Hydrothermal Synthesis of Sodium and Potassium Bismuth Titanates

A thermodynamic model of electrolyte solutions has been used as a tool for equilibrium calculations in the Na-Bi-Ti-H2O and K-Bi-Ti-H2O systems. To perform the calculations, unknown standard-state properties for Na0.5Bi0.5TiO3 (NBT), K0.5Bi0.5TiO3 (KBT) and other solid compounds that exist in these systems were established. As a result of the calculations, stability and yield diagrams of the investigated systems were computed. Thus, the optimum conditions (i.e., temperature, synthesis precursor concentrations, solution pH) for the hydrothermal synthesis of phase-pure KBT and NBT are presented in a graphical form. They show the concentrations of precursors and pH-adjusting agents required for the formation of phase-pure alkaline bismuth titanates. The computed diagrams show that the formation of phase-pure sodium and potassium bismuth titanates takes place in highly alkaline solutions (e.g., at pH above 11.4 and 12.2, respectively, at 473 K). Results of our experiments validated the newly developed yield diagrams and demonstrated, for the first time, that alkaline bismuth titanates can be hydrothermally synthesized. Experimental synthesis was carried out using both conventional and microwave-heated reactors.