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YMgNi4-based alloys exhibit reversible hydrogen absorption and desorption reactions at near room temperature. Here, we report that Co-substituted YMgNi4-based alloys exhibited higher hydrogen contents and lower hydrogen absorption and desorption reaction pressures than unsubstituted alloys. The effects of Co-substitution viewed from atomic arrangements were particularly clarified by synchrotron radiation powder X-ray diffraction, neutron diffraction, and inelastic neutron scattering. Powder neutron diffraction of the Co-substituted alloy at 5 MPa of D-2 pressure suggested the formation of gamma-phase deuteride (higher deuterium content) from beta-phase deuteride (lower deuterium content). However, no gamma-phase deuteride was observed in the unsubstituted alloys at 5 MPa. Therefore, the gamma-phase deuteride formation of the Co-substituted alloy at lower pressure led to higher hydrogen contents than the unsubstituted alloys. The combined results of powder neutron diffraction and inelastic neutron scattering suggested that the gamma-phase hydride of the Co-substituted alloy was continuously generated due to additional H atoms at the H atom sites in the beta-phase hydride because of the disordered H atomic arrangement involving H-H interactions. As a result, hydrogen absorption and desorption reaction pressures for the gamma-phase deuteride formation with higher hydrogen storage capacity were lowered.

Water-in-salt systems, i.e., super-concentrated aqueous electrolytes, such as lithium bis(trifluoromethanesulfonyl)imide (21 mol/kgwater), have been recently discovered to exhibit unexpectedly large electrochemical windows and high lithium transference numbers, thus paving the way to safe and sustainable charge storage devices. The peculiar transport features in these electrolytes are influenced by their intrinsically nanoseparated morphology, stemming from the anion hydrophobic nature and manifesting as nanosegregation between anions and water domains. The underlying mechanism behind this structure-dynamics correlation is, however, still a matter of strong debate. Here, we enhance the apolar nature of the anions, exploring the properties of the aqueous electrolytes of lithium salts with a strongly asymmetric anion, namely, (trifluoromethylsulfonyl)(nonafluorobutylsulfonyl) imide. Using a synergy of experimental and computational tools, we detect a remarkable level of structural heterogeneity at a mesoscopic level between anion-rich and water-rich domains. Such a ubiquitous sponge-like, bicontinuous morphology develops across the whole concentration range, evolving from large fluorinated globules at high dilution to a percolating fluorous matrix intercalated by water nanowires at super-concentrated regimes. Even at extremely concentrated conditions, a large population of fully hydrated lithium ions, with no anion coordination, is detected. One can then derive that the concomitant coexistence of (i) a mesoscopically segregated structure and (ii) fully hydrated lithium clusters disentangled from anion coordination enables the peculiar lithium diffusion features that characterize water-in-salt systems.

We introduce and study the notion of contact dual pair adopting a line bundle approach to contact and Jacobi geometry. A contact dual pair is a pair of Jacobi morphisms defined on the same contact manifold and satisfying a certain orthogonality condition. Contact groupoids and contact reduction are the main sources of examples. Among other properties, we prove the Characteristic Leaf Correspondence Theorem for contact dual pairs which parallels the analogous result of Weinstein for symplectic dual pairs., v2: 36 pages, minor revisions, final version to appear in IMRN, comments still welcome!

International audience; In this study the correlation between bulk chemistry and optical properties for a set of Eu-bearing phosphate glasses, containing different alkali elements, synthetized in air and under NH3 flux, was investigated. The chemistry of the glass strongly influences several properties and the effects of bulk chemistry and synthesis conditions on glass structure and on thermal and optical properties are discussed. In alkali-phosphate glasses studied by absorption and photoluminescence spectroscopy we verified that Eu2+ emission band shifts toward higher wavelengths by substituting the alkali cation (Na

LiMn 2O 4 is a promising candidate for a cathode material in lithium-ion batteries because of its ability to intercalate lithium ions reversibly through its three-dimensional manganese oxide network. One of the promising techniques for depositing LiMn 2O 4 thin-film cathodes is atomic layer deposition (ALD). Because of its unparalleled film thickness control and film conformality, ALD helps to fulfill the industry demands for smaller devices, nanostructured electrodes, and all-solid-state batteries. In this work, the intercalation mechanism of Li + ions into an ALD-grown β-MnO 2 thin film was studied. Samples were prepared by pulsing LiO tBu and H 2O for different cycle numbers onto about 100 nm thick MnO 2 films at 225 °C and characterized with X-ray absorption spectroscopy, X-ray diffraction, X-ray reflectivity, time-of-flight elastic recoil detection analysis, and residual stress measurements. It is proposed that for tBu/H 2O, the Li + ions penetrate only to the surface region of the β-MnO 2 film, and the samples form a mixture of β-MnO 2 and a lithium-deficient nonstoichiometric spinel phase Li xMn 2O 4 (0 < x < 0.5). When the lithium concentration exceeds x ≈ 0.5 in Li xMn 2O 4 (corresponding to 100 cycles of LiO tBu/H 2O), the crystalline phase of manganese oxide changes from the tetragonal pyrolusite to the cubic spinel, which enables the Li + ions to migrate throughout the whole film. Annealing in N 2 at 600 °C after the lithium incorporation seemed to convert the films completely to the pure cubic spinel LiMn 2O 4.

A detailed microscopic study of the local geometry was performed for the cation sites in fluoroelpasolite Cs2NaYF6 crystal doped with 3d ions (Cr3+ and Fe3+). Electronic structure of pure, Crdoped and Fe-doped Cs2NaYF6 has been studied by determining various properties: substitution energies of dopant ions, ionic Mulliken charges, atomic coordinates and unit cell parameters for the optimized structures. Also, the optimized local geometry of all three cation sites before and after the possible dopant ions substitutions has been deduced by DFT calculations. It is shown that the local symmetry of both Y3+ and Na+ sites maintains its cubic form after both Fe and Cr substitution; however, Fe ions lead quite larger distortion on ligand lengths at Y-site. The optimized structure has been used independently to calculate zero-field splitting parameters (ZFSPs) and crystal field parameters (CFPs) of both Cr3+ and Fe3+ centers. The calculated ZFSPs of Fe(3+ )ions indicate that the results predicted by SPM for both Y3+ site and Na+ site agree well with the previous experimental observations of ZFSPs. It can be inferred from the findings regarding both electronic structure and local optimized geometry that substitution of both Cr3+ and Fe3+ ions for Y3+ is more likely in C(s)2NaYF(6). (C) 2019 Elsevier Ltd. All rights reserved.

Erbahar, Doğan (Dogus Author) Allowing high transition metal (TM) ions doping concentrations makes the fluoroelpasolite crystals Cs2NaMF6 (M=Al, Ga) quite interesting in various laser applications. In this study, we explored the influence of the dopant ions on the electronic and structural properties of Cs2NaAlF6. Particular attention is paid to the influence of trivalent TM ions (Cr3+ and Fe3+) on the local structure around the possible substitutional cation sites by investigating the optimized geometry around impurity sites due to Cr3+ and Fe3+ using density functional theory (DFT). The obtained structural data are utilized for the calculations of zero-field splitting (ZFS) parameters (ZFSPs) and crystal field parameters (CFPs) of both dopant ions independently in the frame of semi empirical superposition model (SPM). It is shown that remarkable change on the ZFSPs comes not from the distortion on metal-ligand distance but from that on the angular positions of the ligands around the impurity centers. It is clearly shown that the substitution of both Cr3+ and Fe3+ ions take place with significant influence on electronic structure and geometry for the trivalent Al3+ sites. The presented results here have significant potential to better understand the low-symmetry effects in especially optical applications of Cs2NaAlF6:Cr3+ and Cs2NaYF6:Fe3+ systems. Particularly, the calculated CFPs for Cs2NaYF6:Fe3+ system may serve as the only record in literature.

GeTe-Sb2Te3 superlattices have attracted major interest in the field of phase-change memories due to their improved properties compared with their mixed counterparts. However, their crystal structure and resistance-switching mechanism are currently not clearly understood. In this work epitaxial GeTe-Sb2Te3 superlattices have been grown with different techniques and were thoroughly investigated to unravel the structure of their crystalline state with particular focus on atomic stacking and van-der-Waals bonding. It is found that, due to the bonding anisotropy of GeTe and Sb2Te3, the materials intermix to form van-der-Waals heterostructures of Sb2Te3 and stable GeSbTe. Moreover, it is found through annealing experiments that intermixing is stronger for higher temperatures. The resulting ground state structure contradicts the dominant ab-initio results in the literature, requiring revisions of the proposed switching mechanisms. Overall, these findings shed light on the bonding nature of GeTe-Sb2Te3 superlattices and open a way to the understanding of their functionality.

Gehlenite glass microspheres, doped with a different concentration of Bi3+ions (0.5, 1, 3 mol%), were prepared by a combination of solid-state reaction followed by flame synthesis. The prepared glass microspheres were characterized from the point of view of surface morphology, phase composition, thermal and photoluminescence (PL) properties by optical and scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and PL spectroscopy. The closer inspection of glass microsphere surface by SEM confirmed a smooth surface. This was further verified by XRD. The basic thermal characteristics of prepared glasses, i.e.Tg(glass transition temperature),Tx(onset of crystallization peak temperature),Tf(temperature of the inflection point of the crystallization peak) andTp(maximum of crystallization peak temperature), were estimated from the DSC records. High-temperature XRD experiments in the temperature interval range 600–1100°C were also performed. The PL emission properties of prepared glasses and their polycrystalline analogues (glass crystallized at 1000°C for 10 h) were studied in the visible and near-infrared (NIR) spectral range. When excited at 300 nm, the glasses, as well as their polycrystalline analogues, exhibit broad emission in the visible spectral range from 350 to 650 nm centred at about 410–450 nm, corresponding to Bi3+luminescence centres. The emission intensity of polycrystalline samples was found to be at least 30 times higher than the emission of their glass analogues. In addition, a weak emission band was observed around 775 nm under 300 nm excitation. This band was attributed to the presence of a minor amount of Bi2+species in prepared samples. In the NIR spectral range, the broad band emission was observed in the spectral range of 1200–1600 nm with the maxima at 1350 nm. The chemistry of Bi and its oxidation state equilibrium in glasses and polycrystalline matrices is discussed in detail.

The underlying mechanism driving the structural amorphous-to-crystalline transition in Group VI chalcogenides is still a matter of debate even in the simplest GeTe system. We exploit the extreme sensitivity of 57Fe emission Mössbauer spectroscopy, following dilute implantation of 57Mn (T½ = 1.5 min) at ISOLDE/CERN, to study the electronic charge distribution in the immediate vicinity of the 57Fe probe substituting Ge (FeGe), and to interrogate the local environment of FeGe over the amorphous-crystalline phase transition in GeTe thin films. Our results show that the local structure of as-sputtered amorphous GeTe is a combination of tetrahedral and defect-octahedral sites. The main effect of the crystallization is the conversion from tetrahedral to defect-free octahedral sites. We discover that only the tetrahedral fraction in amorphous GeTe participates to the change of the FeGe-Te chemical bonds, with a net electronic charge density transfer of ~ 1.6 e/a0 between FeGe and neighboring Te atoms. This charge transfer accounts for a lowering of the covalent character during crystallization. The results are corroborated by theoretical calculations within the framework of density functional theory. The observed atomic-scale chemical-structural changes are directly connected to the macroscopic phase transition and resistivity switch of GeTe thin films., This work was supported by the European Union Seventh Framework through ENSAR (Contract No. 262010). R. Fallica, C. Wiemer, and M. Longo acknowledge the SYNAPSE project (“SYnthesis and functionality of chalcogenide NAnostructures for PhaSE change memories”), which received funding from the European Union Seventh Framework Programme (FP7/2007–2013), under grant agreement n° 310339. H. P. Gunnlaugsson acknowledges support from the Fund for Scientific Research-Flanders and the KU Leuven BOF (SF/14/013, CREA/14/13, and STRT/14/002). K. Bharuth-Ram, H. Masenda, D. Naidoo, and M. Ncube acknowledge support from the South African National Research Foundation and the Department of Science and Technology. T. E. Mølholt, H. P. Gislason, and S. Ólafsson acknowledge support from the Icelandic Research Fund (Grant No. 110017021-23). The authors acknowledge Numonyx for the GeTe samples and Dr. Enrico Varesi (now at Micron), Dr. Davide Erbetta (now at STMicroelectronics) and Dr. Roberto Bez (now at LFoundry) for scientific discussion.

Er, Er-Ag and Er-Mg co-doped ZnO nanopowders were prepared by wet chemical precipitation method. The structure and morphology of the prepared samples were studied by XRD and SEM, respectively. The effect of Ag and Mg on visible and IR emission properties of Er doped ZnO nanopowders have been systematically investigated. We found that Mg did not change the visible and IR emission properties of Er doped ZnO nanopowders. But, addition of Ag drastically decreases the visible emission and enhances the IR emission at 1536 nm. (C) 2014 Elsevier B.V. All rights reserved.

The structure parameters of temperature and humidity are important in scintillometry as they determine the structure parameter of the refractive index of air, the primary atmospheric variable obtained with scintillometers. In this study, we investigate the variability of the logarithm of the Monin-Obukhov-scaled structure parameters (denoted as $$\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$$ ) of temperature and humidity. We use observations from eddy-covariance systems operated at three heights (2.5, 50, and 90 m) within the atmospheric surface layer under unstable conditions. The variability of $$\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$$ depends on instability and on the size of the averaging window over which $$\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$$ is calculated. If instability increases, differences in $$\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$$ between upward motions (large $$C_{s}^2$$ ) and downward motions (small $$C_{s}^2$$ ) increase. The differences are, however, not sufficiently large to result in a bimodal probability density function. If the averaging window size increases, the variances of $$\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$$ decrease. A linear regression of the variances of $$\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$$ versus the averaging window size for various stability classes shows an increase of both the offset and slope (in absolute sense) with increasing instability. For temperature, data from the three heights show comparable results. For humidity, in contrast, the offset and slope are larger at 50 and 90 m than at 2.5 m. In the end we discuss how these findings could be used to assess whether observed differences in $$C_{s}^2$$ along a scintillometer path or aircraft flight leg are just within the range of local variability in $$C_{s}^2$$ or could be attributed to surface heterogeneity. This is important for the interpretation of data measured above a heterogeneous surface.

Scientific reports 6, 32895 (2016). doi:10.1038/srep32895, Published by Nature Publishing Group, London

International audience; The last few years have seen a great deal of progress in the development of transmission electron microscopy based techniques for strain mapping. New techniques have appeared such as dark field electron holography and nanobeam diffraction and better known ones such as geometrical phase analysis have been improved by using aberration corrected ultra-stable modern electron microscopes. In this paper we apply dark field electron holography, the geometrical phase analysis of high angle annular dark field scanning transmission electron microscopy images, nanobeam diffraction and precession diffraction, all performed at the state-of-the-art to five different types of semiconductor samples. These include a simple calibration structure comprising 10-nm-thick SiGe layers to benchmark the techniques. A SiGe recessed source and drain device has been examined in order to test their capabilities on 2D structures. Devices that have.been strained using a nitride stressor have been examined to test the sensitivity of the different techniques when applied to systems containing low values of deformation. To test the techniques on modern semiconductors, an electrically tested device grown on a SOI wafer has been examined. Finally a GaN/AlN superlattice was tested in order to assess the different methods of measuring deformation on specimens that do not have a perfect crystalline structure. The different deformation mapping techniques have been compared to one another and the strengths and weaknesses of each are discussed. (C) 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Zeolite catalysis is determined by a combination of pore architecture and Bronsted acidity. As Bronsted acid sites are formed by the substitution of AlO4 for SiO4 tetrahedra, it is of utmost importance to have information on the number as well as the location and neighbouring sites of framework aluminium. Unfortunately, such detailed information has not yet been obtained, mainly due to the lack of suitable characterization methods. Here we report, using the powerful atomic-scale analysis technique known as atom probe tomography, the quantitative spatial distribution of individual aluminium atoms, including their three-dimensional extent of segregation. Using a nearest-neighbour statistical analysis, we precisely determine the short-range distribution of aluminium over the different T-sites and determine the most probable Al-Al neighbouring distance within parent and steamed ZSM-5 crystals, as well as assess the long-range redistribution of aluminium upon zeolite steaming.

Different oxide host matrices of ABO(4) with A(3+) (Eu) and B5+ (Nb, Ta, and Sb) were prepared to investigate the solid state luminescence behavior of Eu3+ as a self-activated emitter in orthoceramics. Crystal structures, phonon modes, metal valence states, optical excitation, and emission luminescence properties including emission decay curves, colorimetry, and nonstoichiometry defects were studied using X-ray diffraction (XRD), Raman spectroscopy, photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR). Classical intraconfigurational (f-f transitions), interconfigurational (4f(n-1)5d -> 4f(n)), and charge transfer bands (X5+-O2-) were observed, besides split Stark levels indicating low local symmetry. The influence of host matrices on the optical behavior of orthocompounds was noticed by changes in the energy of the typical Eu3+ transitions. In this case, the larger blue-shifted peaks were observed for EuSbO4 and the larger red-shifted positions for EuNbO4, with respect to emission spectra of orthocompounds studied here. These results were correlated to the respective fifth ionization energies and covalent fraction as well as to the crystal structures exhibited by the different host matrices (I2/a: Nb, Ta; P(2)1/c: Sb). The presence of more than one component for the D-5(0) -> F-7(0) transition in each compound was observed at low temperature, and it could be justified by nonstoichiometric defects like the oxygen vacancy and Eu2+. These typical defects are observed in nonstoichiometric oxide materials, and they were analyzed in the orthoceramics by EPR and XPS spectroscopy. Finally, the lifetime of the D-5(0) state and chromaticity diagrams confirmed our orthoceramics as good optical emitters in the red-end region.

A full understanding of the structural glass variations and of the nucleation and growth of crystalline phases in glass-ceramics (GCs) induced by compositional changes is highly relevant for applications such as sealants for solid oxide fuel cells, bio- and dental materials, and photonics. Systematic substitution of Ca with Sr in diopside (Di-CaMgSi2O6) has a significant impact on the crystalline-phase assemblage after heat treating at 850 degrees C for 500 h. A high percentage of Di crystallized from Sr-free glass; however, with increasing Sr content the tendency toward crystallization decreased, and Sr-akermanite (Sr-Ak) was preferentially crystallized. In this study, the structural transformations in the Di-based-GC system upon the substitution of calcium with strontium have been examined by X-ray powder diffraction (XRD), Si-29 and Al-27 magic angle spinning (MAS)-nuclear magnetic resonance (NMR), and Raman spectroscopies. XRD Rietveld refinement results indicate that up to a maximum of similar to 25 at. % of Sr occupies the Ca site of the Di structure. Al-27 MAS NMR spectra reveal that most of the Al exists in amorphous glassy phase after crystallization. Measured mean quadrupolar coupling constant values augment with increasing Sr content, indicating an increase in order in the glassy phase, which is manifested by a strong preference of Mg2+ for nonbridging oxygens (NBO), resulting in the formation MgO4 tetrahedra. At higher Sr substitution levels (>50%), the decrease in Mg2+ coordination from six to four is evident from the decrease in average Mg-O bond lengths and the systematic variation in intensity of characteristic Mg-O Raman vibrational bands.

Structural investigations of various aliquots of Ce2Zr2O8 were conducted after application of well-defined redox treatments. The results indicate that specific redox procedures induce modifications in the ceria-zirconia mixed oxide, which lead to notable effects on the hydrogen uptake temperature during Temperature Programmed Reduction (TPR). Pyrochlore-type cation ordering is evident for an aliquot which exhibits a single low temperature peak during TPR. However, the cation ordering is not complete in this case. Furthermore, the analysis indicates less extensive pyrochlore-type cation ordering for an aliquot which shows a TPR profile in which there are two peaks, one at high temperature and one at low temperature. Finally, an aliquot that exhibits a single high temperature TPR peak shows no evidence of cation ordering. The trend in the promotion of reduction to low temperature is associated with the extent of the pyrochlore-type cation ordering.

The relationship between optical properties and local structure has been studied in a niobium tellurite glass of molar composition 4.0Nb2O5–87.0TeO2–8.5Al2O3–0.5Eu2O3 using both broadband and laser-induced fluorescence line narrowing techniques. From the broad band excitation and emission spectra information about the local symmetry and the main lanthanide–ligand vibrational modes of the Eu3+ ions in this matrix was obtained. Under laser selective excitation in the 7F0 → 5D0 inhomogeneous band the site-dependent luminescence of the Eu3+ ions allow us to analyze the distribution of local environments for the lanthanide ion in the niobium tellurite glass. Moreover, the energies of the Stark components of the 7F1 and 7F2 multiplets were obtained as a function of the excitation energy and a crystal field analysis was carried out assuming a C2v site symmetry. From the calculated crystal field parameters Bkq the ratios B22/B20 and B44/B40 for each excitation energy were obtained and compared with the values calculated using the geometric model proposed by Brecher and Riseberg.

Two siloxane-based di-urethanesil frameworks incorporating poly(oxyethylene) (POE) chains have been synthesized by the sol-gel process and doped with magnesium triflate (Mg(CF3SO3)(2)) with the goal of developing electrolytes for the fabrication of solid-state rechargeable magnesium batteries. In these matrices, short POE chains are covalently bonded to the siloxane network via urethane linkages. The xerogels have been represented by the notation d-Ut(Y) (n) Mg(CF3SO3)(2), where Y = 300 and 600 represents the average molecular weight of the POE chains and n stands for salt composition (molar ratio of OCH2CH2 units per Mg2+). Xerogels with compositions ranging from 2 a parts per thousand currency signaEuro parts per thousand n < a were prepared. A crystalline POE/Mg(CF3SO3)(2) complex of unknown stoichiometry is formed in the d-Ut(300) (n) Mg(CF3SO3)(2) materials with n a parts per thousand currency signaEuro parts per thousand 6 and in the d-Ut(600) (n) Mg(CF3SO3)(2) materials with n a parts per thousand currency signaEuro parts per thousand 5. The organically modified silicate electrolytes with the highest conductivity of the d-Ut(300) (n) Mg(CF3SO3)(2) and d-Ut(600) (n) Mg(CF3SO3)(2) series are the samples with n = 6 (3.9 x 10(-8) S cm(-1) at 26 A degrees C and 8.7 x 10(-5) S cm(-1) at 97 A degrees C) and n = 100 (2.63 x 10(-7) S cm(-1) at 20 A degrees C and 1.4 x 10(-5) S cm(-1) at 85 A degrees C), respectively. Since the electrolytes for Mg batteries that have been proposed up to now have many intrinsic problems and although the room temperature conductivity values exhibited by the systems developed in the present study are still low in view of practical application, this work opens new directions for the development of solid-state Mg ion electrolytes.

Seven prototypical microporous titanosilicates have been studied by multinuclear solid-state NMR (SSNMR) spectroscopy, representing four typical Ti environments: square-pyramidal TiO5 units (natisite, AM-1, ETS-4), edge-shared brookite-type TiO6 chains (AM-4), cubane-type Ti4O16 clusters (sitinakite, GTS-1), and corner-shared TiO6 chains (ETS-10, ETS-4). Ti-47/49 SSNMR spectra at 21.1 T are related to the coordination, crystal symmetry, and local environment of Ti. Distortions in TiO bond lengths and OTiO coordination angles are reflected via C-Q(Ti-47/49) values that range from 8 to 16 MHz. Several titanosilicates feature axially symmetric Ti-47/49 electric field gradient (EFG) tensors that permit facile spectral assignment and detection of deviations in local symmetry. This study uses Si-29 NMR experiments to assess phase purity and crystallinity. Na-23 NMR is used to probe the location and mobility of the sodium ions in the framework. The potential of K-39 SSNMR for investigation of extra-framework counter cations is demonstrated by ETS-10, with increased spectral resolution and enhanced sensitivity to changes in local environment versus Na-23 experiments. Plane-wave DFT calculations predicted Ti-47/49 NMR parameters assisting in spectral assignments and help correlate Na-23 and Si-29 NMR resonances to crystallographic sites. The approach described in this work should promote further SSNMR investigations of microporous solids, such as titanosilicates, with unknown or poorly defined structures.

We report high-resolution x-ray-absorption near-edge structure measurements at the Mn K edge as a function of temperature, for La1−xCaxMnO3 samples, with a focus mainly on the pre-edge region. Small peaks labeled A1–A3 are observed which correspond to 1s−3d dipole transitions, made weakly allowed via a hybridization of Mn 4p states with Mn 3d states on neighboring atoms. Adjusting the parameters in a local spin-density approximation calculation to approximately match the experimental A1−A2 splitting yields U=4 eV and JH=0.7 eV. For colossal magnetoresistance samples, A1 decreases with T while A2 increases with T below Tc, which shows that the 3d bands change significantly as T moves through Tc. There are also small changes in the shape of the main edge (1s−4p transitions).

The intra-4f transition close to 1.54 mu m of Er implanted into Si shows rich fine structure due to the crystal field of different defect types. Making use of the influence of implantation and annealing parameters, additional doping, temperature, and excitation power, we identify groups of lines belonging to different Er-related, optically active defects: the isolated interstitial Er, axial symmetry Er complexes with oxygen, and Er complex centers containing residual radiation defects. We show that the exciton binding energies as well as nonradiative quenching rates differ for different Er centers. Under optimum annealing conditions, the isolated interstitial Er has the highest photoluminescence yield at temperatures above 100 K.

Anelastic and dielectric spectroscopy measurements on PZT close to the morphotropic (MPB) and antiferroelectric boundaries provide new insight in some controversial aspects of its phase diagram. No evidence is found of a border separating monoclinic (M) from rhombohedral (R) phases, in agreement with recent structural studies supporting a coexistence of the two phases over a broad composition range x < 0.5, with the fraction of M increasing toward the MPB. It is also discussed why the observed maximum of elastic compliance appears to be due to a rotational instability of the polarisation and therefore cannot be explained by extrinsic softening from finely twinned R phase alone, but indicates the presence also of M phase, not necessarily homogeneous. A new diffuse transition is found within the ferroelectric phase near x ~ 0.1, at a temperature T_IT higher than the well established boundary T_T to the phase with tilted octahedra. It is proposed that around T_IT the octahedra start rotating in a disordered manner and finally become ordered below T_T. In this interpretation, the onset temperature for octahedral tilting monotonically increases up to the antiferroelectric transition of PbZrO3, and the depression of T_T(x) below x = 0.18 would be a consequence of the partial relieve of the mismatch between the cation radii with the initial stage of tilting below T_IT., submitted to J. Phys.: Condens. Matter

On the basis of density functional theory and coupled-cluster CCSD(T) calculations we propose a mechanism of the dehydrogenation of transition metal doped sodium alanate. Insertion of two early 3d-transition metals, scandium and titanium, both of which are promising catalysts for reversible hydrogen storage in light metal hydrides, is compared. The mechanism is deduced from studies on the decomposition of a model system consisting of one transition metal atom and two NaAlH(4) units. Subsequently, the significance of such minimal cluster model systems to the real materials is tested by embedding the systems into the surface of the NaAlH(4) crystal. It is found that the dehydrogenation proceeds via breaking of the bridge H-Al bond and consequent formation of intermediate coordination compounds in which the H(2) molecule is side-on (eta(2)-) bonded to the transition metal centre. The total barrier to the H(2) release is thus dependent upon both the strength of the Al-H bond to be broken and the depth of the coordinative potential. The analogous mechanism applies for the recognized three successive dehydrogenation steps. The gas-phase model structures embedded into the surface of the NaAlH(4) crystal exhibit an unambiguous kinetic stability and their general geometric features remain largely unchanged.

International audience; Using real space pair distribution functions derived from high precision x-ray diffraction data, the local atomic structure of Zr-Ni and Zr-Ni-Al metallic glasses was investigated. Unlike Zr-Cu metallic glasses, the structure of Zr-Ni metallic glasses cannot be approached with an ideal solution model, due to strong attractive interactions between Zr- and Ni atoms, which promote chemical short range order. Addition of Al can be beneficial for the glass forming ability of Zr-Ni metallic glasses. The atomic size of Al, being intermediate to those of Zr- and Ni and the strongly attractive interactions between Zr-Al and Zr-Ni atoms can lead to highly negative volumes of mixing Delta V/(mix), and to denser atomic structures, reduced atomic mobility in the liquid and easier suppression of crystallization in the undercooled liquid state

International audience; The high-pressure structure and dynamics of PbZr0.40Ti0.60O3 were investigated by means of neutron diffraction, X-ray diffraction, and resonance Raman spectroscopy. The complex (P4mm, Cm, Cc, F1, F1̅) phase transition sequence is characterized by these techniques. On the basis of the results of structure refinements, the high-pressure behavior of the spontaneous polarization, the (Zr,Ti)O6 rotation angles, and the polarization rotation angle are obtained. Moreover, resonance Raman spectra combined with previous Raman data in the literature provide evidence that the pressure-induced transition to the monoclinic Cm space group and the above transition sequence terminating in a paraelectric state are general features of Pb(Zr1−xTix)O3 (0.48 ≤ x ≤ 1).

The special magnetotransport properties of hole doped manganese perovskites originate from a complex interplay among structural, magnetic and electronic degree of freedom. In this picture the local atomic structure around Mn ions plays a special role and this is the reason why short range order techniques like X-ray absorption spectroscopy (XAS) have been deeply exploited for studying these compounds. The analysis of near edge region features (XANES) of XAS spectra can provide very fine details of the local structure around Mn, complementary to the EXAFS, so contributing to the full understanding of the peculiar physical properties of these materials. Nevertheless the XANES analysis is complicated by the large amount of structural and electronic details involved making difficult the quantitative interpretation. This work exploits the recently developed MXAN code to achieve a full structural refinement of the Mn K edge XANES of LaMnO3 and CaMnO3 compounds; they are the end compounds of the doped manganite series LaxCa1-xMnO3, in which the Mn ions are present only in one charge state as Mn3+ and Mn4+ respectively. The good agreement between the results derived from the analysis of near edge and extended region of the XAS spectra demonstrates that a quantitative picture of the local structure call be obtained from structural refinement of Mn K edge XANES data in these crystalline compounds. The XANES analysis offers, in addition.. the possibility to directly achieve information on the topology of local atomic structure around the absorber not directly achievable from EXAFS. (c) 2005 Elsevier B.V. All rights reserved.

The interfaces between Fe and different high-kappa oxides are investigated by means of conversion electron Mossbauer spectroscopy (CEMS). Information on the magnetic ordering at the interface is obtained from the magnetic hyperfine splitting of the Mossbauer spectra. The reactivity of the Fe atoms at the interface (intermixing) is also estimated by CEMS. X-ray diffraction (XRD) and X-ray reflectivity (XRR) provide additional information on the intermixing and different phases present at the interface. CEM-spectra show the presence of both ferromagnetic and paramagnetic phases. CEMS and XRD results show that the Fe/HfO2 and Fe/Al2O3 interfaces are the least reactive. The degree of intermixing between Fe and the high-kappa oxide is determined by the oxide surface roughness.

Thin films of mixed Ta and Re oxides have been produced by reactive dc magnetron co-sputtering of pure Ta and Re metal targets in Ar-O-2 atmosphere. The structural evolution of these films has been studied as a function of the composition, starting from a pure tantalum oxide film up to about 82% rhenium content. The composition and the structure of the films have been investigated by using X-ray diffraction and micro-Raman spectroscopy. For low Re content (20%), islands of a well crystallized phase, based on ReO4 groups, appear in the films still composed by pure amorphous tantalum oxide, while a mixed disordered solid phase is found for the highest Re concentration (82%). (c) 2006 Elsevier B.V. All rights reserved.

We have investigated by extended x-ray absorption fine structure spectroscopy the local order around Er atoms introduced by ion implantation in substoichiometric silica films prepared by plasma enhanced chemical vapor deposition, where Si nanoclusters have been formed by different preimplantation annealing processes. The results show that Er atoms are surrounded by a first shell of O atoms and no Er-Si direct correlations are observed; moreover, while the variation of the preimplantation annealing temperature has no effect on the Er site, it is observed that the increase of the Er concentration determines an increase of both the Er first shell coordination number and the Er-O interatomic distance, becoming more similar to those of Er2O3. In the presence of an extensive phase separation between Si and SiO2 the local environment around Er plays a crucial role on the efficiency of the photoluminescence emission at 1.54 mu m, which is significantly increased when the first shell of atoms around Er is closer to that one of Er2O3.

One of the limits of using silicate glasses as a host for Er 3+ is the emission bandwidth, which is smaller than in the case of other oxide glasses, such as phosphates and tellurites. The bandwidth can be increased by adding aluminum to the composition. Here we present an experimental investigation of the effect of increasing aluminum content on optical and spectroscopic properties of this glass. Glass samples were prepared by melting at 1550 � C reagent grade SiO2 ,A l 2O3 ,N a 2CO3 ,K 2CO3, CaCO3, (NH4)3PO4 and Er2O3 ;A l 2O3 concentration ranged from 0.99 to 20 mol%. Both heating and cooling rates were equal to 0.17 � Cs � 1 . Measurements of the fluorescence bandwidth were then made and as a function of alumina content. Judd–Ofelt analysis was performed and calculated radiative lifetimes were compared with experimental ones. Planar waveguides were also fabricated by using ion-exchange technology, and some optical properties measured by the prism coupling technique. These waveguides had losses of the order of 1 dB/cm at 635 nm.

The applicability of ultrasoft pseudopotentials to the problem of rare-earth incorporation in silicates is investigated using the compound Er2Si2O7 as a test case. It is found that density-functional theory within the generalized gradient approximation provides a good description of the structural parameters, when treating the Er 4f states as a partially occupied core shell. The density of states and the distribution of electronic charge are analyzed, and it is concluded that the presence of Er tends to increase the covalency of neighboring Si-O bonds.

We report high-resolution x-ray-absorption near-edge structure measurements at the Mn K edge as a function of temperature, for La1−xCaxMnO3 samples, with a focus mainly on the pre-edge region. Small peaks labeled A1–A3 are observed which correspond to 1s−3d dipole transitions, made weakly allowed via a hybridization of Mn 4p states with Mn 3d states on neighboring atoms. Adjusting the parameters in a local spin-density approximation calculation to approximately match the experimental A1−A2 splitting yields U=4 eV and JH=0.7 eV. For colossal magnetoresistance samples, A1 decreases with T while A2 increases with T below Tc, which shows that the 3d bands change significantly as T moves through Tc. There are also small changes in the shape of the main edge (1s−4p transitions).

The intra-4f transition close to 1.54 mu m of Er implanted into Si shows rich fine structure due to the crystal field of different defect types. Making use of the influence of implantation and annealing parameters, additional doping, temperature, and excitation power, we identify groups of lines belonging to different Er-related, optically active defects: the isolated interstitial Er, axial symmetry Er complexes with oxygen, and Er complex centers containing residual radiation defects. We show that the exciton binding energies as well as nonradiative quenching rates differ for different Er centers. Under optimum annealing conditions, the isolated interstitial Er has the highest photoluminescence yield at temperatures above 100 K.

In the last few decades, X-ray Absorption Spectroscopy (XAS) has recorded a remarkable amount of interest in the study of glass structure for its sensitivity to the Short Range Order (SRO) around a selected atomic species. XAS greatest evolution started in the seventies of the last century thanks to an adequate and structurally useful theoretical interpretation of the Extended X-ray Absorption Fine Structure (EXAFS) beyond the absorption edge and to the contemporaneous advent of synchrotron radiation facilities. EXAFS probes the local structure of liquid, vitreous, amorphous, and crystalline materials; it is particularly important for noncrystalline systems since they do not give rise to the Bragg diffraction. EXAFS is also used to determine the local environment of impurities contained in glasses, even if they are very diluted. Today, a reliable theoretical description of glassy system structure through modern modelling techniques should be carried out on the basis of x-ray and/or neutron scattering patterns, along with a careful use of the information derived from SRO techniques like EXAFS, NMR, etc. The use of EXAFS in the case of nanocrystalline glass-ceramic systems is very important, since the obtained information are complemental to the ones determined by the Bragg diffraction. The paper gives a brief description of EXAFS theory and analysis methods in the case of disordered systems. Examples of state-of-the-art studies in different glassy systems will be presented along with a critical assessment of EXAFS potential for such systems.

Temperature dependences of the optic phonons in PbSc1/2Ta1/2O3 (PST) sol-gel films deposited on sapphire substrates were studied by means of Fourier transform far-infrared transmission spectroscopy in the temperature range of 20-900 K. Four films displaying different B-site order with both ferroelectric and relaxor behavior were studied. In all cases the TO mode near 80 cm(-1) at 10 K softens on heating to approximate to 45 cm(-1) following the Cochran law with extrapolated critical temperature near 700 K which is 400 K above the temperature of dielectric maximum, T-m. Above 600 K the TO1 mode remains stabile. This mode can be assigned to the A(1) component of the ferroelectric soft-mode inside polar clusters which form below the Burns temperature near 700 K. In the ordered PST film another mode activates below T-m in the infrared spectra near 60 cm(-1), also exhibiting an anomalous temperature dependence due to its coupling with the former mode. It is assigned to the A(1) component of the F-2g Raman active mode. The central mode, which appears below the Burns temperature in the terahertz range, is assigned to the dynamics of polar clusters. It slows down on cooling and vanishes from our spectral range below T-m. Another overdamped excitation assigned to the E component of the soft mode appears near 30 cm(-1) at low temperature. (c) 2005 American Institute of Physics.

The use of synthetic aperture radar (SAR) to investigate the ocean surface provides a wealth of useful information that is very seldom used to its full potential. Here we will discuss the application of multifractal techniques to detect oil spills and the dynamic state of the sea regarding turbulent diffusion. We present different techniques in order to relate the shape of the multifractal spectral functions and the maximum fractal dimension to the behaviour of the ocean surface. We compare eddy and sheared dominated flows with convective driven flows and discuss the different features and observation methods. We also compare the scaling of different oil spills detected by means of SAR images. Recent spills and weathered ones are selected and compared to investigate their behaviour in different spatial and temporal ranges. We calculate the partition function based on the grey intensity value of each SAR pixel deriving several types of multifractal spectra as a function of spill residence time estimated for each image. Image manipulations are seen to reduce the speckle noise and thus distinguish much better the texture of the oil spill images. The results are used to discuss how eddy diffusivity may be estimated and used in a description of the ocean surface using a simple turbulence kinematic simulation model to predict the shape of oil spills. Differences in the multifractal spectrum among SAR images may detect the slicks due to plankton and also provide information on the age of the oil spills, on the Lagrangian turbulent structure and on ocean surface diffusivity.