Synthetic Mechanism of Perovskite-Type KNbO₃ by Modified Solid-State Reaction Process

Perovskite-type KNbO 3 could be synthesized at low temperature (<600 °C) via a modified solid-state reaction method in which urea played an important role. The synthetic mechanism of KNbO 3 via the modified solid-state reaction process has been studied by high-temperature X-ray diffraction (HT-XRD), thermogravimetry/differential thermal analysis/mass spectrometry (TG/DTA/MS), and time-resolved energy-dispersive X-ray absorption fine structure (DXAFS) to elucidate the role of the urea. The HT-XRD showed that the urea was decomposed at 100–150 °C and KNbO 3 was formed at 500–550 °C via a metastable *KNbO 3 phase as an intermediate. From the DXAFS analysis, the structural change of the Nb 2 O 5 started at 480 °C to form the perovskite-type KNbO 3 . TG/DTA/EGA analysis showed that the urea decomposed at <400 °C. On the other hand, most K 2 C 2 O 4 was decomposed at 452–480 °C and K 2 O would be generated. The formed K 2 O reacted with Nb 2 O 5 at 480–550 °C to form the perovskite-type KNbO 3, because the exothermic signal could be observed at ∼452 °C. From the above results, we propose a synthetic mechanism of perovskite KNbO 3 via the modified solid-state reaction process. In addition, we found that the urea contributes to the decomposition of K 2 C 2 O 4 at low temperatures.