Characterization of sodium cobalt oxides related to P3-phase superconductor

This paper reports characterization of sodium cobalt oxides related to a P3-phase superconductor (space group R (3) over barm) newly found other than P2 phase (space group P6(3)/MVIC). alpha-NaCO02 used as a parent compound for the synthesis of the superconductor was oxidized to be Na0.42CoO2, in which the rhombohedral structure in the parent phase providing octahedral coordination environment for Na+ ions changed to a monoclinic lattice with trigonal prismatic sites. Hydration of the as-oxidized Na0.42CoO2) to the superconducting bilayer-hydrate phase was accompanied by the ion exchange between Na+ and H3O+ ions and reductive insertion of H3O+ ions as observed in the formation of P2-phase superconductor. Partial dehydration of the superconducting phase transformed its bilayer-hydrate structure into a monolayer-hydrate one, in which the superconductivity disappeared again. Influence of the decreasing interlayer distance caused by the dehydration upon the guest species is discussed in combination with the results of Raman spectroscopy.