Nickel hydroxide and derived phases obtained by chimie douce from NaNiO2

Sodium nickelate NaNiO2, exhibits NiO2 slabs made of edge-sharing NiO6 octahedra. As these typical structural units are found in nickel oxyhydroxide and hydroxide, NaNiO2 can be used as parent compound to prepare a lot of well crystallized oxyhydroxide and hydroxide derivatives. Moreover Co3+ ions can be substituted for Ni3+ in NaNiO2. Chimie douce reactions (exchange, oxidation and reduction) on this material lead to a large variety of new layered compounds. According to the reaction conditions the oxyhydroxides and the hydroxides can be unhydrated or intercalated with one or two layers of water molecules. When Ni is replaced by cobalt, which is less reducible, the substituting cation stays in the hydroxide at the trivalent state. The excess of positive charges is compensated by intercalation of negative species like CO2-3 between the slabs. Some of these materials have a practical interest as they are involved in the nickel electrode of the nickel-cadmium battery for example. The topotactic character of the chimie douce reactions has been shown by electron microscopy.