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

Perovskite-type KNbO3 could be synthesized at low temperature (<600 degrees C) via a modified solid-state reaction method in which urea played an important role. The synthetic mechanism of KNbO3 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 degrees C and KNbO3 was formed at 500-550 degrees C via a metastable *KNbO3 phase as an intermediate. From the DXAFS analysis, the structural change of the Nb2O5 started at 480 degrees C to form the perovskite-type KNbO3. TG/DTA/EGA analysis showed that the urea decomposed at <400 degrees C. On the other hand, most K2C2O4 was decomposed at 452-480 degrees C and K2O would be generated. The formed K2O reacted with Nb2O5 at 480-550 degrees C to form the perovskite-type KNbO3, because the exothermic signal could be observed at similar to 452 degrees C. From the above results, we propose a synthetic mechanism of perovskite KNbO3 via the modified solid-state reaction process. In addition, we found that the urea contributes to the decomposition of K2C2O4 at low temperatures.