Symmetric Sites Research Articles

Microstructural evolution of Ni–Mn–Ga microwires during the melt-extraction process

Small size Ni–Mn–Ga microwires have numerous potential applications in micro-sensors and actuators. In this paper, polycrystalline Ni–Mn–Ga microwires with a diameter of 35–80 μm and length of 3–20 cm were fabricated using a melt-extraction technique. The microstructural evolution of the microwires during the process was evaluated in detail. Such melt-extracted microwires have a semicircular cross-section with a flattened side giving an arc-chord length ratio varying from 1.8 to 3.6. Nucleation occurred on the flattened side that was in contact with the copper wheel, with columnar grains growing towards the semicircular surface. Fine equiaxed grains (0.4–2 μm) were observed at the nucleation sites. With an increase in the copper wheel temperature, two symmetric nucleation sites may appear on the flattened surface, accompanied by a decrease in the arc-chord length ratio. Unique grain growth behavior was observed along the radial direction with columnar grains initially growing along the crystal <001> direction with a fan-like texture and then gradually evolving into <001> direction in perpendicular to the flattened surface in the late stage of crystallization. Such grain structure and texture may favor the magnetic-field-induced strain (MFIS) in Ni–Mn–Ga microwires.

First principles investigation of the diffusion of interstitial Cu, Ag and Au in ZnTe

The diffusion is of great significance in many applications when the impurities are employed to tune the semiconductor's electrical or optical properties. It is necessary to understand how dopant defects diffuse in semiconductors. Using first-principles calculations, we consider interstitial diffusion mechanisms and calculate the migration barrier energies of interstitial Cu, Ag and Au atoms in II–VI compounds ZnTe. We find that the relative size of dopant and bulk atoms is an important factor which affects the diffusion behavior. The high symmetry Tc site, which is tetrahedrally coordinated by four cation atoms, is the global minimum energy location for Ag and Au interstitials. The size of Cu adatom is small, so Cu is more stable when it locates at the Ta site which is tetrahedrally coordinated by four anion atoms. But the global minimum energy location for Cu interstitials is M site which is of smaller space than Ta. Cu adatoms show an asymmetric curve of energy diffusion barrier with two energetically distinct extremum in the pathway. Ag diffuses along nearly straight line paths along [111] or equivalent directions. Diffusion for Cu or Au deviates from the straight line paths along 〈111〉 avoiding high symmetric sites.

Ag29(BDT)12(TPP)4: A Tetravalent Nanocluster.

The bottom-up assembly of nanoparticles into diverse ordered solids is a challenge because it requires nanoparticles, which are often quasi-spherical, to have interaction anisotropy akin to atoms and molecules. Typically, anisotropy has been introduced by changing the shape of the inorganic nanoparticle core. Here, we present the design, self-assembly, optical properties, and total structural determination of Ag29(BDT)12(TPP)4, an atomically precise tetravalent nanocluster (NC) (BDT, 1,3-benzenedithiol; TPP, triphenylphosphine). It features four unique tetrahedrally symmetrical binding surface sites facilitated by the supramolecular assembly of 12 BDT (wide footprint bidentate thiols) in the ligand shell. When each of these sites was selectively functionalized by a single phosphine ligand, particle stability, synthetic yield, and the propensity to self-assemble into macroscopic crystals increased. The solid crystallized NCs have a substantially narrowed optical band gap compared to that of the solution state, suggesting strong interparticle electronic coupling occurs in the solid state.

Local electronic and magnetic properties of pure and Mn-containing magnetocaloric LaFe13−xSix compounds inferred from Mössbauer spectroscopy and magnetometry

Manganese containing La–Fe–Si alloys are important magnetocaloric compounds, since Mn atoms prevent segregation of hydrogen in partially hydrogenated La–Fe–Mn–Si alloys when their Curie temperature is tuned to room temperature by hydrogen. The effect of Mn alloying on the Fe atomic magnetic moment μFe is still rather unexplored. Therefore, we investigated the (local) magnetic and electric hyperfine interactions in the strongly magnetocaloric compound LaFe11.3Mn0.3Si1.4 and, for comparison, LaFe11.6Si1.4 by 57Fe Mössbauer spectroscopy, and the global magnetic properties by vibrating sample magnetometry. The NaZn13 structure was confirmed by x-ray diffraction. Two non-equivalent Fe lattice sites are known to exist in this material: the (96i) sites (FeII) of low local symmetry, and the highly symmetrical (8b) sites (FeI). At room temperature in the paramagnetic state, the electric hyperfine parameters of Fe atoms on both sites were obtained. At low temperatures (4.8 K), the observed magnetically split nuclear Zeeman sextets with broad apparent lines were analyzed in terms of a distribution P(Bhf) of hyperfine magnetic fields Bhf. The average hyperfine field 〈Bhf〉, originating predominantly from FeII sites, was found to be rather high (30.7(1) T at 4.8 K) for LaFe11.6Si1.4, and the approximate relation 〈Bhf〉 = AμFe is confirmed for FeII sites, with A = 14.2 T/μB. 〈Bhf〉 is significantly reduced (to 27.7(1) T at 4.8 K) for the Mn-containing sample LaFe11.3Mn0.3Si1.4, providing evidence for a reduction by 9.7% of the average Fe atomic moment μFe from ~2.16 μB to a value of ~1.95 μB by Mn substitution of Fe. Our Mössbauer results are in good agreement with magnetometry, which reveals a reduction of the saturation magnetization of Ms = 163.1(1) Am2 kg−1 of LaFe11.6Si1.4 by 10.5% due to Mn substitution.

Tactile, thermal, and electrical thresholds in patients with and without phantom limb pain after traumatic lower limb amputation.

PurposeTo examine whether there is central sensitization in patients with phantom limb pain (PLP) after traumatic limb amputation.MethodsSeventeen patients after unilateral lower limb amputation secondary to trauma were enrolled. Ten patients had chronic PLP, while the other seven patients had no PLP. Tactile-sensation threshold, cold- and warm-sensation thresholds, cold- and heat-pain thresholds, electrical-sensation threshold (EST), and electrical-pain threshold on the distal residual limb and the symmetrical site on the sound limb were measured in all tested patients. Their thresholds were compared within the PLP and non-PLP group, and between the groups.ResultsThe novel findings included: 1) electrical-pain threshold was only decreased in the sound limb in the PLP group and there was no difference between two limbs in the non-PLP group, suggesting central sensitization in patients with PLP; and 2) EST was increased on the affected limb as compared to the sound limb within the PLP group, but there were no significant differences in EST between the PLP and non-PLP group. There were in general no significant differences in other tested thresholds within the groups and between groups.ConclusionOur results demonstrate central sensitization in the patients with PLP after traumatic limb amputation.

Charge distribution and hyperfine interactions in the vicinity of impurity sites in In2O3 doped with Fe, Co, and Ni

In this paper, first-principles calculations based on density functional theory (DFT) were used to determine TM (TM=Fe, Ni, Co) and Cd impurity locations in the In2O3 host structure, their charge states, the electronic and structural relaxations induced in the host lattice as well as to interpret previous and supplementary experimental results of hyperfine interactions. Different techniques were carried out to characterize TM-doped In2O3 bulk samples prepared by the sol–gel method starting from very pure metals. Perturbed angular correlation (PAC) spectroscopy, a sensitive nuclear technique capable of measuring interactions from electronic charge and spins within an atomic distance, was used to experimentally determine hyperfine interactions at cation sites of In2O3 doped with Co and Ni using In111→Cd111 as probe nuclei. Room temperature results of magnetization measurements in In2O3 doped with Fe, Co and Ni show ferromagnetic ordering coexisting with a paramagnetic behavior for all samples. Results of PAC spectroscopy and DFT calculations show that TM atoms locate as second nearest neighbors of Cd probes preferentially occupy symmetric sites of the doped In2O3 crystal structure with lattice parameters slightly different from that of pure In2O3. Moreover, while a major population of 111Cd probes observes almost the same hyperfine interactions measured for pure In2O3, a small population detects magnetic dipole interactions with magnetic hyperfine field at Cd probes of 2.6T, 3.1T, and 4.6T, respectively for Ni, Co, and Fe doping presenting an almost linear dependence on the number of unpaired 3d electrons of the transition metal impurity.

Bilateral Extradural Hematoma: A Case Report

The occurrence of bilateral extradural hematomas is an uncommon consequence of cranio-cerebral trauma. In the literature bilateral, extradural hematomas have rarely been reported. This article is about a case of 45-year-old female who was admitted in our hospital with a history of road traffi c accident. On admission, she was conscious but irritable having Glasgow coma scale 14/15. In stable condition, she was found to have visual agnosia for left eye. Her computed tomography scan showed a bilateral symmetrical site with asymmetrical dimensions. Patient underwent left occipito-posterio-parietal craniotomy with complete removal of extradural hematoma was done. Right occipital extradural hematoma was not removed.

Sulphur shuttling across a chaperone during molybdenum cofactor maturation.

Formate dehydrogenases (FDHs) are of interest as they are natural catalysts that sequester atmospheric CO2, generating reduced carbon compounds with possible uses as fuel. FDHs activity in Escherichia coli strictly requires the sulphurtransferase EcFdhD, which likely transfers sulphur from IscS to the molybdenum cofactor (Mo-bisPGD) of FDHs. Here we show that EcFdhD binds Mo-bisPGD in vivo and has submicromolar affinity for GDP-used as a surrogate of the molybdenum cofactor's nucleotide moieties. The crystal structure of EcFdhD in complex with GDP shows two symmetrical binding sites located on the same face of the dimer. These binding sites are connected via a tunnel-like cavity to the opposite face of the dimer where two dynamic loops, each harbouring two functionally important cysteine residues, are present. On the basis of structure-guided mutagenesis, we propose a model for the sulphuration mechanism of Mo-bisPGD where the sulphur atom shuttles across the chaperone dimer.

Effect of B site coordination environment in the ORR activity in disordered brownmillerites Ba2In2-xCexO5+δ.

Ba2In2O5 brownmillerites in which the In site is progressively doped with Ce exhibit excellent oxygen reduction activity under alkaline conditions. Ce doping leads to structural changes advantageous for the reaction. Twenty-five percent doping retains the ordered structure of brownmillerite with alternate layers of tetrahedra and octahedra, whereas further increase in Ce concentration creates disorder. Structures with disordered oxygen atoms/vacancies are found to be better oxygen reduction reaction catalysts probably aided by isotropic ionic conduction, and Ba2In0.5Ce1.5O5+δ is the most active. This enhanced activity is correlated to the more symmetric Ce site coordination environment in this compound. Stoichiometric perovskite BaCeO3 with the highest concentration of Ce shows very poor activity emphasizing the importance of oxygen vacancies, which facilitate O2 adsorption, in tandem with catalytic sites in oxygen reduction reactions.

Correlation and spectral analysis of vascular tone regulator mechanisms in paired formations during postnatal ontogenesis in rats.

The study focused on the changes of microcirculation indices and components of the vascular tone of endothelial, neural, and myogenic nature in symmetrical skin sites of temples, forehead, auricles, scapulae, and groin during postnatal ontogeny. Initially, microcirculation indices in the symmetrical regions decreased to postnatal day 10, thereafter they surpassed the initial level at the age of 30 days. The endothelial, neural, and myogenic indices of vascular tone changed differently in the symmetrical left and right sites. On the right side, all indices increased by the postnatal day 21 ("redundancy" phase), and by postnatal day 30 surpassed the initial level by two times. On the left side, the redundancy phase manifested in the endothelial component on postnatal day 10, but to postnatal day 30 all three indices dropped below the initial level.

Asymmetric mutations in the tetrameric R67 dihydrofolate reductase reveal high tolerance to active-site substitutions.

Type II R67 dihydrofolate reductase (DHFR) is a bacterial plasmid-encoded enzyme that is intrinsically resistant to the widely-administered antibiotic trimethoprim. R67 DHFR is genetically and structurally unrelated to E. coli chromosomal DHFR and has an unusual architecture, in that four identical protomers form a single symmetrical active site tunnel that allows only one substrate binding/catalytic event at any given time. As a result, substitution of an active-site residue has as many as four distinct consequences on catalysis, constituting an atypical model of enzyme evolution. Although we previously demonstrated that no single residue of the native active site is indispensable for function, library selection here revealed a strong bias toward maintenance of two native protomers per mutated tetramer. A variety of such "half-native" tetramers were shown to procure native-like catalytic activity, with similar KM values but kcat values 5- to 33-fold lower, illustrating a high tolerance for active-site substitutions. The selected variants showed a reduced thermal stability (Tm ∼12°C lower), which appears to result from looser association of the protomers, but generally showed a marked increase in resilience to heat denaturation, recovering activity to a significantly greater extent than the variant with no active-site substitutions. Our results suggest that the presence of two native protomers in the R67 DHFR tetramer is sufficient to provide native-like catalytic rate and thus ensure cellular proliferation.

Phonon instability of Co single crystal in uniaxial tension and nanoindentation

Although hexagonal close-packed (HCP) metals have significant industrial importance, little attention has been given to their crystal instability. In this work, the uniaxial tension and nanoindentation of Co single crystal were simulated by a multiscale lattice dynamical finite-element method (LDFEM) and molecular dynamics (MD) to investigate its crystal instability. Simulation results show that the crystal instability of Co is associated with loading modes and crystallographic orientations. In uniaxial tension, the stress–strain curves obtained from LDFEM simulations are in good agreement with those in MD simulations before crystal instability. The critical stress for crystal instability in [0001] tension is 21.04GPa, larger than 16.00GPa in [011¯0] tension and 8.31GPa in [21¯1¯0] tension. In nanoindentation, the crystal instability and mechanical behavior of Co are affected by crystallographic orientations. The load–displacement curves and stress distribution obtained from LDFEM simulations match well with those in MD simulations. The crystal instability of Co mainly leads to dislocation nucleation on basal plane. Six symmetric dislocation nucleation sites appeared in the spherical nanoindentation of Co (0001) surface.

Site-selective analyses of battery reactivities in Li1+xNi1-xO2by powder DAFS

Deep understanding of a battery reactivity of transition metals in a positive electrode is essential for further development of lithium ion batteries (LIBs). Li1-xNi1+xO2(LNO, 0 &lt; x &lt; 1) is one of the typical positive-electrodes with a layered rock-salt structure. LNO often contains significant antisite defects of Ni at Li site, i.e., interlayer gallery, in addition to Ni site in the host layer. Though a relationship between such structural disorder and capacity fading of the battery has been reported[1], its degradation mechanism is still unclear presumably because of insufficient insight into electrochemical behavior of Ni at each site in LNO. In the present work, powder diffraction anomalous fine structure (P-DAFS)[2] has been applied to LNO to analyze the valence and local structure of Ni at both different symmetric sites for tackling this degradation issue. We prepared pellets of as-synthesized and charged LNOs for P-DAFS measurements. The composition of the samples is [LiyNi0.11]3a[Ni]3b[O2]6c(y = 0.89, 0.40) in R-3m space group. DAFS measurements were carried out at undulator beamline BL28XU, SPring-8 in Japan. The DAFS spectra were measured with use of 003 and 104 diffractions in ~1 keV energy range around Ni K-edge (8.331 keV). The XAFS-like spectra were extracted from DAFS spectra with a priori structural information from Rietveld analyses. Figure 1 shows (a) the DAFS spectra of the as-synthesized sample, XAFS-like spectra of Ni occupying the Ni site (b) and the Li site (c) in the as-synthesized (solid line) and the charged (broken line) samples. The spectrum at the Ni site shows a shift to the higher energy with charging, indicating the oxidation of Ni accompanied with Li extraction. In contrast, Ni at the Li site shows no significant shift with charging. Thus, Ni at the Li site hardly compensates the charge for delithiation, which indicates that the electrochemically inactive Ni occupying the Li sites would cause irreversibility in this system.

Insights into copper coordination in the EcoRI–DNA complex by ESR spectroscopy

The EcoRI restriction endonuclease requires one divalent metal ion in each of two symmetrical and identical catalytic sites to catalyse double-strand DNA cleavage. Recently, we showed that Cu2+ binds outside the catalytic sites to a pair of new sites at H114 in each sub-unit, and inhibits Mg2+-catalysed DNA cleavage. In order to provide more detailed structural information on this new metal ion binding site, we performed W-band (∼94 GHz) and X-band (∼9.5 GHz) electron spin resonance spectroscopic measurements on the EcoRI–DNA–(Cu2+)2 complex. Cu2+ binding results in two distinct components with different gzz and Azz values. X-band electron spin echo envelope modulation results indicate that both components arise from a Cu2+ coordinated to histidine. This observation is further confirmed by the hyperfine sub-level correlation results. W-band electron nuclear double resonance spectra provide evidence for equatorial coordination of water molecules to the Cu2+ ions.

Synthesis and luminescent properties of trivalent rare-earth (Eu3+, Tb3+) ions doped nanocrystalline AgLa(PO3)4 polyphosphates

The AgLa(PO3)4 phosphors activated with trivalent rare-earth (Eu3+, Tb3+) ions were prepared by a sol–gel synthesis method. The crystal structure of the compound was studied by X-ray diffraction patterns and found to be crystallized in the monoclinic system with a space group P21/c, indicating the calculated lattice parameters of a=10.08Å, b=13.12Å, and c=7.314Å. The Fourier-transform infrared spectrum, photoluminescence excitation/emission spectra, and decay curves were examined to study the optical properties. The analysis of the Eu3+ ions related emission spectrum revealed the presence of highly symmetric sites for the activator ions. The Tb3+ ions related emission spectrum exhibited a 5D3 emission due to the prolonged calcination at high temperatures, which reduces the residual hydroxyl ions. The optical properties show that this host material is suitable for phosphor materials and laser crystals.

Genetic and Environmental Influences on the Visual Word Form and Fusiform Face Areas.

Two areas of the occipitotemporal cortex show a remarkable hemispheric lateralization: written words activate the visual word form area (VWFA) in the left fusiform gyrus and faces activate a symmetrical site in the right hemisphere, the fusiform face area (FFA). While the lateralization of the VWFA fits with the leftward asymmetry of the speech processing network, origin of the rightward asymmetry for faces is still unclear. Using fMRI data from 64 subjects (including 16 monozygotic (MZ) and 13 dizygotic (DZ) twin pairs), we investigated how activations evoked by written words, faces, and spoken language are co-lateralized in the temporal lobe, and whether this organization reflects genetic factors or individual reading expertise. We found that the lateralization of the left superior temporal activation for spoken language correlates with the lateralization of occipitotemporal activations for both written words and faces. Behavioral reading scores also modulate the responses to words and faces. Estimation of genetic and environmental contributions shows that activations of the VWFA, the occipital face area, and the temporal speech areas are partially under genetic control whereas activation of the FFA is primarily influenced by individual experience. Our results stress the importance of both genetic factors and acquired expertise in the occipitotemporal organization.

Localized modes in two-dimensional Schro¨dinger lattices with a pair of nonlinear sites

We address the existence and stability of localized modes in the two-dimensional (2D) linear Schrödinger lattice with two symmetric nonlinear sites embedded into it, and a generalization for moderately localized nonlinearity featuring two distinct symmetric maxima. The latter setting admits a much greater variety of localized modes. Symmetric, antisymmetric, and asymmetric discrete solitons are found, and a subcritical bifurcation, accounting for the spontaneous symmetry breaking (SSB) of the symmetric modes and transition to asymmetric ones, is identified. Existence and stability of more complex 2D solutions in the form of discrete symmetric and asymmetric vortices are also discussed.

Molten salt synthesis, characterization, and luminescence of SrWO4, SrWO4:Tb3+ and SrWO4:Eu3+ powders

SrWO4, SrWO4:Tb3+, and SrWO4:Eu3+ powders were synthesized by a method of molten salt. XRD patterns showed that the synthesized powders have a pure tetragonal scheelite structure without the presence of deleterious phases. Scanning electron microscopy images show that powders are in the range of 20–35 nm. The emission spectrum of SrWO4 shows the emission peak in the blue spectral region. The excitation spectra of SrWO4:Tb3+ and SrWO4:Eu3+ show the energy transfer from WO4 2− group to Tb3+ and Eu3+ ions with a high efficiency. The emission spectrum of SrWO4:Tb3+ shows the green emission at 545 nm corresponding to the 5D4 → 7F5 transition of Tb3+. The emission spectrum of SrWO4:Eu3+ shows the red emission located at 612 nm corresponding to the 5D0 → 7F2 transition of Eu3+. The asymmetry ratio of SrWO4:Eu3+ is found to be 5.54, which indicates that the Eu3+ ions are located in a lower symmetric site.

O2, CO2, and H2O Chemisorption on UN(001) Surface: Density Functional Theory Study

We performed density functional theory calculations of O2, CO2, and H2O chemisorption on the UN (001) surface using the generalized gradient approximation and PW91 exchange-correlation functional at non-spin polarized level with the periodic slab model. Chemisorption energies vs. molecular distance from UN(001) surface were optimized for four symmetrical chemisorption sites. The results showed that the bridge parallel, hollow parallel and bridge hydrogen-up adsorption sites were the most stable site for O2, CO2, and H2O molecular with chemisorption energies of 14.48, 4.492, and 5.85 kJ/mol, respectively. From the point of adsorbent (the UN(001) surface), interaction of O2 with the UN(001) surface was of the maximum magnitude, then CO2 and H2O, indicating that these interactions were associated with structures of the adsorbate. O2 chemisorption caused N atoms on the surface to migrate into the bulk, however CO2 and H2O had a moderate and negligible effect on the surface, respectively. Calculated electronic density of states demonstrated the electronic charge transfer between s, p orbital in chemisorption molecular and U6d, U5f orbital.

Ionic exchange and the local structure in the HfO2/Ho2O3 system studied by PAC spectroscopy

The ionic exchange of Hf and Ho atoms in the HfO2/Ho2O3 system was studied at the atomic level applying the nuclear solid-state Time-Differential γ–γ Perturbed-Angular-Correlation (PAC) spectroscopy. This exchange was promoted by a ball-milling-assisted solid-state reaction between Ho2O3 and m-HfO2 initial powders. In order to follow and to elucidate the effect of different variables (milling time, temperature, pressure) on the exchange process and the appearance of new phases, 181Hf(→181Ta) ions were used as local probes in the PAC experiments. The measured hyperfine interactions enabled the electric-field gradient tensor (EFG) characterization at Hf sites at each step of the process. At the final stages of the solid-state reaction, 75–90% Hf-doping at both substitutional defect-free cation sites of Ho2O3 was achieved, being the EFG measured at these sites in excellent agreement with those determined in 181Hf-implanted Ho2O3 samples and to those predicted by the EFG systematics established in rare-earth bixbyites doped by ion-implantation of 181Hf(→181Ta) ions. Ab initio electronic structure calculations of the EFG at Ta impurities localized at both cation sites in Ho2O3 also confirm the 181Hf cationic substitution in both PAC experiments. Additional ab initio calculations at Hf impurity sites in Ho2O3 and Tm2O3 were performed to study the relative Hf preference for the symmetric site of the structure. We showed that high-energy milling plus high temperature treatments are both necessary to achieve a high degree of Hf substitution in the cation sublattice of the Ho2O3 structure. Also, we found that the pressure effect on the crystal structure favors the impurity substitution at cationic sites closer to a homogenous distribution of the probes and with much less local and far disorder. Additional spurious hyperfine interactions that were always present in 181Hf-implanted Ho2O3 samples were not observed when using this solid-state reaction method. The appearance of a modified m-HfO2 phase produced after heavy Ho-doping was also discussed.