Superconductivity, structural features and charge ordering in NaxCoO2·yH2O (0≤y≤1.8) system

Structural and physical properties of NaxCoO2 and superconducting NaxCoO2·yH2O hydrates have been systematically investigated. Raman scattering measurements reveal that the five active phonons in NaxCoO2·yH2O system change systematically with the H2O intercalation and modification of superconductivity, in particular the A1g and E1g modes in correlation with oxygen movements. Incoherent inelastic neutron scattering (IINS) study on the local structure of H2O-layers in NaxCoO2·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.