Effect of Na content and hydration on the excitation spectrum of the cobaltite Na_<i>x</i>CoO₂·<i>y</i>H₂O

Abstract

We report on a Raman scattering study on the superconducting cobaltite\n$Na_xCoO_2\\cdot yH_2O$ as function of Na content and hydration (x$\\approx$1/3,\n3/4 and y$\\approx$0, 2/3, 4/3). The observed phonon scattering and scattering\ncontinua are analyzed in terms of lattice strain due to the structural misfit\nand disorder. Hydration, due to the intercalation of one or two $H_2O$ layers,\nreleases a part of this strain. Our Raman data suggest a connection between\ndisorder on the partly occupied Na sites, the split off of the $a_{1g}$ level\nfrom the other $t_{2g}$ states of $Co^{4+}$ and superconductivity.\n