Atomic Occupancies Research Articles

Spindle Spinel CoFeCoO4 Microparticles/rGO as an Oxygen Reduction and Oxygen Evolution Catalyst

The representative spinel-type materials AB2O4 (both A and B are transition metals) electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) have been investigated and significant improvements have been achieved in the activity and durability for ORR and OER in the alkaline solution. But CoFeCoO4 was not explored widely like ZnCo2O4 (or NiCo2O[Formula: see text] as the ORR electrocatalyst for its relatively complicated atomic site occupation. CoFeCoO4 has a typical cubic spinel structure with Co[Formula: see text] in the tetrahedron and Co[Formula: see text] and Fe[Formula: see text] in the octahedron. A mixture of Co[Formula: see text] and Fe[Formula: see text] in the B site makes the oxide have a wider overlap between transition metal 3d orbit and O 2p orbit, which can lead to an effective charge transfer in the rate-determining steps of ORR process and then enhance the ORR activity. The high electronic conductivity and specific surface area of rGO can accelerate charger transfer and provide more catalytic sites, which would contribute to a faster ORR process. In this work, the porous spindle CoFeCoO4 microparticles which were synthesized by hydrothermal technology, were assembled on the rGO surface to obtain the CoFeCoO4/rGO composite, which exhibited enhanced ORR activity and catalytic stability comparable to that of Pt/C. On the other hand, the OER catalytic activity of the prepared samples was also studied to explore the potential of CoFeCoO4/rGO as a bifunctional oxygen catalyst.

Integrated modeling of molar volume of the sigma phase aided by first-principles calculations

The volume modeling of the sigma phase is an indispensable complement to the integrated computational material design of technologically important materials, such as high-alloy steels and Ni-based superalloys. The molar volume of the sigma phase is influenced by both the atomic mixing (the volume variation affected by this factor is caused by composition alteration rather than site occupation change) and atomic order (i.e. atomic constituent distribution or site occupancy preference on inequivalent sites of a crystal). In the present work, we developed a new integrated thermodynamic and molar volume model to consider physically both mixing and order factors. The integrated model was built within the compound energy formalism (CEF), enabling the thermodynamic calculations to determine equilibrium site occupancies for the subsequent volume calculations. The model parameters of the CEF were assigned by using the first-principles calculated energies and molar volumes of the complete sets of ordered configurations of the sigma phase, as well as the extrapolated molar volumes of the pure elements in the hypothetic sigma phase structure. Such extrapolation for pure elements is based on the experimental data from the literature and the first-principles calculations. We applied the integrated model to study the binary compounds, e.g. Cr-Co, Cr-Fe, Cr-Mn, Mo-Fe, Mo-Mn, Mo-Re, Re-Cr, Re-Fe, Re-Mn, Nb-Al, Ta-Al, V-Fe, V-Ni, and ternary compounds Cr-Fe-X (X = Co and Ni). The integrated thermodynamic and molar volume databases can predict successfully the molar volume of the binary and ternary sigma compounds. As most experimental volume data were measured at room temperature and atmospheric pressure, and the first-principles calculations were performed at 0 K, the present model parameters are valid at about room temperature and atmospheric pressure.

Formation of mixed metal sulfides of NixCu1−xCo2S4 for high-performance supercapacitors

A series of mixed metal sulfides of NixCu1−xCo2S4 were synthesized by a facile two-step hydrothermal method and they were investigated as electrode materials for supercapacitor. A substitutional solid solution can be easily formed due to the same atom occupation and similar crystal radius of Ni and Cu in the metal sulfides. The electrochemical performances of the mixed metal sulfides are strongly influenced by the chemical compositions, though they have the same crystal structure. Among all the as-prepared materials, Ni0.67Cu0.33Co2S4 shows the best electrochemical performances with the highest specific capacitance of 1340.48Fg−1 at 1 A g−1, outstanding rate capability and excellent cycling stability. The improved performances can be attributed to the high crystallinity, modified microstructure, increased electrochemical sites and improved electroconductivity caused by lattice distortion as well as the synergistic effect in the Cu-substituted NiCo2S4. Furthermore, the assembled two-electrode hybrid device Ni0.67Cu0.33Co2S4//activated carbon can deliver a high energy density of 40Whkg−1 at the power density of 412.5Wkg−1, showing a great potential for energy storage.

A Tungsten-Based Nanolaminated Ternary Carbide: (W,Ti)4C4- x.

Nanolamellar transition metal carbides are gaining increasing interests because of the recent developments of their two-dimensional (2D) derivatives and promising performance for a variety of applications from energy storage, catalysis to transparent conductive coatings, and medicine. To develop more novel 2D materials, new nanolaminated structures are needed. Here we report on a tungsten-based nanolaminated ternary phase, (W,Ti)4C4- x, synthesized by an Al-catalyzed reaction of W, Ti, and C powders at 1600 °C for 4 h, under flowing argon. X-ray and neutron diffraction, along with Z-contrast scanning transmission electron microscopy, were used to determine the atomic structure, ordering, and occupancies. This phase has a layered hexagonal structure ( P63 /mmc) with lattice parameters, a = 3.00880(7) Å, and c = 19.5633(6) Å and a nominal chemistry of (W,Ti)4C4- x (actual chemistry, W2.1(1)Ti1.6(1)C2.6(1)). The structure is comprised of layers of pure W that are also twin planes with two adjacent atomic layers of mixed W and Ti, on either side. The use of Al as a catalyst for synthesizing otherwise difficult to make phases, could in turn lead to the discovery of a large family of nonstoichiometric ternary transition metal carbides, synthesized at relatively low temperatures and shorter times.

Doping effect investigation of Li-doped CdS thin films

ABSTRACTLi-doped CdS (CdS:Li) thin films with 0, 1, 2 and 3 wt-% Li concentrations were prepared by the spray technique using a perfume atomiser. The effect of Li doping concentration on the structural, optical, electrical and magnetic properties was systematically investigated and the results are reported in this paper. Analysis of the microstructure indicates that all the films consist of a hexagonal CdS phase with a strong (0 0 2) preferential growth texture. Nanoneedles were evinced from the SEM images of the doped films. Optical studies reveal that the transparency and band gap were dependent on Li-doping content. Hall effect measurements reveal that all the doped films exhibit p-type conductivity and the resistivity values strongly depend on the occupancy of Li atoms on the substitutional and interstitial sites of the CdS lattice. Room temperature ferromagnetism was observed for the doped films.

Growth and physical properties of Ce(O,F)Sb(S,Se)2 single crystals with site-selected chalcogen atoms

Ce(O,F)Sb(S,Se)2 single crystals were successfully grown using a CsCl/KCl flux method. The obtained crystals have a plate-like shape with the typical size of 1–2 mm and well-developed ab-plane, which enables X-ray single crystal structural analysis. The Ce(O,F)Sb(S,Se)2 crystallizes in a monoclinic space group, P21/m, with lattice parameters of a = 4.121(7) Å, b = 4.109(7) Å, c = 13.233(15) Å, β = 97.94(7) °. It is composed of alternate stacking of Ce-(O,F) and Sb-SSe layers, and the Sb-SSe layer includes selective occupation of Se atoms in its in-plane site. The valence state of Ce is estimated to be Ce3+ by X-ray absorption fine spectroscopy analysis. The single crystals show an insulating behavior, and a magnetic ordering around 6 K.

Concentration and duration dependence of a new prototype polycarbonate/activated-carbon fabric individual and environmental radon monitor

The basic design of new multi-purpose polycarbonate track detector (PCTD)/activated-carbon-fabric (ACF) detector for radon monitoring in particular for parametric studies has been recently self-invented in our laboratory by enhancing PCTD's sensitivity by using the ACF adsorptive layer as external alpha radiator. The PCTD/ACF monitor basically consists of a PCTD; half bare and half covered with ACF in order to adsorb radon on its carbon active sites. From parameters studied, radon concentration and exposure duration were discovered to be the most important parameters affecting the monitor's sensitivity enhancement. An amplification factor (AF), defined as ratio of net PCTD/ACF to PCTD/bare alpha track density responses after background track density subtraction, was found being highly dependent on the two stated parameters. The AF versus duration response is flat with a value ∼1.5 constant up to 32 days in ∼180±20 Bq.m−3 radon environment and also flat with a constant value ∼7.5 up to 8 days in ∼25±1.1 kBq.m−3 radon environment after which the AF decreases down to ∼3.0 up to 32 days exposure. This phenomenon seems be due to full occupancy of radon atoms on the ACF active sites at high concentrations with an equilibrium between adsorption and desorption processes. The latter significantly dominates after 8 days in ∼25±1.1 kBq.m−3 environment leaving behind non-active sites requiring regeneration. The prototype monitor developed can be applied to normal level individual and/or environmental radon monitoring for an extended period of time but to high radon concentrations for short-term such as a week as a “grab sampler” with a high prospect for determination of radon equilibrium factor with its progeny in the radon environment under measurement. The actual radon monitor and its parametric studies are under further development.

A Chain Model of a Zigzag Contact of Lateral Graphene-Like Heterostructures

A simple structural model is proposed for the zigzag interface formed by contacting two-dimensional graphene-like compounds AB and CD (both free and formed on a metal). For the graphene–hexagonal boron nitride system, analytical expressions for the electron spectrum, density of states, and atom occupation numbers at the interface are obtained. The results of calculating the densities of states and occupation numbers within two alternative approximations are in good agreement.

Explaining the Different Geometries of the Water Oxidising Complex in the Nominal S3 State Crystal Structures of Photosystem II at 2.25 Å and 2.35 Å.

Recently two atomic resolution crystal structures of Photosystem II, in the double flashed, nominal S3 intermediate state of its Mn4 Ca water oxidising complex (WOC), have been presented (Young et al., Nature 2016, 540, 453; Suga et al., Nature 2017, 543, 131). These structures are at 2.25 Å and 2.35 Å resolution, respectively. Although highly similar in most respects, the structures differ in a key region within the WOC catalytic site. In the 2.25 Å structure, one oxy species (O5) is observed within the WOC cavity, weakly associated with the Mn centres, similar to that seen earlier in the 1.95 Å XRD structure of the S1 intermediate (Suga et al., Nature, 2015, 517, 99). In the 2.35 Å structure, two such species are seen (O5, O6), with the Mn centres and O5 positioned as in the 2.25 Å structure and an O5-O6 separation of ∼1.5 Å, consistent with peroxo formation. This suggests O5 and O6 are substrate water derived species in this double flashed form. Recently we have presented (Petrie, et al., Chem. Phys. Chem., 2017) a large scale (220 atom) quantum chemical model of the Young et al. 2.25 Å structure, which quantitatively explains all significant features within the WOC region of that structure, particularly the positions of the metal centres and O5 group. Critical to this was our assumption of a 'low' Mn oxidation paradigm (mean S1 Mn oxidation level of +3.0, Petrie et al., Angew. Chem. Int. Ed., 2015), rather than a 'high' oxidation model (mean S1 oxidation level of +3.5), widely assumed in the literature. Here we show that our same oxidation state model predicts two classes of energetically close S3 structural forms, analogous to the S1 state, one with the metal centres and O5 positioned as in the 2.25 Å structure, and the other with the metals similarly placed, but with O5 located in the O6 position of the 2.35 Å structure. We show that the Suga et al. 2.35 Å structure is likely a superposition of two such forms, one from each class, which is consistent with reported atomic occupancies for that structure and the relative total energies we calculate for the two structural forms.

Microscopic Phase-field Simulation for the Influence of Aging Process on the Precipitation Process of Ni75Al15Ti10 Alloy

The precipitation process of Ni75Al15Ti10 alloy was simulated by a microscopic phase-field kinetics model. The microscopic morphology evolution was studied, which indicates that the precipitation process can be divided into two stages: the first is the transformation from L10 phase to L12 phase, and the second is the formation of the stoichiometric L12 phase. The effect of single aging temperature on the microstructure, atomic occupation probability and atomic diffusion was investigated. The result shows that the shape of γ′ precipitates transforms from irregular shape to regular cuboid and the degree of orientation growth increases with increasing aging temperature. But the higher the aging temperature, the lower the occupation probability and the order degree. Meanwhile, the path of diffusion of Al atom is from the interface of the γ′ phase to the internal, and the diffusion of Ti atom is the opposite. Furthermore, we studied the dual aging to improve occupation probability of γ′ precipitates and to discuss their influence on the γ′ phase. Dual aging can not only obtain a stable γ′ phase with regular shape, but also increase the occupation probability of Al and Ti and inhibit the formation of anti-site defects.

First-principles calculations and thermodynamic modelling of long periodic stacking ordered (LPSO) phases in Mg-Al-Gd

In the present work, thermodynamic modelling of four long periodic stacking ordered (LPSO) phases, i.e., 10H, 18R, 14H, and 24R, in the Mg-Al-Gd system was performed using the CALPHAD (calculation of phase diagram) approach with input from the present first-principles calculations and experimental data in the literature. Sublattice models were developed to describe these LPSO phases. Especially, an L12-type clusters in the FCC stacking layers of LPSO phases and the atomic occupancy in the center of L12 cluster were considered based on experimental observations and energetics from first-principles calculations. The calculated phase equilibrium results are in good agreement with experiments about the phase stability of 14H and 18R and the mole fraction of Gd and Al in these LPSO phases. The present modeling provides a new approach to describe the thermodynamic properties of LPSO phases and can be applied to other alloy systems.

Site occupancy of alloying elements in the L12 structure determined by channeling enhanced microanalysis in γ/γ’ Co-9Al-9W-2X alloys

Knowledge about the sublattice site preference of alloying elements in the L12-γ′ phase of novel Co-base superalloys is a necessary pre-requisite to understand their influence on the properties of the alloys in general and the γ′ phase in particular. In the present study, the atomic site occupancy of the alloying elements in the L12-γ′ structure in Co-9Al-9W-2X quaternary alloys after long-term annealing at 900 °C for 5000 h was determined using the atom location by channeling enhanced microanalysis (ALCHEMI) technique in combination with energy-dispersive X-ray spectroscopy (EDX) composition analysis in a transmission electron microscope (TEM). The experimental ALCHEMI data were evaluated by comparing them with those calculated by the program ‘Inelastic Cross Section Calculator’ (ICSC). The results show that Co mainly occupies one sublattice site and Al/W are located at the other sublattice site in the L12 unit cell in the ternary alloy. The additional elements Ti, V, Mo and Ta which partition strongly to the γ′ phase tend to occupy the Al/W sublattice site, and Cr which partitions more to the γ phase also favors the Al/W sublattice site, while Ni weakly partitions into the γ′ phase and favors the Co sublattice site. The results of this study can provide evidence to the predictions on the site preference in literature based on the phase composition or on theoretical studies.

Interfacial stability of θ′/Al in Al-Cu alloys

Designing low-energy interface structures is crucial in growth and coarsening studies of Al2Cu (θ′) precipitates. Given new experimental insights into the θ′/Al system, we use Density Functional Theory (DFT) calculations to investigate the energetics of the coherent (001)θ′//(001)Al and semi-coherent (010)θ′//(010)Al interfaces. We find that the recently proposed occupancy of interstitial Cu atoms at the coherent interface by Bourgeois et al. increases interfacial energy, and hence, is likely due to kinetic effects. For the semi-coherent interface, the semi-coherent interfacial energy does not significantly depend on interfacial configurations or misfit strains up to 10-unit cells of Al.

Structure determination and crystal chemistry of large repeat mixed-layer hexaferrites.

Hexaferrites are an important class of magnetic oxides with applications in data storage and electronics. Their crystal structures are highly modular, consisting of Fe- or Ba-rich close-packed blocks that can be stacked in different sequences to form a multitude of unique structures, producing large anisotropic unit cells with lattice parameters typically >100 Å along the stacking axis. This has limited atomic-resolution structure solutions to relatively simple examples such as Ba2Zn2Fe12O22, whilst longer stacking sequences have been modelled only in terms of block sequences, with no refinement of individual atomic coordinates or occupancies. This paper describes the growth of a series of complex hexaferrite crystals, their atomic-level structure solution by high-resolution synchrotron X-ray diffraction, electron diffraction and imaging methods, and their physical characterization by magnetometry. The structures include a new hexaferrite stacking sequence, with the longest lattice parameter of any hexaferrite with a fully determined structure.

In Situ Electron Diffraction Tomography Using a Liquid-Electrochemical Transmission Electron Microscopy Cell for Crystal Structure Determination of Cathode Materials for Li-Ion batteries.

We demonstrate that changes in the unit cell structure of lithium battery cathode materials during electrochemical cycling in liquid electrolyte can be determined for particles of just a few hundred nanometers in size using in situ transmission electron microscopy (TEM). The atomic coordinates, site occupancies (including lithium occupancy), and cell parameters of the materials can all be reliably quantified. This was achieved using electron diffraction tomography (EDT) in a sealed electrochemical cell with conventional liquid electrolyte (LP30) and LiFePO4 crystals, which have a well-documented charged structure to use as reference. In situ EDT in a liquid environment cell provides a viable alternative to in situ X-ray and neutron diffraction experiments due to the more local character of TEM, allowing for single crystal diffraction data to be obtained from multiphased powder samples and from submicrometer- to nanometer-sized particles. EDT is the first in situ TEM technique to provide information at the unit cell level in the liquid environment of a commercial TEM electrochemical cell. Its application to a wide range of electrochemical experiments in liquid environment cells and diverse types of crystalline materials can be envisaged.

Effect of heating rate on atom migration, phase structure and magnetic properties of the Fe82Si2B11P4Cu1 alloy

Atom migration, phase structure and magnetic properties in the Fe82Si2B11P4Cu1 amorphous alloy isothermally annealed at various heating rates are investigated. Annealing the samples at a high heating rate promotes the formation of Cu-rich regions and delays the phase separation process to high temperature, which is conductive to increase the volume fraction of the nanocrystalline phase and refine nanograins during the crystallization. When the alloy is isothermally annealed at 758 K for 180 s, the saturation magnetic flux density increases from 1.74 T to 1.79 T and the coercivity decreases from 7.8 A/m to 4.4 A/m with the heating rate increasing from 0.5 K/s to 20 K/s. Meanwhile, it confirms that the occupation of Si atoms in the lattice of bcc-Fe can be suppressed by the annealing with high heating rate. Increasing the heating rate is widely regarded as an effective method to improve soft magnetic properties of Fe-based nanocrystalline alloys. However, it is found that the content of the nonmagnetic atom enriched regions rises with the increase of heating rate, which can reduce the magnetic interaction in the nanocrystalline ribbons. Therefore, the exorbitant heating rate reaching 40 K/s will dramatically promote the aggregation of nonmagnetic atoms and deteriorate the soft magnetic properties.

Magnetic properties of single rare-earth atoms on graphene/Ir(111)

We employed x-ray absorption spectroscopy and x-ray magnetic circular dichroism to study the magnetic properties of single rare-earth (RE) atoms (Nd, Tb, Dy, Ho, and Er) adsorbed on the graphene/Ir(111) surface. The interaction of RE atoms with graphene results for Tb in a trivalent state with $4{f}^{n\ensuremath{-}1}$ occupancy, and in a divalent state with $4{f}^{n}$ occupancy for all other studied RE atoms ($n$ corresponds to the $4f$ occupancy of free atoms). Among the studied RE on graphene/Ir(111), Dy is the only one that shows magnetic hysteresis and remanence at 2.5 K. By comparing measured spectra and magnetization curves with multiplet calculations, we determine the energy diagram of the magnetic states and show for each element the magnetization reversal process that determines the timescale of its magnetic bistability.

Atomic-scale investigation into precipitated phase thickening in Al-Si-Mg-Cu alloy

The microstructure variation in Al-Si-Mg alloys added with high content of Cu during aging process is investigated by aberration-corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). At the early stage of aging process, there are needle-like GP zone and β″ containing Cu precipitating in succession. A kind of bright contrast step type structure unit (step-unit), consisting of Cu atoms, is observed in the edge of these precipitates. The amount of step-unit increases with precipitates thickening. Step-unit is resulted by the translation distribution of a kind of atomic coordination geometry and the specific occupation of Cu atoms in the precipitates. This kind of atomic coordination is called Unit T in β'' and Unit T' in GP zone, respectively. The thickening process of these needle-like precipitates is realized by the formation of Unit T(T′) and local diffusion of solute atoms.

Li and Co Ordering in the Nitridocobaltate(I) SrLi2{Li[CoN2

SrLi2{Li[CoN2]}, an isostructural variant of Li4SrN2, has been synthesised as black single crystals from a reaction between Li2[(Li,Co)N] and Sr2N, at 973 K using a Li flux in a sealed tantalum ampoule. Single crystal diffraction refinements gave a tetragonal unit cell, which upon closer inspection showed a monoclinic supercell. This supercell allowed, for the first time, the refinement of the occupation of metal atoms along the infinite chains in the structure, resulting in the chemical formula SrLi2{Li0.65Co0.35[Co0.65Li0.35N2]}. This revealed a clear preference for the Li and Co atoms to alternate along the chains. Magnetic measurements showed the sample to be a Curie paramagnet, with Co(I) being in a high-spin S = 1 configuration.

Intrinsic magnetic properties of Ce2(Fe, Co)14B and its modifications by Ni and Cu

The intrinsic magnetic properties of Ce2Fe14-xCoxB (x ≤ 4.76) are studied at 25 °C using key alloys annealed at 900 °C for 25 days. The saturation magnetization (Ms) and the Curie temperature (Tc) of Ce2Fe14-xCoxB increase with Co content. However, the anisotropy field (Ha) of Ce2Fe14-xCoxB diminishes precipitously with Co content. The process of crystal structure refinement indicates that the saturation magnetization of Ce2Fe14-xCoxB is related to the site occupancy of Co atoms at different Fe atomic sites. Co atoms prefer to occupy 8j2 site, followed by 16k2, 4e and 16k1 sites sequentially. Moreover, Co atoms occupying 8j2 site are more effective leading to an increase in the Ms. The individual effects of Ni or Cu on the intrinsic magnetic properties of Ce2Fe12.98-xCo1.02NixB and Ce2Fe12.98-yCo1.02CuyB are evaluated. The maximum solid solubilities of Ni and Cu in Ce2Fe12.98Co1.02B at 900 °C are found to be 8 at.% and 0.8 at.%, respectively. Ni or Cu enhances Tc, but decreases both Ms and Ha of Ce2Fe12.98Co1.02B. The paper also discussed the combined effects of Ni and Cu on the intrinsic magnetic properties of Ce2Fe12.98Co1.02B. The Ms of Ce2Fe12.98-x-yCo1.02NixCuyB (0 ≤ x ≤ 0.41, y ≈ 0.119) increases after doping with both Ni and Cu, reaching around 155 emu/g. Meanwhile, the Ha and the Tc are measured to be near 24 kOe and 280 °C, respectively.