Superconducting single crystal samples of Na0.3CoO2.1.3H(2)O have been produced using an electrochemical technique which dispenses with the usual bromine chemical deintercalation method. In fully hydrated crystals, susceptibility and specific heat measurements confirm that bulk superconductivity has been achieved. The extracted normal state density of states indicates Fermi-liquid behavior with strong mass enhancement and a modest Wilson ratio. Measurements of H(c2) for H parallel c and H parallel ab reveal significant anisotropy, and the extracted value for the coherence length is about 100 A, consistent with an extremely narrow bandwidth.

We have measured the magnetic susceptibility of single crystal samples of nonhydrated Na x CoO 2 ͑x Ӎ 0.75, 0.67, 0.5, 0.3͒ and hydrated Na 0.3 CoO 2 · yH 2 O ͑y Ӎ 0 , 0.6, 1.3͒. Our measurements reveal considerable anisotropy between the susceptibilities with H ʈ c and H ʈ ab. The derived anisotropic g-factor ratio ͑g ab / g c ͒ decreases significantly as the composition is changed from the Curie-Weiss metal with x = 0.75 to the paramagnetic metal with x = 0.3. Fully hydrated Na 0.3 CoO 2 · 1.3H 2 O samples have a larger susceptibility than nonhydrated Na 0.3 CoO 2 samples, as well as a higher degree of anisotropy. In addition, the fully hydrated compound contains a small additional fraction of anisotropic localized spins.

We report neutron powder diffraction measurements on a series of Na x CoO 2 samples with x near 0.75 which were prepared under different synthesis conditions. Two different crystal structures for the samples are observed at room temperature. The two structures belong to the space group P6 3 / mmc and differ primarily by a shift of a large fraction of the Na ions from a high-symmetry position to a lower-symmetry position. Close inspection of the refinement parameters indicates that the presence of either structure depends sensitively on the Na content x, with x Ӎ 0.75 as the critical concentration separating the two phases. By raising the temperature to around T Ӎ 323 K, the high-symmetry structure can be converted to the lower-symmetry structure. The transition is reversible, but there is significant hysteresis. We discuss the effects of this structural transition on the bulk magnetic and transport properties.