In situ monitoring of the formation of crystalline phases during conventional hydrothermal synthesis is experimentally challenging. Here, we report an in situ time-resolved synchrotron X-ray diffraction study during hydrothermal synthesis of NaNbO 3 using a high-pressure custom-made capillary cell penetrable to X-rays. The high time resolution (0.1 s) revealed a sequence of transient intermediate phases, including several unknown phases, before the final perovskite NaNbO 3 was formed. These new findings highlight the complexity of the hydrothermal synthesis of NaNbO 3 and demonstrate the potential for obtaining in-depth knowledge of the reactions taking place by time-resolved in situ X-ray diffraction.

In situ techniques are powerful for providing insight into the determining factors when preparing KxNa1−xNbO3 nanoparticles with a designed composition, structure and size.

Sodium niobate (NaNbO 3 ) attracts attention for its great potential in a variety of applications, for instance, due to its unique optical properties. Still, optimization of its synthetic procedures is hard due to the lack of understanding of the formation mechanism under hydrothermal conditions. Through in situ X-ray diffraction, hydrothermal synthesis of NaNbO 3 was observed in real time, enabling the investigation of the reaction kinetics and mechanisms with respect to temperature and NaOH concentration and the resulting effect on the product crystallite size and structure. Several intermediate phases were observed, and the relationship between them, depending on temperature, time, and NaOH concentration, was established. The reaction mechanism involved a gradual change of the local structure of the solid Nb 2 O 5 precursor upon suspending it in NaOH solutions. Heating gave a full transformation of the precursor to HNa 7 Nb 6 O 19 ·15H 2 O, which destabilized before new polyoxoniobates appeared, whose structure depended on the NaOH concentration. Following these polyoxoniobates, Na 2 Nb 2 O 6 ·H 2 O formed, which dehydrated at temperatures ≥285 °C, before converting to the final phase, NaNbO 3 . The total reaction rate increased with decreasing NaOH concentration and increasing temperature. Two distinctly different growth regimes for NaNbO 3 were observed, depending on the observed phase evolution, for temperatures below and above ≈285 °C. Below this temperature, the growth of NaNbO 3 was independent of the reaction temperature and the NaOH concentration, while for temperatures ≥285 °C, the temperature-dependent crystallite size showed the characteristics of a typical dissolution–precipitation mechanism.

The present challenge with all-oxide thermoelectric modules is their poor durability at high temperatures caused by the instability of the metal-oxide interfaces at the hot side. This work explains a new module concept based on a hybrid p−n junction, fabricated in one step by spark plasma co-sintering of Ca 3 Co 4−x O 9+δ (CCO, p-type) and CaMnO 3−δ /CaMn 2 O 4 (CMO, n-type). Different module (unicouple) designs were studied to obtain a thorough understanding of the role of the in situ formed hybrid p−n junction of Ca 3 CoMnO 6 (CCMO, p-type) and Co-oxide rich phases (ptype) at the p−n junction (>700 °C) in the module performance. A time-enhanced performance of the modules attributed to this p−n junction formation was observed due to the unique electrical properties of the hybrid p−n junction being sufficiently conductive at high temperatures (>700 °C) and nonconductive at moderate and low temperatures. The alteration of module design resulted in a variation of the power density from 12.4 (3.1) to 28.9 mW/cm 2 (7.2 mW) at ΔT ∼ 650 °C after 2 days of isothermal hold (900 °C hot side). This new concept provides a facile method for the fabrication of easily processable, cheap, and high-performance hightemperature modules.