Structure and dynamics of superconducting Na_xCoO₂ hydrate and its unhydrated analog

Neutron scattering has been used to investigate the crystal structure and lattice dynamics of superconducting Na_0.3CoO₂·1.4(H/D)₂O, and the “parent” Na_0.3CoO₂ material. The structure of Na_0.3CoO₂ consists of alternate layers of CoO₂ and Na and is the same as the structure at higher Na concentrations. For the superconductor, the water forms two additional layers between the Na and CoO₂, increasing the c-axis lattice parameter of the hexagonal P6₃/mmc space group from 11.16 Å to 19.5 Å. The Na ions are found to occupy a different configuration from the parent compound, while the water forms a structure that replicates the structure of ice. Both types of sites are only partially occupied. The CoO₂ layer in these structures is robust, on the other hand, and we find a strong inverse correlation between the CoO₂ layer thickness and the superconducting transition temperature (T_C increases with decreasing thickness). The phonon density of states for Na_0.3CoO₂ exhibits distinct acoustic and optic bands, with a high-energy cutoff of ∼100 meV. The lattice dynamical scattering for the superconductor is dominated by the hydrogen modes, with librational and bending modes that are quite similar to ice, supporting the structural model that the water intercalates and forms ice-like layers in the superconductor.