytically underlines the important role that disorder scattering plays even when the grains are of the order of a few nm. This is in contrast to non-alloyed systems, where phonon transport is governed mainly by the boundary scattering. The temperature dependence is weak, in agreement with experimental measurements. The effect of angle misorientation is also small, which stresses the main role pla...

Using ab-initio calculations, we argue that the apparent frequency dependence of the thermal conductivity measured by time-domain thermoreflectance (TDTR) is majorly determined by the balance between harmonic (elastic) and anharmonic (inelastic) phonon channels, and not by ballistic phonons. The match between this theory and experiment is closer than that obtained with previous models. Concrete trends in frequency and temperature dependence are predicted. Reinterpreting frequency modulated time-domain thermoreflectance in terms of elastic vs. inelastic channels explains the markedly different behavior between alloys and non-alloys, and the approach can in principle be applied to other nanostructured material systems.

Silicon (Si) nanoparticles embedded in a Mg2Si matrix (Mg2Si/xSi) have been successfully synthesized at 623 K from MgH2 and Bi containing Si nanoparticle powders. The use of MgH2 in this synthetic route avoids the formation of oxides through the generation of hydrogen and provides a route to homogeneously mixed Si nanoparticles within a doped Mg2Si matrix. The samples were characterized by powder X-ray diffraction, thermogravimetry/differential scanning calorimetry (TG/DSC), electron microprobe analysis (EMPA), and scanning transmission electron microscopy (STEM). The final crystallite size of Mg2Si obtained from the XRD patterns is about 50 nm for all the samples and the crystallite size of Si inclusions is approximately 17 nm. Theoretical calculations indicate that ∼5 mol% concentrations of Si nanoparticles with diameters in the 5–50 nm range could decrease the lattice thermal conductivity of Mg2Si by about 1–10% below the matrix value. Reduction in thermal conductivity was observed with the smallest amount of Si, 2.5 mol%. Larger amounts, x = 10 mol%, did not provide any further reduction in thermal conductivity. Analysis of the microstructure of the Bi doped Mg2Si/xSi nanocomposites showed that the Bi dopant has a higher concentration at grain boundaries than within the grains and Bi preferentially substitutes the Mg site at the boundaries. The nanocomposite carrier concentration and mobility depend on the amount of Bi and Si inclusions in a complex fashion. Agglomerations of Si start to show up clearly in the Bi doped 5 mol% nanocomposite. While the inclusions result in a lower thermal conductivity, electrical resistivity and Seebeck are negatively affected as the presence of Si inclusions influences the amount of Bi dopant and therefore the carrier concentration. The x = 2.5 mol% nanocomposite shows a consistently higher zT throughout the measured temperature range until the highest temperatures where a dimensionless figure of merit zT ∼ 0.7 was obtained at 775 K for Mg2Si/xSi with x = 0 and 2.5 mol%. With optimization of the electronic states of the matrix and nanoparticle, further enhancement of the figure of merit may be achieved.

In this paper, we show that combining Multiwavelength Anomalous Diffraction (MAD) and Diffraction Anomalous Fine Structure (DAFS) spectroscopy, in grazing incidence geometry, allows to obtain structural properties (strain and composition) of semiconductor nanostructures. We report results obtained on dome-shaped Ge nano-islands grown on Si(001) surfaces and AlGaN nanowires grown on Si(100). It is shown that, in the case of sharp interfaces, MAD alone can not determine the mean Ge content in the region of the substrate-island interface and needs to be combined with Extented-DAFS measurements.