Synergy of diffraction and spectroscopic techniques to unveil the crystal structure of antimonic acid

The elusive crystal structure of the so-called 'antimonic acid' has been investigated by means of robust and state-of-the-art techniques. The synergic results of solid-state magic-angle spinning nuclear magnetic resonance spectroscopy and a combined Rietveld refinement from synchrotron X-ray and neutron powder diffraction data reveal that this compound contains two types of protons, in a pyrochlore-type structure of stoichiometric formula (H₃O)_1.20(7)H_0.77(9)Sb₂O₆. Some protons belong to heavily delocalized H₃O⁺ subunits, while some H⁺ are directly bonded to the oxygen atoms of the covalent framework of the pyrochlore structure, with O-H distances close to 1 Å. A proton diffusion mechanism is proposed relying on percolation pathways determined by bond-valence energy landscape analysis. X-ray absorption spectroscopy results corroborate the structural data around Sb⁵⁺ ions at short-range order. Thermogravimetric analysis and differential scanning calorimetry endorsed the conclusions on the water content within antimonic acid. Additional 0.7 water molecules per formula were assessed as moisture water by thermal analysis.

Strongly reduced lattice thermal conductivity in Sn-doped rare-earth (M) filled skutterudites M x Co4Sb12-y Sn y , promoted by Sb-Sn disordering and phase segregation

CoSb3 thermoelectric skutterudite has been filled with rare-earth metals (M = La, Ce, Yb) and partially doped with Sn in specimens of M x Co4Sb12-y Sn y stoichiometry. This has been achieved under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. A structural investigation using synchrotron X-ray diffraction data reveals a phase segregation in twin skutterudite phases with filling fraction fluctuation and different unit-cell sizes. As a result of three effects acting as phonon scatterers, namely the rattling effect of M at the wide 8a cages of the cubic Im3̄ structure, the phase segregation, and the intrinsic disorder introduced by Sn substitution at the Sb sublattice, the total thermal conductivity (κ) dramatically falls to reach minimum values under 2 W m-1 K-1, well below those typically exhibited by other thermoelectric materials based upon single-filled skutterudites. The power factor is substantially enhanced to 1.11 mW m-1 K-2 in Yb0.5Co4Sb11.6Sn0.4 with respect to the unfilled composition, as a result of the charge transfer promoted by the filler.

Correction and verification of x-ray imaging crystal spectrometer analysis on Wendelstein 7-X through x-ray ray tracing

X-ray ray tracing is used to develop ion-temperature corrections for the analysis of the X-ray Imaging Crystal Spectrometer (XICS) used at Wendelstein 7-X (W7-X) and perform verification on the analysis methods. The XICS is a powerful diagnostic able to measure ion-temperature, electron-temperature, plasma flow, and impurity charge state densities. While these systems are relatively simple in design, accurate characterization of the instrumental response and validation of analysis techniques are difficult to perform experimentally due to the requirement of extended x-ray sources. For this reason, a ray tracing model has been developed that allows characterization of the spectrometer and verification of the analysis methods while fully considering the real geometry of the XICS system and W7-X plasma. Through the use of ray tracing, several important corrections have been found that must be accounted for in order to accurately reconstruct the ion-temperature profiles. The sources of these corrections are described along with their effect on the analyzed profiles. The implemented corrections stem from three effects: (1) effect of sub-pixel intensity distribution during de-curving and spatial binning, (2) effect of sub-pixel intensity distribution during forward model evaluation and generation of residuals, and (3) effect of defocus and spherical aberrations on the instrumental response. Possible improvements to the forward model and analysis procedures are explored, along with a discussion of trade-offs in terms of computational complexity. Finally, the accuracy of the tomographic inversion technique in stellarator geometry is investigated, providing for the first time a verification exercise for inversion accuracy in stellarator geometry and a complete XICS analysis tool-chain.

Structural, electrical characterization and oxygen-diffusion paths in LaSrGa1−xMgxO4−δ (x = 0.0–0.2) layered perovskites: an impedance spectroscopy and neutron diffraction study

We present the diffusion pathways in perovskite-like structures. The modules of Gfourir and BondStr of Fullprof are feasible to obtain different paths in first approximation.

Defective Sr0.9Mo0.9O3−δ perovskites with exsolved Ni nanoparticles as high-performance composite anodes for solid-oxide fuel cells

An A-site deficient perovskite with metallic Ni in exsolution, Ni–Sr_0.9Mo_0.9O_3−δ, has been prepared, characterized and tested as an anode material in intermediate-temperature solid-oxide fuel cells (IT-SOFCs).

Topotactic Oxidation of Perovskites to Novel SrMo1-xMxO4-δ (M = Fe and Cr) Deficient Scheelite-Type Oxides

New polycrystalline SrMo_1−xM_xO_4−δ (M = Fe and Cr) scheelite oxides have been prepared by topotactical oxidation, by annealing in air at 500 °C, from precursor perovskites with the stoichiometry SrMo_1−xM_xO_3−δ (M = Fe and Cr). An excellent reversibility between the oxidized Sr(Mo,M)O_4−δ scheelite and the reduced Sr(Mo,M)O_3−δ perovskite phase accounts for the excellent behavior of the latter as anode material in solid-oxide fuel cells. A characterization by X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) has been carried out to determine the crystal structure features. The scheelite oxides are tetragonal, space group I4₁/a (No. 88). The Rietveld-refinement from NPD data at room temperature shows evidence of oxygen vacancies in the structure, due to the introduction of Fe³⁺/Cr⁴⁺ cations in the tetrahedrally-coordinated B sublattice, where Mo is hexavalent. A thermal analysis of the reduced perovskite (SrMo_1−xM_xO_3−δ) in oxidizing conditions confirms the oxygen stoichiometry obtained by NPD data; the stability range of the doped oxides, below 400–450 °C, is lower than that for the parent SrMoO₃ oxide. The presence of a Mo⁴⁺/Mo⁵⁺ mixed valence in the reduced SrMo_1−xM_xO_3−δ perovskite oxides confers greater instability against oxidation compared with the parent oxide. Finally, an XPS study confirms the surface oxidation states of Mo, Fe, and Cr in the oxidized samples SrMo_0.9Fe_0.1O_4-δ and SrMo_0.8Cr_0.2O_4-δ.

Charge exchange recombination spectroscopy at Wendelstein 7-X

The Charge Exchange Recombination Spectroscopy (CXRS) diagnostic has become a routine diagnostic on almost all major high temperature fusion experimental devices. For the optimized stellarator Wendelstein 7-X (W7-X), a highly flexible and extensive CXRS diagnostic has been built to provide high-resolution local measurements of several important plasma parameters using the recently commissioned neutral beam heating. This paper outlines the design specifics of the W7-X CXRS system and gives examples of the initial results obtained, including typical ion temperature profiles for several common heating scenarios, toroidal flow and radial electric field derived from velocity measurements, beam attenuation via beam emission spectra, and normalized impurity density profiles under some typical plasma conditions.

Structural characterization and electrochemical properties of (La,Sr)(Al,Mg)O4−δ perovskites

LaSrAl_1−xMg_xO_4−δ (x = 0.0–0.3) layered perovskites were synthesized by a nitrate–citrate route followed by annealing in air at 1100 °C, and studied as potential electrolyte materials in solid oxide fuel cells (SOFCs).

Water insertion and combined interstitial-vacancy oxygen conduction in the layered perovskites La1.2Sr0.8−xBaxInO4+δ

H₂O molecules are split within the structure with protons bonded to the axial oxygens of the InO₆ octahedra, and with OH units occupying the interstitial space.

Prospects of X-ray imaging spectrometers for impurity transport: Recent results from the stellarator Wendelstein 7-X (invited)

This paper reports on the design and the performance of the recently upgraded X-ray imaging spectrometer systems, X-ray imaging crystal spectrometer and high resolution X-ray imaging spectrometer, installed at the optimized stellarator Wendelstein 7-X. High resolution spectra of highly ionized, He-like Si, Ar, Ti, and Fe as well as H-like Ar have been observed. A cross comparison of ion and electron temperature profiles derived from a spectral fit and tomographic inversion of Ar and Fe spectra shows a reasonable match with both the spectrometers. The also measured impurity density profiles of Ar and Fe have peaked densities at radial positions that are in qualitative agreement with the expectations from the He-like impurity fractional abundances, given the measured temperature profiles. Repeated measurements of impurity decay times have been demonstrated with an accuracy of 1 ms via injection of non-recycling Ti, Fe, and Mo impurities using a laser blow-off system.

Usefulness of the twinkling artifact on Doppler ultrasound for the detection of breast microcalcifications

OBJECTIVE

To determine whether the twinkling artifact on Doppler ultrasound imaging corresponds to microcalcifications previously seen on mammograms and to evaluate the usefulness of this finding in the ultrasound management of suspicious microcalcifications.

MATERIAL AND METHODS

We used ultrasonography to prospectively examine 46 consecutive patients with groups of microcalcifications suspicious for malignancy identified at mammography, searching for the presence of the twinkling artifact to identify the microcalcifications. Once we identified the microcalcifications, we obtained core-needle biopsy specimens with 11G needles and then used X-rays to check the specimens for the presence of microcalcifications. We analyzed the percentage of detection and obtainment of microcalcifications by core-needle biopsy with this technique and the radiopathologic correlation. Microcalcifications that were not detected by ultrasound or discordant lesions were biopsied by stereotaxy at another center. We also used ultrasound guidance for preoperative marking with clips, usually orienting them radially.

RESULTS

We identified and biopsied 41 of the 46 lesions under ultrasound guidance, including 24 of 25 carcinomas (17 in situ). B-mode ultrasound was sufficient for biopsying the microcalcifications in 14 patients, although the presence of the twinkling artifact increased the number of microcalcifications detected and thus enabled more accurate preoperative marking. Thanks to the twinkling sign, we were able to identify 27 additional groups of microcalcifications (89% vs. 30%; p < 0.05). All the surgical specimens had margins free of disease.

CONCLUSIONS

The twinkling artifact is useful for microcalcifications in ultrasound examinations, enabling a significant increase in the yield of ultrasound-guided biopsies and better preoperative marking of groups of microcalcifications.

Technical aspects and complications in the surgical treatment of poliomyelitis-affected lower limb fractures

OBJECTIVE

Post-polio patients present problems such as small and deformed bones, with narrow intramedullary canal and osteoporosis, affecting surgical treatment. The aim of this article is to describe the main preoperative and intraoperative complications of the surgical treatment of fractures in this population.

MATERIAL AND METHODS

A retrospective analysis was conducted between 1995 and 2014. Data obtained from the medical records included patient age, fracture pattern (AO/OTA), device used, technical aspects of the surgery that changed compared to a standard procedure, and the presence of intraoperative skeletal complications.

RESULTS

Sixty-four patients with 78 fractures were included in the study. Forty-seven percent of the fractures were at the proximal femur. The main complications of hip arthroplasty (14 patients) were absent hip abductors and intraoperative instability (3), bad cup fixation (3) and intraoperative periprosthetic fracture (2). The main problems of intramedullary nailing were due to a narrow canal and previous bone deformity. Main problems reported when plating included difficulty to fit a precontoured plate, and oversized hardware.

CONCLUSION

Given the large number of intraoperative complications, in preoperative planning we must include nails of small diameter and length, locking plates and external fixators, and, in the case of hip arthroplasty, long and thin stems and restrictive or dual mobility acetabular systems.

Experimental evidence for bipolaron condensation as a mechanism for the metal-insulator transition in rare-earth nickelates

Many-body effects produce deviations from the predictions of conventional band theory in quantum materials, leading to strongly correlated phases with insulating or bad metallic behavior. One example is the rare-earth nickelates RNiO3, which undergo metal-to-insulator transitions (MITs) whose origin is debated. Here, we combine total neutron scattering and broadband dielectric spectroscopy experiments to study and compare carrier dynamics and local crystal structure in LaNiO3 and NdNiO3. We find that the local crystal structure of both materials is distorted in the metallic phase, with slow, thermally activated carrier dynamics at high temperature. We further observe a sharp change in conductivity across the MIT in NdNiO3, accompanied by slight differences in the carrier hopping time. These results suggest that changes in carrier concentration drive the MIT through a polaronic mechanism, where the (bi)polaron liquid freezes into the insulating phase across the MIT temperature.

Observation of Oscillatory Radial Electric Field Relaxation in a Helical Plasma

Measurements of the relaxation of a zonal electrostatic potential perturbation in a nonaxisymmetric magnetically confined plasma are presented. A sudden perturbation of the plasma equilibrium is induced by the injection of a cryogenic hydrogen pellet in the TJ-II stellarator, which is observed to be followed by a damped oscillation in the electrostatic potential. The waveform of the relaxation is consistent with theoretical calculations of zonal potential relaxation in a nonaxisymmetric magnetic geometry. The turbulent transport properties of a magnetic confinement configuration are expected to depend on the features of the collisionless damping of zonal flows, of which the present Letter is the first direct observation.

Structural origin of the enhanced ionic conductivity upon Nb doping in Sr11Mo4O23 defective double perovskite

We report a substantial enhancement of the oxide-ion conductivity in Sr₁₁Mo₄O₂₃ achieved by Nb doping the Mo sites. This series responds to the formula: Sr₁₁Mo_4-xNb_xO_23-δ (with x = 0.0, 0.5 and 1.0). The original structure can be related to the conventional double perovskite; however, it presents a broken corner sharing connectivity of the octahedral framework, hence leading to a complex and highly defective network. The samples were prepared via citrate precursor method, followed by thermal treatments at 1300 °C for 12 hours in air. The crystal structures were refined from X-ray and neutron powder diffraction (NPD) data. A phase transition from tetragonal to cubic symmetry is identified in a temperature-dependent NPD study, driven by an oxygen delocalization effect. The ionic conductivity measured by impedance spectroscopy is enhanced upon Nb-doping; the x = 1 doped phase exhibits a threefold increase compared to the pristine Sr₁₁Mo₄O₂₃ oxide, with conductivity values of 7.6 × 10^-3 and 2.7 × 10^-2 S·cm^-1 at 650 and 800 °C, which are even greater than for YSZ in the 650-800 °C temperature range, and close to those reported for other state-of-the art solid-oxide electrolytes.

Ionic conductivity enhancement in Ti-doped Sr11Mo4O23 defective double perovskites

A substantially higher ionic motion can be achieved by partially replacing Mo(vi) by Ti(iv) cations in the novel Sr₁₁Mo_4−xTi_xO_23−δ (with x = 0.0, 0.5 and 1.0) electrolyte oxides, successfully enhancing the oxygen vacancy level.

Adsorption and growth of palladium clusters on graphdiyne

The density functional formalism has been used to investigate the stability and the properties of small palladium clusters supported on graphdiyne layers.

Nanostructured Bi2Te3 Prepared by a Straightforward Arc-Melting Method

Thermoelectric materials constitute an alternative source of sustainable energy, harvested from waste heat. Bi2Te3 is the most utilized thermoelectric alloy. We show that it can be readily prepared in nanostructured form by arc-melting synthesis, yielding mechanically robust pellets of highly oriented polycrystals. This material has been characterized by neutron powder diffraction (NPD), scanning electron microscopy (SEM), and electronic and thermal transport measurements. A microscopic analysis from NPD data demonstrates a near-perfect stoichiometry of Bi2Te3 and a fair amount of anharmonicity of the chemical bonds. The as-grown material presents a metallic behavior, showing a record-low resistivity at 320 K of 2 μΩ m, which is advantageous for its performance as a thermoelectric material. SEM analysis shows a stacking of nanosized sheets, each of them presumably single-crystalline, with large surfaces perpendicular to the c crystallographic axis. This nanostructuration notably affects the thermoelectric properties, involving many surface boundaries that are responsible for large phonon scattering factors, yielding a thermal conductivity as low as 1.2 W m(-1) K(-1) around room temperature.

Structural phase transition in polycrystalline SnSe: a neutron diffraction study in correlation with thermoelectric properties

SnSe has been recently reported as a promising and highly efficient thermoelectric intermetallic alloy. The present material has been prepared by arc melting, as mechanically robust pellets, consisting of highly oriented polycrystals. The evolution of its orthorhombic GeS-type structure (space groupPnma) and phase transition to TlI-type structure (space groupCmcm) at high temperature has been studiedin situby neutron powder diffraction (NPD) in the temperature range 295–873 K. This transition has been identified by differential scanning calorimetry measurements, yielding sharp peaks at 795 K. In addition, thermal transport properties were measured in a similar temperature range, and large Seebeck coefficients, as high as 1050 µV K−1at 625 K, were found. The analysis from NPD data demonstrates an almost perfect stoichiometry, Sn0.998(8)Se, that does not evolve with temperature, and a progressive decrease of the anharmonicity of the chemical bonds upon entering the domain of theCmcmstructure.

Giant Seebeck effect in Ge-doped SnSe

Thermoelectric materials may contribute in the near future as new alternative sources of sustainable energy. Unprecedented thermoelectric properties in p-type SnSe single crystals have been recently reported, accompanied by extremely low thermal conductivity in polycrystalline samples. In order to enhance thermoelectric efficiency through proper tuning of this material we report a full structural characterization and evaluation of the thermoelectric properties of novel Ge-doped SnSe prepared by a straightforward arc-melting method, which yields nanostructured polycrystalline samples. Ge does not dope the system in the sense of donating carriers, yet the electrical properties show a semiconductor behavior with resistivity values higher than that of the parent compound, as a consequence of nanostructuration, whereas the Seebeck coefficient is higher and thermal conductivity lower, favorable to a better ZT figure of merit.

Effect of the Pb(2+) lone electron pair in the structure and properties of the double perovskites Pb2Sc(Ti0.5Te0.5)O6 and Pb2Sc(Sc0.33Te0.66)O6: relaxor state due to intrinsic partial disorder

We describe the preparation, the crystal structure refined from neutron powder diffraction (NPD) data, and study of the permittivity of two related double perovskites, Pb2Sc(Ti0.5Te0.5)O6 and Pb2Sc(Sc0.33Te0.66)O6. These compounds were synthesized by standard ceramic procedures; Rietveld refinements from room temperature NPD data show that the crystal structures are well defined in a cubic unit cell (space group Fm3m) with double parameter, a = 2a0 ≈ 8 Å. They contain a completely ordered array of ScO6 and (B,Te)O6 (B = Sc, Ti) octahedra sharing corners; the PbO12 polyhedra present an off-center displacement of the lead atoms along the [1 1 1] directions, due to the electrostatic repulsion between the Pb(2+) 6 s electron lone-pair and the Pb-O bonds of the cuboctahedron. Both compounds present a low temperature, highly dispersive maximum in permittivity, the position of which follows the Vogel-Fulcher relation with freezing temperatures of 156 and 99 K for Pb2Sc(Ti0.5Te0.5)O6 and Pb2Sc(Sc0.33Te0.66)O6, respectively, exhibiting a typical phenomenology of relaxors.

Experimental visualization of the diffusion pathway of sodium ions in the Na3[Ti2P2O10F] anode for sodium-ion battery

Sodium-ion batteries have attracted considerable interest as an alternative to lithium-ion batteries for electric storage applications because of the low cost and natural abundance of sodium resources. The materials with an open framework are highly desired for Na-ion insertion/extraction. Here we report on the first visualization of the sodium-ion diffusion path in Na₃[Ti₂P₂O₁₀F] through high-temperature neutron powder diffraction experiments. The evolution of the Na-ion displacements of Na₃[Ti₂P₂O₁₀F] was investigated with high-temperature neutron diffraction (HTND) from room temperature to 600°C; difference Fourier maps were utilized to estimate the Na nuclear-density distribution. Temperature-driven Na displacements indicates that sodium-ion diffusion paths are established within the ab plane. As an anode for sodium-ion batteries, Na₃[Ti₂P₂O₁₀F] exhibits a reversible capacity of ~100 mAh g⁻¹ with lower intercalation voltage. It also shows good cycling stability and rate capability, making it promising applications in sodium-ion batteries.

Visualization by neutron diffraction of 2D oxygen diffusion in the Sr(0.7)Ho(0.3)CoO(3-δ) cathode for solid-oxide fuel cells

Sr0.7Ho0.3CoO3-δ oxide has been recently described as an excellent cathode material (1274 mW cm(-2) at 850 °C with pure H2 as fuel1) for solid oxide fuel cells (SOFCs) with LSGM as electrolyte. In this work, we describe a detailed study of its crystal structure conducted to find out the correlation between the excellent performance as a cathode and the structural features. The tetragonal crystal structure (e.g., I4/mmm) basically contains layers of octahedrally coordinated Co2O6 units alternated with layers of Co1O4 tetrahedra sharing corners. An "in situ" neutron power diffraction (NPD) experiment, between 25 and 800 °C, reveals the presence of a high oxygen deficiency affecting O4 oxygen atoms, with large displacement factors that suggest a large lability and mobility. Difference Fourier maps allow the visualization at high temperatures of the 2D diffusion pathways within the tetrahedral layers, where O3 and O4 oxygens participate. The measured thermal expansion coefficient is 16.61 × 10(-6) K(-1) between 300 and 850 °C, exhibiting an excellent chemical compatibility with the electrolyte.

Synthesis and Characterization of the Oxynitride Y2Mo2O4.5N2.5 Pyrochlore: A Neutron Diffraction and Magnetic Study

A new molybdenum oxynitride Y2Mo2O4.5N2.5 with cubic pyrochlore structure (a = 10.3350(2) Å, space group Fd3̅m, Z =8) has been synthesized by heating the parent Y2Mo2O7 oxide in flowing ammonia at 898 K. The polycrystalline sample has been characterized by thermal analysis, X-ray and neutron diffraction (NPD), and magnetic susceptibility measurements. The analysis of high resolution NPD data, based on the contrast existing between the scattering lengths of O and N, shows that both atoms are distributed at random at the anion substructure; the refined crystallographic formula implies an oxidation state of +5.25 for Mo cations. The thermogravimetric curve shows a weight gain of 7.5% at 1000 K in air, corresponding to the complete elimination of N2 and oxidation to Mo(VI) oxide, in good agreement with the proposed composition. The magnetic susceptibility exhibits a Pauli-like, temperature-independent term which derives from the partial delocalization of Mo electrons on Mo-(O,N) bands with a broader bandwidth, as a consequence of the significant opening of the Mo-(O,N)-Mo angle and strengthening of the Mo-(O,N) interactions with respect to the parent Y2Mo2O7 oxide. As in this oxide, a reminiscent spin-glass behaviour is observed at low temperature.

Synthesis, Crystal and Magnetic Structure of the New Double Perovskite Ba2MnMoO6

We describe the preparation and characterization of a new double perovskite of formula Ba2MnMoO6. It has been obtained in polycrystalline form by thermal treatment, in a H2/N2 flow, of previously decomposed citrate precursors. This material has been studied by X-ray (XRD) and neutron powder diffraction (NPD): it crystallizes, at room temperature, in the cubic space group Fm3̄̄̄̄̄m, and shows an almost perfect ordering between Mn2+ and Mo6+ cations at the B substructure. Below TN = 10.8 K, it experiences a long range antiferromagnetic ordering that was followed from sequential NPD data. The low-temperature magnetic structure is defined by the propagation vector k = (1⁄2, 1⁄2, 1⁄2). The ordered magnetic moment of Mn is found to be 4.04(8) μB at 2 K, suggesting a divalent oxidation state for Mn cations, in a high spin t3 2ge2 g (S = 5/2) electronic configuration.

Semiempirical Theory of Solid Solubility in Transition Metal Alloys

A semiempirical theory of solid solubility in transition metal alloys is presented. The theory is based on writing the energy of formation of the solid solution as a sum of several contributions: a) a chemical contribution due to electronic redistribution in forming the alloy, b) an elastic contribution arising from the difference in size between solute and solvent atoms and c) a structural contribution due to the difference in crystal structure between solute and solvent and to the change in average valence when disolving foreign atoms. Special care is taken in evaluating these contributions in the concentrated alloy regime. The influence of charge transfer on the effective atomic volumes of the components in the alloy is also considered and found to be important. The results obtained are in good agreement with experiment.

Ab initio studies of propene epoxidation on oxidized silver surfaces

Pathways for propene oxide, acrolein and propanone formation on oxidized silver surfaces are studied using DFT simulations.

Metallicity enhancement in core–shell SiO2@RuO2nanowires

SiO₂@RuO₂composite nanowires exhibit an enhanced metallic nature of the external RuO₂shell, reflected in a significantly improved electronic conductance.

Tailoring structural and electronic properties of RuO2 nanotubes: a many-body approach and electronic transport

The electrical conduction properties of ruthenium oxide nanocables are of high interest. These cables can be built as thin shells of RuO2 surrounding an inner solid nanowire of a dielectric insulating silica material. With this motivation we have investigated the structural, electronic and transport properties of RuO2 nanotubes using the density functional formalism, and applying many-body corrections to the electronic band structure. The structures obtained for the thinnest nanotubes are of the rutile type. The structures of nanotubes with larger diameters deviate from the rutile structure and have in common the formation of dimerized Ru-Ru rows along the axial direction. The cohesive energy shows an oscillating behavior as a function of the tube diameter. With the exception of the thinnest nanotubes, there is a correlation such that the electronic band structures of tubes with high cohesive energies show small gaps at the Fermi energy, whereas the less stable nanotubes exhibit metallic behavior, with bands crossing the Fermi surface. The electronic conductance of nanotubes of finite length connected to gold electrodes has been calculated using a Green-function formalism, and correlations have been established between the electronic band structure and the conductance at zero bias.

Influence of the Bi3+ electron lone pair in the evolution of the crystal and magnetic structure of La(1-x)Bi(x)Mn2O5 oxides

La(1-x)Bi(x)Mn2O5 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1) oxides are members of the RMn2O5 family. The entire series has been prepared in polycrystalline form by a citrate technique. The evolution of their magnetic and crystallographic structures has been investigated by neutron powder diffraction (NPD) and magnetization measurements. All the samples crystallize in an orthorhombic structure with space group Pbam containing infinite chains of Mn(4+)O6 octahedra sharing edges, linked together by Mn(3+)O5 pyramids and (La/Bi)O8 units. These units become strongly distorted as the amount of Bi increases, due to the electron lone pair of Bi(3+). All the members of the series are magnetically ordered below TN = 25-40 K and they present different magnetic structures. For the samples with low Bi content (x = 0.2 and 0.4) the magnetic structure is characterized by the propagation vector k = (0,0,1/2). The magnetic moments of the Mn(4+) ions placed at octahedral sites are ordered according to the basis vectors (Gx, Ay, 0) whereas the Mn(3+) moments, located at pyramidal sites, are ordered according to the basis vectors (0, 0, Cz). When the content of Bi increases, two different propagation vectors are needed to explain the magnetic structure: k1 = (0,0,1/2) and k2 = (1/2,0,1/2). For x = 0.6 and 0.8, k2 is predominant over k1 and for this propagation vector (k2) the magnetic arrangement is defined by the basis vectors (Gx, Ay,0) and (Fx, Cy, 0) for Mn(4+) and Mn(3+) ions, respectively.

Ferromagnetic Cu-O-Cu coupling in CaCu3Sn4O12 probed by neutron diffraction

The A-site ordered perovskite oxide with the formula CaCu(3)Sn(4)O(12) has been synthesized in polycrystalline form under moderate pressure conditions (3.5 GPa) in combination with high temperature (1000 °C). This oxide crystallizes in the cubic space group [Formula: see text] (no. 204) with the unit-cell parameter a = 7.64535(6) Å at 300 K. The SnO(6) network is extremely tilted, giving rise to a square planar coordination for Cu(2+) cations. The non-magnetic character of Sn(4+) offers an excellent opportunity to probe the magnetism of Cu(2+) at the A sublattice in CaCu(3)Sn(4)O(12). Magnetic susceptibility shows that this compound is ferromagnetic below T(C) = 10 K, which is an unusual magnetic behaviour in cuprates. This peculiar aspect has been examined by neutron powder diffraction. The refinement of the magnetic structure at 4 K indeed indicates a parallel coupling between Cu(2+) spins with a magnetic moment of 0.5 μ(B)/Cu atom.

Vanishing neoclassical viscosity and physics of the shear layer in stellarators

The drift kinetic equation is solved for low density TJ-II plasmas employing slowly varying, time-dependent profiles. This allows us to simulate density ramp-up experiments and describe from first principles the formation and physics of the radial electric field shear layer. The main features of the transition are perfectly captured by the calculation, and good quantitative agreement is also found. The results presented here, that should be valid for other nonquasisymmetric stellarators, provide a fundamental explanation for a wealth of experimental observations connected to the shear layer emergence in TJ-II. The key quantity is the neoclassical viscosity, which is shown to go smoothly to zero when the critical density is approached from below. This makes it possible for turbulence-related phenomena, and particularly zonal flows, to arise in the neighborhood of the transition.

Evolution of the crystal and magnetic structure of the R2MnRuO7 (R = Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y) family of pyrochlore oxides

The members of the family of pyrochlore oxides with the formula R(2)MnRuO(7) (R = Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) have been synthesized and characterized. Polycrystalline samples were prepared by a soft chemistry procedure involving citrates of the different metal ions, followed by thermal treatments in air or O(2) pressure. The crystallographic and magnetic structures have been analysed from X-ray diffraction (XRD) and neutron powder diffraction (NPD) data, in complement with magnetic measurements; the evolution along the series of the crystallographic parameters (unit-cell parameters, bond distances and angles) is discussed. In R(2)MnRuO(7) pyrochlores, Mn and Ru ions statistically occupy the 16c sites in a cubic unit cell with space group Fd ̅3m, which defines an intrinsic frustrated three-dimensional system. In all the cases, the low-temperature NPD data unveils an antiferromagnetic coupling of two subsets of Mn(4+)/Ru(4+) spins, indicating that the magnetic frustration is partially relieved by the random distribution of Mn and Ru over the 16c sites. At lower temperatures there is a polarization of the R(3+) magnetic moments, which also participate in the magnetic structure, when a magnetic rare earth is present.

On the magnetic structure of PrMn2O5: a neutron diffraction study

The long-range magnetic ordering of PrMn(2)O(5) has been studied on polycrystalline samples from neutron diffraction and specific heat measurements. The onset of antiferromagnetic ordering is observed at T(N) ≈ 25 K. In the temperature interval 18 K < T < 25 K the magnetic structure is defined by the propagation vector k(1) = (1/2,0,0). Below 18 K, some additional magnetic satellites appear in the NPD patterns, which are indexed with k(2) = (0,0,1/2). Therefore, below 18 K the magnetic structure consists of two independent magnetic domains, defined by the propagation vectors k(1) and k(2). The magnetic structure of the k(1)-domain is given by the basis vectors (C(x),0,0) and (C(x)',0,0) for Mn(4h) and Mn(4f), respectively. In the k(2)-domain, the magnetic structure is defined by the basis vectors (0,0,G(z)) and (F(x)',G(y)',0) for Mn(4h) and Mn(4f), respectively. At T = 1.5 K, for the magnetic phase associated with k(1), the magnetic moments of the Mn atoms at the 4h and 4f sites are 1.82(7) and 1.81(6) μ(B), respectively; for the magnetic phase associated with k(2), the magnetic moments for the Mn(4h) and Mn(4f) atoms are 0.59(5) and 2.62(5) μ(B), respectively.

Evolution of cobalt spin states and magnetic coupling along the SrCo(1-x)Sb(x)O(3-δ) system: correlation with the crystal structure

The oxides of the SrCo(1-x)Sb(x)O(3-δ) perovskite family have been recently designed, characterized and described as cathode materials for solid-oxide fuel cells with competitive power performance in the temperature range 750-850 °C. They feature a number of interesting properties including a good electronic conductivity, low electrode polarization resistance and adequate thermal expansion; the crystal structure adopts a 3C corner-linked perovskite network with a considerable number of oxygen vacancies. This paper reports on the effects of Sb-doping on the crystal structure features, the Co oxidation state and magnetic properties related to the presence of spin-state transitions in the Co cations. A phase transition was observed from the tetragonal P4/mmm space group for x≤ 0.15 to the cubic Pm ̅3m space group in the x = 0.2 composition from neutron powder diffraction data. For the tetragonal phases the oxygen vacancies were found to be ordered and localized in the axial O2 and equatorial O3 atoms surrounding the Co2 positions. A noticeable distortion of CoO(6) octahedra is observed for x = 0.05 and 0.1, exhibiting a charge-ordering with a mixed oxidation state of Co(3+/4+) at Co1 sites and Co(3+) at Co2 positions: the Jahn-Teller Co(3+) in an intermediate-spin configuration is responsible for the octahedral distortion. Increasing Sb contents promotes a higher average oxidation state of cobalt, from a valence of 3.2+ for x = 0.05 to 3.4+ for x = 0.2, inducing a decrease of the oxygen vacancies and favouring a random distribution over a Pm ̅3m cubic symmetry. All the samples present an antiferromagnetic behaviour with a G-type (k = 0) magnetic structure. The increase of the Sb content induces the weakening of the crystal field (Δ(cf)) in the octahedral environment promoting the Co spin-transition from the intermediate-spin to the high-spin configuration, as evidenced by the decrease of the octahedral distortion, increment of the unit-cell volume and enhancement of the ordered magnetic moment.

Magnetic and structural features of the NdNi(1 - x)Mn(x)O3 perovskite series investigated by neutron diffraction

Selected members of the perovskite series NdNi(1 - x)Mn(x)O(3) (0 ≤ x ≤ 1) have been prepared by a soft chemistry technique, followed by thermal treatments either under high oxygen pressure (x ≤ 0.5) or in air (x > 0.5). The crystal and magnetic structures have been studied by means of neutron diffraction, complemented with magnetic susceptibility measurements. For x = 0.25, 0.75, the crystal structure of the perovskites can be defined in the orthorhombic Pbnm space group, with Ni and Mn distributed at random over the octahedral sites of the structure. In contrast, the x = 0.5 compound crystallizes in a monoclinic P 2(1)/n structure containing two different octahedral positions, occupied by Ni and Mn, respectively. This is a result of the charge disproportionation of Ni(3+) + Mn(3+) to give Ni(2+) + Mn(4+) cations. The Ni(2+)O(6) octahedra are considerably larger than the Mn(4+)O(6) octahedra. This compound can be considered as a double perovskite of composition Nd(2)NiMnO(6). Unlike NdNiO(3) and NdMnO(3), which exhibit an antiferromagnetic ordering at low temperatures, the intermediate samples for x = 0.25, 0.50, 0.75 exhibit a ferromagnetic arrangement of (Ni, Mn) spins, with the moments aligned along the z axis, as probed using neutron diffraction. A maximum T(C) of 200 K is observed for x = 0.5, whereas T(C) = 150 K and 130 K are observed for x = 0.25 and 0.75, respectively. While NdNiO(3) is metallic above 200 K, a semiconducting behavior is determined between 120-300 K for the intermediate compositions.

Magnetic structure of LaCrO3 perovskite under high pressure from in situ neutron diffraction

The temperature-pressure phase diagram for both the crystal and magnetic structures of LaCrO(3) perovskite has been mapped out by in situ neutron-diffraction experiments under pressure. The system offers the opportunity to study the evolution of magnetic order, spin direction, and magnetic moment on crossing the orthorhombic-rhombohedral phase boundary. Moreover, a microscopic model of the superexchange interaction has been developed on the basis of the crystal structure obtained in this work to account for the behavior of T(N) under high pressure.

Magnetic and electronic properties of RNiO₃ (R = Pr, Nd, Eu, Ho and Y) perovskites studied by resonant soft x-ray magnetic powder diffraction

Soft x-ray resonant magnetic powder diffraction of the ([Formula: see text]) reflection at the Ni L(2, 3) edges is used to study the magnetic and electronic properties of a series of RNiO(3) materials (with R = Pr, Nd, Eu, Ho and Y) below the metal-insulator transition. The polarization and energy dependence of the reflection gives further support for a non-collinear magnetic structure and charge disproportionation in the whole RNiO(3) series. Only small changes in the spectra of the magnetic ([Formula: see text]) reflection and in the absorption spectra could be detected. The results are discussed with comparison to charge transfer multiplet calculations. Our results emphasize that the lighter and heavier RNiO(3) compounds are very similar from the point of view of their local electronic and magnetic state despite the strong change of the metal-to-insulator transition temperature.

Evidence of kinetically arrested supercooled phases in the perovskite oxide NdNiO(3)

We report the time and temperature dependent response of thermopower in the non-magnetic perovskite oxide NdNiO(3). We find that on cooling below the metal-insulator transition temperature the system evolves into a phase separated state which consists of supercooled metallic and insulating phases. This phase separated state exhibits out of equilibrium features, such as cooling rate dependence and time dependence. The existence of these dynamical features is attributed to the transformation of supercooled metallic phases to the insulating state. On cooling, a small fraction of the supercooled phases get kinetically arrested in a glassy state and these supercooled phases remain in that state down to low temperature. In the heating cycle the arrested states de-arrest above 150 K and this results in the reappearance of time dependent features.

Slow dynamics in hard condensed matter: a case study of the phase separating system NdNiO(3)

We report the time dependent response of electrical resistivity in the non-magnetic perovskite oxide NdNiO(3) in its phase separated state and provide a physical explanation of the observations. We also model the system and make an accurate Monte Carlo simulation of the observed behavior. While cooling, a phase separation takes place in the system below its metal-insulator transition temperature and in this state the material exhibits various dynamical phenomena such as relaxation of resistivity, dependence of resistivity on cooling rate and rejuvenation of the material after ageing. These phenomena signal that the phase separated state of NdNiO(3) is not in thermodynamic equilibrium, and we conjecture that it consists of supercooled paramagnetic metallic and antiferromagnetic insulating phases. The supercooled phases are metastable and they switch over to the insulating equilibrium state stochastically, and this can account for the slow dynamics observed in our system. We also verify the predictive power of our model by simulating the result of a new experiment and confirming it by actual measurements.