Abstract
Synthesis of lanthanoid cobaltates, LnCoO3 (Ln: La, Pr, Dy), via the mechanochemically prepared complex (oxy-)hydroxides has been studied. The hydroxides are prepared by grinding Co(OH)z or Co3O4 with Ln2O3 (Ln: La, Dy) or Pr6O11 in acetone containing a very small amount of H2O in LnCoO3 • 4H2O composition. The starting hydroxides act as the reactive grinding aids to each other. La2O3 and Co(OH)2 form the complex hydroxide, LaCo(OH)5, that is a sort of inorganic polymers having La-O-Co bond. The hydration affinity of trivalent Co3+ ions is not high enough to form complex hydroxide LaCo(OH)6. Then, the addition of H2O2 to the grinding liquid results in the formation of oxyhydroxide, LaCoO(OH)4. These (oxy-)hydroxides are directly converted to pseudo-tetragonal LaCoO3 at 600°C and rhombohedral LaCoO3 above 800°C without forming any by-product. The redox interaction between Co(OH)2 and Pr6O11, which shows the poor reactivity with Fe2O3, enables the formation of PrCoO(OH)4. Contrary, the reaction with Dy2O3 becomes poor.
