Search

Abstract: Structural and physical properties of Na x CoO2 and superconducting Na x CoO2·yH2O hydrates have been systematically investigated. Raman scattering measurements reveal that the five active phonons in Na x CoO2·yH2O system change systematically with the H2O intercalation and modification of superconductivity, in particular the A 1g and E 1g modes in correlation with oxygen movements. Incoherent inelastic neutron scattering (IINS) study on the local structure of H2O-layers in Na x CoO2·yH2O (0≤y≤1.8) suggests the presence of three types of clusters, H3O+, H5O+ and ice-Ih like weakly bonded water, among CoO2 sheets. Electron energy loss (EEL) analysis on Na0.33CoO2·yH2O (0≤y≤1.3) indicates certain notable changes in the electronic structure during hydration. Microstructure and structural phase transitions accompanied with local atomic ordering in the charge-ordered phase Na0.5CoO2 have been investigated along with the hexagonal–orthorhombic phase transition. [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: Transmission electron microscopy (TEM) observations reveal the presence of complex superstructures and remarkable phase separation in association with Na-ordering phenomenon in γ-Na0.5CoO2. Resistivity and magnetization measurements indicate that three phase transitions at the temperatures of 25, 53 and 90K, respectively, appear commonly in γ-Na0.5CoO2 samples. Under a high pressure up to 10kbar, the low-temperature transport properties show certain changes below the charge order transition; under an applied magnetic field of 7T, phase transitions at around 25 and 53K, proposed fundamentally in connection with alternations of magnetic structure and charge ordering maintain almost unchanged. [Copyright &y& Elsevier]

Abstract: The superconductivity in Na0.3Co1−x M x O2 ·1.3H2O and the charge ordering in Na0.5Co1−x M x O2 have been investigated for M=Mn and Ti substitution for Co. Single-phased Na0.5Co1−x M x O2 (0⩽ x ⩽0.1, M=Mn, Ti) materials were prepared by Na deintercalation from Na0.75Co1−x M x O2; the hydrated Na0.3Co1−x M x O2 ·1.3H2O (0⩽ x ⩽0.1, M=Mn, Ti) materials were prepared by Na deintercalation and then water intercalation. Measurements of physical properties demonstrated that the superconductivity in Na0.3Co1−x M x O2 ·1.3H2O is evidently influenced by the Co-site doping and completely ceases at x =0.02 for Mn doping and x =0.01 for Ti doping. Analysis of resistivity and magnetization on Na0.5Co1−x M x O2 revealed that Mn doping has much stronger influence than Ti doping on both the charge ordering and the low-temperature physical properties. [Copyright &y& Elsevier]