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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]