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.