1,917 research outputs found

    Anisotropic Small-Polaron Hopping In W:Bivo4 Single Crystals

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    DC electrical conductivity, Seebeck and Hall coefficients are measured between 300 and 450 K on single crystals of monoclinic bismuth vanadate that are doped n-type with 0.3% tungsten donors (W:BiVO4). Strongly activated small-polaron hopping is implied by the activation energies of the Arrhenius conductivities (about 300 meV) greatly exceeding the energies characterizing the falls of the Seebeck coefficients' magnitudes with increasing temperature (about 50 meV). Small-polaron hopping is further evidenced by the measured Hall mobility in the ab-plane (10(-1) cm(2) V-1 s(-1) at 300 K) being larger and much less strongly activated than the deduced drift mobility (about 5 x 10(-5) cm(2) V-1 s(-1) at 300 K). The conductivity and n-type Seebeck coefficient is found to be anisotropic with the conductivity larger and the Seebeck coefficient's magnitude smaller and less temperature dependent for motion within the ab-plane than that in the c-direction. These anisotropies are addressed by considering highly anisotropic next-nearest-neighbor (approximate to 5 angstrom) transfers in addition to the somewhat shorter (approximate to 4 angstrom), nearly isotropic nearest-neighbor transfers. (C) 2015 AIP Publishing LLC.U.S. Department of Energy (DOE), DE-FG02-09ER16119Welch Foundation Grant F-1436Hemphill-Gilmore Endowed FellowshipNSF MIRT DMR 1122603Chemical EngineeringTexas Materials InstituteChemistr

    Range size and seasonal movement for female woodland caribou in the boreal forest of northeastern Ontario

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    A preliminary examination was conducted of range size and distribution of female woodland caribou (Rangifer tarandus caribou) in northeastern Ontario. Annual and seasonal ranges were calculated using satellite telemetry data collected for 30 female caribou between 1998 and 2001. The mean annual home range size of collared females was 4026 km2. Seasonal ranges varied in size depending on time of year (P<0.05). Calving and summer ranges were significantly smaller than autumn and late winter ranges. Early winter ranges were significantly larger than calving ranges and smaller than late winter ranges. Overall, range sizes of female woodland caribou in northeastern Ontario were larger than those reported for caribou in other Boreal Forest regions across Canada

    Staying in touch in the digital era: New social work practice

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    The findings of a small-scale empirical study are drawn upon to explore the concept of social presence and the way in which it can contribute to meeting service users’ expectations of relationship-based social work. Findings from the study highlight the role of mobile communication technologies in establishing social presence with service users and an argument is made for the proactive use of mobile devices as a component of direct practice. However, such emerging digital social work practices will require practitioners, and social work organisations, to respond positively to new ethical and organisational challenges

    Engineering Solution-Processed Non-Crystalline Solid Electrolytes for Li Metal Batteries

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    Non-crystalline Li-ion solid electrolytes (SEs), such as lithium phosphorus oxynitride, can uniquely enable high-rate solid-state battery operation over thousands of cycles in thin film form. However, they are typically produced by expensive and low throughput vacuum deposition, limiting their wide application and study. Here, we report non-crystalline SEs of composition Li-Al-P-O (LAPO) with ionic conductivities > 10-7 S cm-1 at room temperature made by spin coating from aqueous solutions and subsequent annealing in air. Homogenous, dense, flat layers can be synthesized with submicrometer thickness at temperatures as low as 230 °C. Control of the composition is shown to significantly affect the ionic conductivity, with increased Li and decreased P content being optimal, while higher annealing temperatures result in decreased ionic conductivity. Activation energy analysis reveals a Li-ion hopping barrier of ≈0.4 eV. Additionally, these SEs exhibit low room temperature electronic conductivity (< 10-11 S cm-1) and a moderate Young’s modulus of ≈54 GPa, which may be beneficial in preventing Li dendrite formation. In contact with Li metal, LAPO is found to form a stable but high impedance passivation layer comprised of Al metal, Li-P, and Li-O species. These findings should be of value when engineering non-crystalline SEs for Li-metal batteries with high energy and power densities

    Lithium-sulfur battery diagnostics through distribution of relaxation times analysis

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    Electrochemical impedance spectroscopy (EIS) is widely used in battery analysis as it is simple to implement and non-destructive. However, the data provided is a global representation of all electrochemical processes within the cell and much useful information is ambiguous or inaccessible when using traditional analysis techniques. This is a major challenge when EIS is used to analyse systems with complex cell chemistries, like lithium-sulfur (Li-S), one of the strongest candidates to supersede conventional Li-ion batteries. Here we demonstrate the application of distribution of relaxation times (DRT) analysis for quantitative deconvolution of EIS spectra from Li-S batteries, revealing the contributions of (eight) distinct electrode processes to the total cell polarisation. The DRT profile is shown to be strongly dependent on cell state-of-charge, offering a route to automated and on-board analysis of Li-S cells

    Quasi-Two-Dimensional Heterostructures (KM1 – xTe)(LaTe3) (M = Mn and Zn) with Charge Density Waves

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    Layered heterostructure materials with two different functional building blocks can teach us about emergent physical properties and phenomena arising from interactions between the layers. We report intergrowth compounds KLaM1 – xTe4 (M = Mn and Zn; x ≈ 0.35) featuring two chemically distinct alternating layers [LaTe3] and [KM1 – xTe]. Their crystal structures are incommensurate, determined by single X-ray diffraction for the Mn compound and a transmission electron microscope study for the Zn compound. KLaMn1 – xTe4 crystallizes in the orthorhombic superspace group Pmnm(01/2γ)s00 with lattice parameters a = 4.4815(3) Å, b = 21.6649(16) Å, and c = 4.5220(3) Å. It exhibits charge density wave order at room temperature with a modulation wave vector q = 1/2b* + 0.3478c* originating from electronic instability of Te-square nets in [LaTe3] layers. The Mn analog exhibits a cluster spin glass behavior with spin freezing temperature Tf ≈ 5 K attributed to disordered Mn vacancies and competing magnetic interactions in the [Mn1 – xTe] layers. The Zn analog also has charge density wave order at room temperature with a similar q-vector having the c* component ∼0.346 confirmed by selected-area electron diffraction. Electron transfer from [KM1 – xTe] to [LaTe3] layers exists in KLaM1 – xTe4, leading to an enhanced electronic specific heat coefficient. The resistivities of KLaM1 – xTe4 (M = Mn and Zn) exhibit metallic behavior at high temperatures and an upturn at low temperatures, suggesting partial localization of carriers in the [LaTe3] layers with some degree of disorder associated with the M atom vacancies in the [M1 – xTe] layers

    The subchalcogenides Ir₂In₈Q (Q = S, Se, Te): Dirac semimetal candidates with re-entrant structural modulation

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    Subchalcogenides are uncommon compounds where the metal atoms are in unusually low formal oxidation states. They bridge the gap between intermetallics and semiconductors, and can have unexpected structures and properties because of the exotic nature of their chemical bonding, as they contain both metal-metal and metal-main group (e.g. halide, chalcogenide) interactions. Finding new members of this class of materials presents synthetic challenges, as attempts to make them often result in phase separation into binary compounds. We overcome this difficulty by utilizing indium as a metal flux to synthesize large (mm scale) single crystals of novel subchalcogenide materials. Herein, we report two new compounds Ir2In8Q (Q = Se, Te) and compare their structural and electrical properties to the previously reported Ir2In8S analogue. Ir2In8Se and Ir2In8Te crystallize in the P42/mnm space group and are isostructural to Ir2In8S but also have commensurately modulated (with q-vectors q = 1/6a* + 1/6b* and q= 1/10a* + 1/10b* for Ir2In8Se and Ir2In8Te, respectively) low temperature phase transitions, where the chalcogenide anions in the channels experience a distortion in the form of In-Q bond alternation along the ab plane. Both compounds display re-entrant structural behavior, where the supercells appear on cooling but revert to the original subcell below 100 K, suggesting competing structural and electronic interactions dictate the overall structure. Notably, these materials are topological semimetal candidates with symmetry-protected Dirac crossings near the Fermi level, and exhibit high electron mobilities (~1500 cm2 V-1 s-1 at 1.8 K) and moderate carrier concentrations (~1020 cm-3) from charge transport measurements. This work highlights metal flux as a powerful synthetic route to high quality single crystals of novel intermetallic subchalcogenides

    A two-dimensional type I superionic conductor

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    Superionic conductors possess liquid-like ionic diffusivity in the solid state, finding wide applicability from electrolytes in energy storage to materials for thermoelectric energy conversion. Type I superionic conductors (for example, AgI, Ag2Se and so on) are defined by a first-order transition to the superionic state and have so far been found exclusively in three-dimensional crystal structures. Here, we reveal a two-dimensional type I superionic conductor, α-KAg3Se2, by scattering techniques and complementary simulations. Quasi-elastic neutron scattering and ab initio molecular dynamics simulations confirm that the superionic Ag+ ions are confined to subnanometre sheets, with the simulated local structure validated by experimental X-ray powder pair-distribution-function analysis. Finally, we demonstrate that the phase transition temperature can be controlled by chemical substitution of the alkali metal ions that compose the immobile charge-balancing layers. Our work thus extends the known classes of superionic conductors and will facilitate the design of new materials with tailored ionic conductivities and phase transitions

    Phase transition systematics in BiVO4 by means of high-pressure-high-temperature Raman experiments

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    "We report here high-pressure-high-temperature Raman experiments performed on BiVO4. We characterized the fergusonite and scheelite phases (powder and single crystal samples) and the zircon polymorph (nanopowder). The experimental results are supported by ab initio calculations, which, in addition, provide the vibrational patterns. The temperature and pressure behavior of the fergusonite lattice modes reflects the distortions associated with the ferroelastic instability. The linear coefficients of the zircon phase are in sharp contrast to the behavior observed in the fergusonite phase. The boundary of the fergusonite-to-scheelite second-order phase transition is given by TF-Sch (K) = -166(8)P(GPa) + 528(5). The zircon-to-scheelite, irreversible, first-order phase transition takes place at T-Z-(Sch )(K) = -107(8)P(GPa) + 690(10). We found evidence of additional structural changes around 15.7 GPa, which in the downstroke were found to be not reversible. We analyzed the anharmonic contribution to the wave-number shift in fergusonite using an order parameter. The introduction of a critical temperature depending both on temperature and pressure allows for a description of the results of all the experiments in a unified way.
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