Investigation into La(Fe/Mn)O Perovskites Formation over Time during Molten Salt Synthesis.

Molten salt synthesis (MSS) of complex oxides is generally investigated by characterization of the product phases with no insight into evolution of particle morphology. In this work, LaFeO and LaMnO MSS was investigated in KF-KCl and LiCl-KCl at 850 °C using a "feeding-and-sampling" procedure. By feeding the oxide reagents into a molten salt, the reaction starting point was clearly defined, while subsequent sampling of the melt provided means for tracking the phase composition along with the shape and size of product particles during MSS. Samples taken just after 1 min contained perovskite particles along with reagents and intermediates, which were consumed over time to yield a pure product within 10-30 min. The shape and size of perovskite particles sampled at different times during MSS were virtually unchanged, revealing a lack of notable growth. The observed fast MSS along with prevailing nucleation provided means to control perovskite particle size by varying the extent of reagent dissolution. Thus, increasing the salt/reagent ratio (from 10:1 to 25:1) strikingly reduced the duration required to obtain a pure product, along with decreasing the size of product particles (from 0.5-1.5 μm to 80-200 nm). Furthermore, performing MSS in KF-KCl, which exhibits greater oxide solubility compared to LiCl-KCl, resulted in a shorter duration and smaller perovskite particles (80-200 nm and 0.6-2.0 μm, respectively). This insight into perovskite formation and growth during MSS and its kinetics provides valuable guidelines for tuning MSS conditions to better control synthesis duration and particle size.