Hydrothermal epitaxy of KTaO3 thin films under supercritical water conditions

Potassium tantalate powders were hydrothermally synthesized at 400 °C using Ta₂O₅ and KOH as starting materials with various KOH concentrations (0.1–1.0 M) and heating durations (2–48 h). A defect pyrochlore phase of KTa₂O₅(OH)·nH₂O, was obtained at low KOH concentration and short reaction time. The perovskite phase of KTaO₃ predominated as the heating duration and the KOH concentration increased. Pure KTaO₃ was obtained in 0.5–1.0 M KOH aqueous solutions and reaction times of 8–48 h. Heteroepitaxitial KTaO₃ thin films were achieved on the (100) SrTiO₃ substrate in 0.5 M KOH solution under supercritical water conditions. Based on the XRD, SEM-EDX and EBSP (Electron BackScatter Pattern) results, epitaxial KTaO₃ crystals were grown on the (100) oriented single-crystal SrTiO₃ substrate. Synthesis of perovskite KTaO₃ crystals in supercritical water employed significantly low KOH concentrations (<0.5 M), which was far less than the very high concentrations (>7 M) required for conventional hydrothermal method.