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Abstract: The electronic structure of Sr0.35CoO2, structurally analogous to the layered Na x CoO2, has been evaluated using the local spin density approximation (LSDA). We find that evident c-dispersions appear in both the and a 1g Co-derived bands, demonstrating the existence of a notable interplanar interaction in Sr0.35CoO2. The LSDA+U calculation reveals that the electronic structure, in particular the band splitting between the spin-up and spin-down electrons, changes evidently along with the increase of the effective on-site Coulomb interaction U. Analysis of theoretical and experimental electron energy-loss spectra (EELS) for the oxygen K-edge and the Co L23-edge suggests that the on-site interaction (U) in Sr0.35CoO2 is less than 3eV which is noticeably weaker than the estimated value (from 5 to 8eV) in the Na x CoO2 materials. [Copyright &y& Elsevier]

Abstract: The nonstoichiometric Na x CoO2 system exhibits extraordinary physical properties that correlate with temperature and Na concentration in its layered lattice without evident long-range structure modification when conventional crystallographic techniques are applied. For instance, Na0.7CoO2, a thermodynamically stable phase, shows large thermoelectric power; water-intercalated Na0.33CoO2·1.3H2O is a newly discovered superconductor with T c∼4K, and Na0.5CoO2 exhibits an unexpected charge ordering transition at around T co∼55K. Recent studies suggest that the transport and magnetic properties in the Na x CoO2 system strongly depend on the charge carrier density and local structural properties. Here we report a combined variable temperature transmission electron microscopy and Raman scattering investigation on structural transformations in Na0.5CoO2 single crystals. A series of structural phase transitions in the temperature range from 80 to 1000K are directly identified and the observed superstructures and modulated phases can be interpreted by Na-ordering. The Raman scattering measurements reveal phase separation and a systematic evolution of active modes along with phase transitions. Our work demonstrates that the high mobility and ordering of sodium cations among the CoO2 layers are a key factor for the presence of complex structural properties in Na x CoO2 materials, and also demonstrate that the combination of electron diffraction and Raman spectroscopy measurements is an efficient way for studying the cation ordering and phase transitions in related systems. [Copyright &y& Elsevier]

Abstract: The electronic structure of β-Na0.33V2O5 has been evaluated using the first-principle density functional theory approach. All energy bands near the Fermi surface (FS) disperse principally along the b-axis direction indicating the quasi-one-dimensionality of this system. The theoretical simulation of the optical property yields reasonable explanations for the notable features revealed in the measurements of the optical spectroscopy. Superconductivity appearing under a pressure of 8GPa has been discussed in connection with the pressure-induced structural and bands alternations. It is suggested that the strong interchain coupling could play a key role in the appearance of superconductivity. The electron correlation effects on the electronic structure have also been calculated and discussed in comparison with photoemission data. [Copyright &y& Elsevier]

Abstract: A new inverse spinel ScMn2O4 has been synthesized and characterized by measurements of structural and physical properties. The crystal structure of ScMn2O4, similar with the spinel Mn3O4, is made up of Mn2+ located at tetrahedral site and Mn3+/Sc3+ ions randomly located at the octahedral site. Experimental results of magnetic susceptibility and heat capacity demonstrate that ScMn2O4 undergoes a ferrimagnetic phase transition at the temperature of about 58K. Extensive analyses on the data obtained from structural refinement, electronic structural calculation and EELS spectra measurement suggests that substitution of Sc for Mn in MnO6 octahedron could greatly suppress Jahn–Teller distortions in comparison with what observed in Mn3O4. [Copyright &y& Elsevier]