In situ X-ray diffraction for millisecond-order dynamics of BaZrO₃ nanoparticle formation in supercritical water

In situ high-energy X-ray diffraction measurements were conducted to elucidate the dynamics of barium zirconate (BaZrO₃) nanoparticle formation in supercritical water. Time-resolved experiments of approximately milliseconds were achieved using a continuous-flow reactor and high-energy X-ray diffraction, which allows us to probe the sample in a stainless-steel tube. The size and structure of BaZrO₃ in the early stage of crystallization under supercritical conditions (400 °C, 30 MPa) and on the order of milliseconds (46-66 ms) were successfully observed. An increase in the lattice parameter of BaZrO₃ owing to a decrease in the number of defect sites in the time range was observed for the first time. A first-principles calculation confirmed the relationship between Ba defects and the lattice parameter and supported the formation mechanism of BaZrO₃ in which nuclei, consisting of mainly zirconium, absorb the barium ion in supercritical water during crystallization.