PREPARATION AND CHARACTERIZATION OF COBALT-SUBSTITUTED ALPHA-NICKEL HYDROXIDES STABLE IN KOH MEDIUM .1. ALPHA'-HYDROXIDE WITH AN ORDERED PACKING

An alpha-cobalt-substituted nickel hydroxide has been prepared from a beta(II)-substituted nickel hydroxide by an oxidation to a gamma-type phase followed by a reduction. This alpha'-Co3+-hydroxide crystallizes in the hexagonal system with a P3-type hydroxyl packing (a = 3.1 angstrom, c = 23.5 angstrom). Beta(II)- and alpha'-cobalted phases have been characterized by chemical analysis, TGA, u.v.-visible and i.r. spectroscopies. In both materials cobalt ions remain in the trivalent state. In alpha'-Co3+-hydroxide, the excess of positive charge is compensated by carbonate anions which are either inserted between the M(OH)2 slabs, or adsorbed on the particles. The infrared study shows that inserted anions are totally hydrogen bonded with intercalated water molecules (D3h Symmetry), while the adsorbed ions are linked to the nickel or cobalt ions in substitution of hydroxyl groups. In beta-Co3+-hydroxide, the charge compensation due to the presence of trivalent cobalt ions is provided by chemisorbed carbonate anions and by a proton deficiency in the sheets. The bonding between the inserted carbonate anions and the Co3+ cations increases the slab cohesion; as a result the alpha'-Co3+-phase (x(Co) greater-than-or-equal-to 0.2) is stable indefinitely in KOH medium.