Electronic phase diagram of the layered cobalt oxide system, LixCoO2 (0.0 <= x <= 1.0)

Here we report the magnetic properties of the layered cobalt oxide system, LixCoO2, in the whole range of Li composition, 0 <= x <= 1. Based on dc-magnetic susceptibility data, combined with results of 59Co-NMR/NQR observations, the electronic phase diagram of LixCoO2 has been established. As in the related material NaxCoO2, a magnetic critical point is found to exist between x = 0.35 and 0.40, which separates a Pauli-paramagnetic and a Curie-Weiss metals. In the Pauli-paramagnetic regime (x <= 0.35), the antiferromagnetic spin correlations systematically increase with decreasing x. Nevertheless, CoO2, the x = 0 end member is a non-correlated metal in the whole temperature range studied. In the Curie-Weiss regime (x >= 0.40), on the other hand, various phase transitions are observed. For x = 0.40, a susceptibility hump is seen at 30 K, suggesting the onset of static AF order. A magnetic jump, which is likely to be triggered by charge ordering, is clearly observed at Tt = 175 K in samples with x = 0.50 (= 1/2) and 0.67 (= 2/3), while only a tiny kink appears at T = 210 K in the sample with an intermediate Li composition, x = 0.60. Thus, the phase diagram of the LixCoO2 system is complex, and the electronic properties are sensitively influenced by the Li content (x).Comment: 29 pages, 1 table, 9 figure

Impact of lithium composition on the thermoelectric properties of the layered cobalt oxide system LixCoO2

Thermoelectric properties of the layered cobalt oxide system LixCoO2 were investigated in a wide range of Li composition, 0.98 >= x >= 0.35. Single-phase bulk samples of LixCoO2 were successfully obtained through electrochemical deintercalation of Li from the pristine LiCoO2 phase. While LixCoO2 with x >= 0.94 is semiconductive, the highly Li-deficient phase (0.75 >= x >= 0.35) exhibits metallic conductivity. The magnitude of Seebeck coefficient at 293 K (S293K) significantly depends on the Li content (x). The S293K value is as large as +70 ~ +100 uV/K for x >= 0.94, and it rapidly decreases from +90 uV/K to +10 uV/K as x is lowered within a Li composition range of 0.75 >= x >= 0.50. This behavior is in sharp contrast to the results of x <= 0.40 for which the S293K value is small and independent of x (+10 uV/K), indicating that a discontinuous change in the thermoelectric characteristics takes place at x = 0.40 ~ 0.50. The unusually large Seebeck coefficient and metallic conductivity are found to coexist in a narrow range of Li composition at about x = 0.75. The coexistence, which leads to an enhanced thermoelectric power factor, may be attributed to unusual electronic structure of the two-dimensional CoO2 block.Comment: 29 pages, 1 table, 8 figure

Study of the effect of the tensor correlation in oxygen isotopes with the charge- and parity-projected Hartree-Fock method

Recently, we developed a mean-field-type framework which treats the correlation induced by the tensor force. To exploit the tensor correlation we introduce single-particle states with the parity and charge mixing. To make a total wave function have a definite charge number and a good parity, the charge number and parity projections are performed. Taking a variation of the projected wave function with respect to single-particle states a Hartree-Fock-like equation, the charge- and parity-projected Hartree-Fock equation, is obtained. In the charge- and parity-projected Hartree-Fock method, we solve the equation selfconsistently. In this paper we extend the charge- and parity-projected Hartree-Fock method to include a three-body force, which is important to reproduce the saturation property of nuclei in mean-field frameworks. We apply the charge- and parity-projected Hartree-Fock method to sub-closed-shell oxygen isotopes (14O, 16O, 22O, 24O, and 28O) to study the effect of the tenor correlation and its dependence on neutron numbers. We obtain reasonable binding energies and matter radii for these nuclei. It is found that relatively large energy gains come from the tensor force in these isotopes and there is the blocking effect by occupied neutron orbits on the tensor correlation

Chiral Sigma Model with Pion Mean Field in Finite Nuclei

The properties of infinite matter and finite nuclei are studied by using the chiral sigma model in the framework of the relativistic mean field theory. We reconstruct an extended chiral sigma model in which the omega meson mass is generated dynamically by the sigma condensation in the vacuum in the same way as the nucleon mass. All the parameters of chiral sigma model are essentially fixed from the hadron properties in the free space. In nuclear matter, the saturation property comes out right, but the incompressibility is too large and the scalar and vector potentials are about a half of the phenomenological ones, respectively. This fact is reflected to the properties of finite nuclei. We calculate N = Z even-even mass nuclei between N = 16 and N = 34. The extended chiral sigma model without the pion mean field leads to the result that the magic number appears at N = 18 instead of N = 20 and the magic number does not appear at N = 28 due to the above mentioned nuclear matter properties. The latter problem, however, could be removed by the introduction of the finite pion mean field with the appearance of the magic number at N = 28. We find that the energy differences between the spin-orbit partners are reproduced by the finite pion mean field which is completely a different mechanism from the standard spin-orbit interaction.Comment: 19 pages, 9 figures. Prog. Theor. Phys. to be publishe

Charge storage mechanism of nanostructured anhydrous and hydrous ruthenium-based oxides

ArticleElectrochimica Acta. 52(4): 1742-1748 (2006)journal articl

Particle growth behavior of carbon-supported Pt, Ru, PtRu catalysts prepared by an impregnation reductive-pyrolysis method for direct methanol fuel cell anodes

ArticleJournal of Catalysis. 229(1):176-184 (2005)journal articl

Effect of the tensor force in the exchange channel on the spin-orbit splitting in 23F in the Hartree-Fock framework

We study the spin-orbit splitting ($ls$-splitting) for the proton d-orbits in 23F in the Hartree-Fock framework with the tensor force in the exchange channel. 23F has one more proton around the neutron-rich nucleus 22O. A recent experiment indicates that the ls-splitting for the proton d-orbits in 23F is reduced from that in 17F. Our calculation shows that the ls-splitting in 23F becomes smaller by about a few MeV due to the tensor force. This effect comes from the interaction between the valence proton and the occupied neutrons in the 0d5/2 orbit through the tensor force and makes the ls-splitting in 23F close to the experimental data

Performance of ternary PtRuRh/C electrocatalyst with varying Pt : Ru : Rh ratio for methanol electro-oxidation

ArticleJournal of Applied Electrochemistryjournal articl

Oxygen reduction behavior of rutile-type iridium oxide in sulfuric acid solution

ArticleELECTROCHIMICA ACTA. 54(2):566-573 (2008)journal articl