MECHANISM OF BATIO3 FORMATION THROUGH GEL-TO-CRYSTALLITE CONVERSIONS

The wet chemical synthesis of BaTiO3 through gel-to-crystallite conver sions involves the reaction of coarse TiO2.xH2O (10<x<120) gels, free of anionic contaminants, with Ba(OH)2 solutions under refluxing condit ions at T less-than-or-equal-to 100-degrees-C, giving rise to nanosize d crystallites. The paper focuses on the mechanism of formation of BaT iO3. The reaction kinetics were monitored for various temperatures and concentrations and were found to be strongly dependent on Ba(OH)2 con centration. Two regions of concentration could be detected: below 0.15 M Ba(OH)2, polytitanates are formed, whereas at higher concentrations , i.e., greater than 0.15 M, perovskite phase is stabilised. Analyses of the kinetic data were carried out using various kinetic models used for heterogeneous reactions. Under higher concentrations of Ba(OH)2, a bulk diffusional process dominates, accompanied by the collapse of t he gel and a large decrease in volume. The present results also indica te the general features of gel-to-crystallite conversions, involving i nstability of the metal hydroxide gel brought about by the upset of io nic pressure in the gel, as a result of faster migration of Ba2+ ions through the solvent cavities within the gel framework. It is proposed that with increasing pH within the gel, de-olation of the bridging gro ups such as Ti-OH-Ti and Ti-O-Ti takes place, followed by oxolation, l eading to the formation of corner-sharing TiO6 octahedra that are char ge-compensated by Ba2+ ions.