Low temperature hydrothermal synthesis and formation mechanisms of lead titanate (PbTiO3) particles using tetramethylammonium hydroxide:: thermodynamic modelling and experimental verification

Thermodynamic modeling was used to predict the optimum synthesis conditions for precipitation of the phase-pure lead titanate (PbTiO3) in the Pb-Ti-tetramethylammonium hydroxide (TMAH) system using a newly developed computer program for automatic generation of stability and yield diagrams. The thermodynamic model has been experimentally validated over a wide range of processing conditions. Like KOH-mineralized systems, it was determined that the pH of the hydrothermal reaction medium and the Pb/Ti ratio are critical factors in forming stoichiometric PbTiO3 powder. Morphological evolution during the reaction suggests that the formation mechanism appears to be controlled by a dissolution and recrystallization process. Two possible growth mechanisms are proposed based on the magnitude of the Pb/Ti ratio. In the case of Pb/Ti ratio= 1.1, at the early stage of the reaction (3 h) excess lead species promote the formation of spherical intermediate pyrochlore phase followed by the formation of primary cubic PbTiO3 crystals. The growth of cubic PbTiO3 crystals proceeds until the intermediate phase acting as a reservoir to provide precipitating ions is consumed. In case of Pb/Ti ratio= 1.25, excess lead condition leads to the formation of a platelet-shaped intermediate pyrochlore phase. These platelet intermediate particles act as a template in which small cubic shaped PbTiO3 grains grew on the surface of these platelets. (C) 2003 Elsevier Ltd. All rights reserved.