Structure
Like other alkali metal, alkali-cobalt oxides, sodium cobaltate has a layer structure. Layers of monovalent sodium cations () alternate with two-dimensional anionic sheets of cobalt and oxygen atoms. Each cobalt atom is bound to six oxygen atoms forming an octahedron, with two faces parallel to the layer plane. The octahedra share edges, resulting in a layer of cobalt atoms sandwiched between two layers of oxygen atoms, all three with regular triangular roughly planar lattice . The structure is reminiscent of cuprate superconductors, except that the copper atom arrangement in the latter is a square lattice. The cobalt atoms have formal oxidation state 4−''x''. Namely, the fully reduced compound can be interpreted as ··. As the compound is oxidized, sodium cations exit the structure and the cobalt formally approaches the state. For ''x'' above 0.5, the sodium ions adopt many different arrangements in which Na ions occupy two inequivalent Wyckoff sites, 2b and 2d, of the space group P63/mmc. In galvanostatic experiments, the arrangements transition at specific values of ''x'' as the sodium content is electrolysis, electrolytically varied. The diffusion rate of the ions, plotted as a function of ''x'', shows sharp dips (from about 10−7 to 10−10 centimeter, cm2/second, s at ambient temperature) at values of ''x'' that correspond to particular regular arrangements, namely 1/3, 1/2, and 5/7. Smaller and broader dips are observed around some other simple ratios, like 5/9. For ''x'' = 0.8, at 100 kelvin, K the vacancies in the sodium layer are arranged in clusters of three. The clusters are arranged in stripes, with a fixed offset between the clusters in adjacent stripes. In those conditions, the diffusion rate of the sodium atoms is minimal. At about 290 K, the structure becomes partially disordered, with the offset of between adjacent stripes becoming random. creating channels that allow their quasi-dimension, unidimensional diffusion. The sodium lattice "melts" at about 370 K, allowing two-dimensional diffusion. As ''x'' increases, the conductivity along the main crystal planes increases, until about ''x'' = 0.85, and is roughly independent of ''x'' thereafter. The temperature dependency at those higher concentrations has metallic character. The Seebeck effect, thermopower ''S'' increases with ''x'' up to 0.97, but drops for higher ''x''. For each composition, as a function of temperature it increases rapidly until about 130 K, and then decreases gradually. The figure of merit ''Z'' = ''S''/ρκ (where ρ is the in-plane resistivity and κ is the thermal conductivity) is maximum for ''x'' about 0.89 at about 65 K.Minhyea Lee, Liliana Viciu, Luli Yayuwang, M. L. Foo, S. Watauchi, R. A. Pascal Jr., R. J. Cava, and N. P. Ong (2006): "Large enhancement of the thermopower in at high Na doping". ''Nature Materials Letters'', volume 6, pages 537–540,Preparation
The fully reduced compound can be prepared by dissolving stoichiometric amounts of sodium acetate and cobalt tartrate in ethanol with a gelling agent, drying and calcinating the resulting gel, and annealing it at 650 °C.Nur Khairani Samin, Roshidah Rusdi, Norashikin Kamarudin, and Norlida Kamarulzaman (2012), "Synthesis and Battery Studies of Sodium Cobalt Oxides, ". ''Advanced Materials Research'', volume 545, pages 185–189. The compound (or ) can be obtained in the form of platelets up to 6 mm wide from metallic cobalt powder, by treatment with molten sodium chloride and sodium hydroxide at 550 °C.Xiaofeng Tang (2005),See also
* Lithium cobalt oxideReferences