Effect of Ta substitution on the synthesis of (K,Na)(Nb,Ta)O₃ powders by hydrothermal reaction: Insight into the combination of alkaline solution and raw powder

Ta substituted (K,Na)NbO₃ powders with K-rich composition across polymorphic phase boundary were synthesized at 200 °C by hydrothermal method. The amount of Ta content in the powders was controlled by changing the nominal composition of raw powders, C = [Ta₂O₅]/([Nb₂O₅] + [Ta₂O₅]). Elemental mappings using a transmission electron microscope showed all the elements (K, Na, Nb, and Ta) were contained in the powders. X-ray diffraction measurement showed that (K,Na)(Nb,Ta)O₃ solid solution powders can be obtained for all nominal compositions (C = 01), and that the orthorhombic-tetragonal phase transition can be induced by controlling C value, yet the diffraction peaks arising from a secondary phase were also detected in the range of C = 0.51. Scanning electron microscopy (SEM) observation revealed that the morphology of powder changed from polymorphic to square, and the size of the square-shaped (K,Na)(Nb,Ta)O₃ powder decreased, with increasing the nominal composition. Temporal evolution of the emerged phases revealed that the range of intermediate phase became shorter with increasing the nominal composition, allowing the perovskite phase to form from early stages. In addition, the cause of the secondary phase was unreacted raw powder. Observations on the powder synthesis with end-member composition (KOHNb₂O₅, NaOHNb₂O₅, KOHTa₂O₅, and NaOHTa₂O₅) has shown that perovskite single-phase was formed in KOH-Nb₂O₅, NaOH-Nb₂O₅, and KOH-Ta₂O₅, while that unreacted raw powder was detected in the combination of NaOH and Ta₂O₅. These results demonstrated that Ta substitution is an effective way to control the morphology of powder and that the synthesis behavior of (K,Na)(Nb,Ta)O₃ is strongly dependent on the combination of alkaline solution and raw powder.