Hydrothermal synthesis of sodium potassium niobate solid solutions at 200 °C

For the first time, a series of sodium potassium niobate solid solutions with compositions around the morphotropic phase boundary (MPB) are hydrothermally synthesized at 200 °C using a simple KOH and NaOH mixture with Nb₂O₅ as a precursor powder. Rietveld refinement of X-ray diffraction data indicated the presence of a second sodium niobate perovskite phase when the concentration of NaOH (compared to the total hydroxyl ion concentration) is above 11.7%. The presence of the second phase is attributed to the different solubilities of the intermediate potassium and sodium hexaniobate species. It is also found that heat-treating the mixed-phase powders for two hours at a temperature of 800 °C is effective in obtaining the desired single-phase solid solution with compositions near the MPB, thereby opening the way to using hydrothermal synthesis in simplifying the laborious solid-state process.