Crystal structure of the sodium cobaltate deuterate superconductorNaxCoO2⋅4xD2O(x≈13)

Neutron and x-ray powder diffraction have been used to investigate the crystal structures of a sample of the newly-discovered superconducting sodium cobaltate deuterate compound with composition ${\mathrm{Na}}_{0.31(3)}{\mathrm{CoO}}_{2}\ensuremath{\cdot}1.25(2){\mathrm{D}}_{2}\mathrm{O}$ and its anhydrous parent compound ${\mathrm{Na}}_{0.61(1)}{\mathrm{CoO}}_{2}.$ The anhydrous parent compound ${\mathrm{Na}}_{0.61(1)}{\mathrm{CoO}}_{2}$ has two partially occupied Na sites sandwiched, in the same plane, between ${\mathrm{CoO}}_{2}$ layers. When Na is removed to make the superconducting composition, the Na site that experiences the strongest Na--Co repulsion is emptied while the occupancy of the other Na site is reduced to about one third. The deuterate superconducting compound is formed by coordinating four ${\mathrm{D}}_{2}\mathrm{O}$ molecules (two above and two below) to each remaining Na ion in a way that gives Na--O distances nearly equal to those in the parent compound. One deuteron of the ${\mathrm{D}}_{2}\mathrm{O}$ molecule is hydrogen bonded to an oxygen atom in the ${\mathrm{CoO}}_{2}$ plane and the oxygen atom and the second deuteron of each ${\mathrm{D}}_{2}\mathrm{O}$ molecule lie approximately in a plane between the Na layer and the ${\mathrm{CoO}}_{2}$ layers. This coordination of Na by four ${\mathrm{D}}_{2}\mathrm{O}$ molecules leads in a straightforward way to ordering of the Na ions and ${\mathrm{D}}_{2}\mathrm{O}$ molecules consistent with the observation of additional shorter-range scattering features in the diffraction data. The sample studied here, which has ${T}_{c}=4.5\mathrm{K},$ has a refined composition of ${\mathrm{Na}}_{0.31(3)}{\mathrm{CoO}}_{2}\ensuremath{\cdot}1.25(2){\mathrm{D}}_{2}\mathrm{O},$ in agreement with the expected 1:4 ratio of Na to ${\mathrm{D}}_{2}\mathrm{O}.$ These results show that the optimal superconducting composition should be viewed as a specific hydrated compound, not a solid solution of Na and ${\mathrm{D}}_{2}\mathrm{O}$ $({\mathrm{H}}_{2}\mathrm{O})$ in ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}\ensuremath{\cdot}{\mathrm{D}}_{2}\mathrm{O}.$ The hydrated superconducting compound may be stable over a limited range of Na and ${\mathrm{D}}_{2}\mathrm{O}$ concentration, but studies of ${T}_{c}$ and other physical properties vs Na or ${\mathrm{D}}_{2}\mathrm{O}$ composition should be viewed with caution until it is verified that the compound remains in the same phase over the composition range of the study.