Investigation into La(Fe/Mn)O₃ Perovskites Formation over Time during Molten Salt Synthesis

Abstract

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, LaFeO3 and LaMnO3 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.

Solubility of MoS2 and Graphite in Molten Salt: Flowers, Faceted Crystals, or Exfoliation?

“…[ 59 ] Molten salt treatment of insoluble layered materials (e.g., graphite) resulted in exfoliation, where the impregnation of the molten salt ions into graphite interlayers introduced gentle shear forces that yielded large‐sized graphene nanoplatelets. [ 3,60 ] Soluble precursors of the 2D material in salt undergo nucleation and growth during the thermal treatment to form thin sheets [ 61–64 ] that further aggregate into a flowerlike structure, [ 57 ] a common feature in few bottom‐up syntheses of MoS 2 in molten salts and other media. [ 38,39,57,58,65–68 ]…”

“…[59] Molten salt treatment of insoluble layered materials (e.g., graphite) resulted in exfoliation, where the impregnation of the molten salt ions into graphite interlayers introduced gentle shear forces that yielded large-sized graphene nanoplatelets. [3,60] Soluble precursors of the 2D material in salt undergo nucleation and growth during the thermal treatment to form thin sheets [61][62][63][64] that further aggregate into a flowerlike structure, [57] a common feature in few bottom-up syntheses of MoS 2 in molten salts and other media. [38,39,57,58,[65][66][67][68] We ask if the molten salt approach could also serve as 1) a medium for top-down exfoliation of insoluble 2D materials (e.g., graphite); or 2) a solvent for bottom-up growth of thin sheets of soluble substances, where we take MoS 2 as a test case.…”