Ca and Co substitutions in (Ca,Co)(OH)2 hydroxides

According to DFT simulation, no mixed hydroxide containing calcium and a small divalent cation with the brucite structure is possible. This experimental study confirms that between portlandite (Ca(OH)2) and β-Co(OH)2, a solid solution does not exist. Samples have been synthesized by coprecipitation under flowing nitrogen at room temperature. However, XRD, TEM and DTA/TG analyses show that a partial solubility exists and that the substitution limit of calcium in β-Co(OH)2 is lower than 12% and lower than 7% for cobalt in Ca(OH)2. The two kinds of particles exhibit similar plate-like morphology with a size between 50 and 200 nm and a thickness of about 10 nm. The origin of such low substitution limits could be the difference in radii between calcium and cobalt generating local stresses. Substitutions by small amounts of different cations to reduce the local stresses such as Al, Sr or Cu have failed to allow improving significantly the substitution limits. An alternative synthesis route consisting in contacting the first cation oxide in a solution containing a salt of the second cation has also failed to reach a complete solid solution between portlandite and β-Co(OH)2.