Structure and dynamics of superconducting NaxCoO2 hydrate and its unhydrated analog
J. W. Lynn, Q. Huang, C. M. Brown, V. L. Miller, M. L. Foo, R. E. Schaak, C. Y. Jones, E. A. Mackey, R. J. Cava
DOI 10.1103/PhysRevB.68.214516 · Physical Review B
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Abstract
Neutron scattering has been used to investigate the crystal structure and lattice dynamics of superconducting Na0.3CoO2⋅1.4(H/D)2O, and the “parent” Na0.3CoO2 material. The structure of Na0.3CoO2 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 (TC increases with decreasing thickness). The phonon density of states for Na0.3CoO2 exhibits distinct acoustic and optic bands, with a high-energy cutoff of ∼100meV. 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.
Source-reported materials — not catalogue approval
| Formula | Reported Tc (K) | Pressure (GPa) | Type |
|---|---|---|---|
| Na0.3CoO2*1.4D2O Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula. | 2.5 | Pressure not reported | onset |
| Na0.3CoO2*1.4(D1.76H0.24O) Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula. | 4.5 | Pressure not reported | onset |
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