R. S. Liu

Pb-for-Bi substitution for enhancing thermoelectric characteristics of [(Bi,Pb)2Ba2O4±ω]0.5CoO2

We report strongly enhanced thermoelectric characteristics for a misfit-layered oxide, [Bi2Ba2O4±ω]0.5CoO2, in a wide temperature range, as achieved through substituting up to 20% of Bi by Pb. The Pb substitution kept the thermal conductivity (κ) unchanged but decreased the electrical resistivity (ρ) and increased the Seebeck coefficient (S) simultaneously, such that a three-fold enhancement in the thermoelectric figure of merit, Z (≡S2∕ρκ), was realized. At the same time x-ray absorption near-edge structure data indicated that the valence and spin states of Co are not affected by the Pb-for-Bi substitution.

Studies of novel deuterides RMn2D6 (R — rare earth) compressed in DAC up to 30 GPa

The exposure of RMn2 (C15 or C14 cubic Laves phase, where R = Y; Dy; Ho or Er) to high deuterium pressure leads to formation of novel, unique YMn2D6, DyMn2D6, HoMn2D6 and ErMn2D6 deuterides with cubic Fm-3m symmetry. In spite of different structures and molecular volumes of parent RMn2 compounds, the molar volumes of RMn2D6 deuterides are almost identical. In this paper, we present results of studies on RMn2Dx (where R = Y, Dy, Ho and Er) submitted to compression up to 30 GPa in diamond anvil cell (DAC) combined with energy dispersive X-ray diffraction. The EOS (equation of state) parameters of the above four RMn2D6 samples and YMn2Dx, with x ? 4 are compared. The EOS parameters of YMn2D6 are very similar to those of other RMn2D6 but very different than those of interstitial deuterides YMn2Dx (x ? 4). The phase transition or segregation was not detected in RMn2D6 up to 30 GPa.

Structural, electronic and magnetic properties of ErFeMn and ErFeMnH 4.7 compounds

ErFeMn intermetallic alloy after exposure to high hydrogen pressure transformed into the ErFeMnH4.7 hydride. Both parent material and hydride were investigated for their structural, electronic and magnetic properties by synchrotron XRD (x-ray diffraction), XANES (x-ray absorption near edge structure) and SQUID (superconducting quantum interference device), respectively. Hydrogenation did not change the structure symmetry but caused large expansion of the lattice parameters. Mn and Fe K-edge XANES study of the parent alloy and its hydride reveals that charge on both Mn and Fe atoms remains the same and slightly increases after hydrogenation. Hydrogenation of ErFeMn alloy also caused decrease in the magnetic moment.

XANES and wet-chemical analyses of the charge balance in (Hg,Pb)(Ba,Sr)2Ca2Cu3O8+z

Abstract Here we present the results revealed from iodometric, coulometric and cerimetric titrations and Cu L-edge and O K-edge X-ray absorption near-edge structure analyses of the concentration and distribution of holes in a series of polycrystalline (Hg,Pb)(Ba,Sr)2Ca2Cu3O8+z [(Hg,Pb)-1(Ba,Sr)223] samples. By means of oxygen-content tuning only, the average valence of copper in the pristine Hg-1(Ba)223 phase can be controlled in the range of 2.03–2.10. Substituting Hg partly with Pb increased considerably the amount of excess oxygen z in the (Hg,Pb)Oz charge-reservoir block. Lead was found to enter the phase with an oxidation state higher in average than III so as to increase the positive charge at the Hg site. However, the increase in the oxygen content counter-balanced the effect of higher-valent (PbIII/IV) for lower-valent (HgII) cation substitution such that the average CuO2-plane hole concentration was found higher for the (Hg,Pb)-1(Ba,Sr)223 sample with Tc=115 K than for the optimally doped pristine Hg-1(Ba)223 sample with Tc=134.5 K.

High-resolution XANES study of Eu(Ba1−xRx)2Cu3O7+δ (R = Eu, Pr)

We have probed the distribution of hole carriers in Eu(Ba1−xEux)2Cu3O7 + δ (x = 0–0.2) and Eu(Ba1−xPrx)2Cu3O7 + δ (x = 0–0.25) by O K-edge and Cu L-edge x-ray absorption spectra. Upon Eu and Pr substitution at the Ba site in Eu(Ba1 − xRx)2Cu3O7 + δ, the concentration of holes in the CuO2 planes becomes greatly diminished. The depletion rate of hole carriers within the CuO2 planes in Eu(Ba1 − xPrx)2Cu3O7 + δ is greater than that in Eu(Ba1 − xEux)2Cu3O7 + δ. The rate of Tc suppression with Pr doping in Eu(Ba1−xPrx)2Cu3O7 + δ is accordingly greater than that in Eu(Ba1 − xEux)2Cu3O7 + δ.

Chemical Control of Underdoped and Overdoped States in Y(Ba2 − ySr y )Cu3O6 + δ

The chemical control of underdoped and overdoped states in the Y(Ba2 − ySry)Cu3O6 + δ (δ∼0.1 and 0.9) compounds has been observed by high-resolution O K-edge X-ray-absorption near-edge-structure spectra. The chemical substitution of Sr for Ba in the fully-oxygenated Y(Ba2−ySry)Cu3O6 + δ (δ∼0.9) compounds gives rise to high hole concentrations within both the CuO2 planes and the out-of-plane sites, leading to the overdoped state and the decrease in the superconducting transition temperature from 92 K for y=0 to 84 K for y=0.8. In contrast, an increase in the Sr content in the oxygen-deficient Y(Ba2 − ySry)Cu3O6 + δ (δ∼0.1) compounds did not indicate superconductivity. The oxygen-deficient compounds exhibit the underdoped state due to the low hole concentration.

A New Anode Material LiVMoO6 for Use in Rechargeable Li-Ion Batteries

The lithiated transition metal oxide LiVMoO6 has been synthesized by solid state reaction. This is the first report of this compound to be studied as an anode material for use in secondary batteries. The synthesized LiVMoO6 powder has been studied by means of X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES) spectroscopy. The electrochemical characteristics of the prepared electrodes assembled in coin cells were also investigated in terms of half-cell performance. It is observed that the cell exhibits three stages of discharge plateaus. The total discharge capacity, averaged over several test runs, is about 1250 mAh/g. This value is much higher than the capacities exhibited by many kinds of anode materials.

Substitution effects in Bi2Sr2(Ca1-xYx)Cu2O8+δ studied by X-ray absorption spectroscopy

Abstract The Bi 2 Sr 2 (Ca 1−x Y x )Cu 2 O 8+δ system over the homogeneity range x = 0 ∼ 1.0 has a maximal superconducting transition temperature (T c ) of around 92 K at x = 0.2. The hole distribution of overdoped, optimum doped and underdoped states in Bi 2 Sr 2 (Ca 1−x Y x )Cu 2 O 8+δ has been investigated by high-resolution O K-edge X-ray-absorption near-edge-structure (XANES) spectra. Near the O 1s edge, a well-pronounced pre-edge peak with maximum at ∼528.3 eV is found, which is ascribed to the excitations of O 1s electron to O 2p hole states located in the CuO 2 planes. The intensity of this pre-edge peak decreases as the Y doping increases, demonstrating that the chemical substitution of Y 3+ for Ca 2+ in Bi 2 Sr 2 (Ca 1−x Y x )Cu 2 O 8+δ gives rise to a decrease in hole concentrations within the CuO 2 planes. Moreover, the correlation between T c and critical current density (J c ) as a function of doping concentration has been studied. It is important to point out that the highest J c across the system is appeared at the overdoped side with x = 0 in Bi 2 Sr 2 (Ca 1−x Y x )Cu 2 O 8+δ .

Soft-x-ray absorption spectroscopy ofNd1+xBa2−xCu3O7+δs(x=0–0.6)

O K-edge and Cu

Soft-x-ray absorption spectroscopy of Nd 1 + x Ba 2 − x Cu 3 O 7 + δ s(x=0–0.6)

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