Superconductivity, structural features and charge ordering in Na x CoO2·yH2O (0≤y≤1.8) system

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 Na_0.33CoO₂·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 Na_0.5CoO₂ have been investigated along with the hexagonal–orthorhombic phase transition. [Copyright &y& Elsevier]