Designing the Solid-State Synthesis of Multiferroic BiFeO3 using In Situ Transmission Electron Microscopy

Solid state reactions form the basis for most new material discovery, especially bulk powder phases, yet the dynamics of the solid-state mechanism, as bonds break and form, particles change shape and sinter, and transient metastable phases fleetingly emerge and are consumed, are too often shrouded by a figurative synthetic “black box” within which knowledge of the key mechanistic factors are lost. Solidstate reactions depend on processes occurring over many length scales, from chemical gradients to processes on the micro, nano, and atomic scale, occurring at grain boundaries, interfaces, defects, and chemical inhomogeneities. The ceramic synthesis of bismuth ferrite, BiFeO3, is one of the most studied solid-state reactions, and is notoriously challenging due to the volatility of bismuth and the existence of several stable secondary phases. BiFeO3 stands out among multiferroic materials due to the rare coexistence of room temperature ferroelectricity and antiferromagnetism, and the consequent potential for magnetoelectric applications. Reports of the solid-state reaction conflict on both the key diffusion pathways, and the role of potential intermediates, however. One approach to opening the synthetic black box for this reaction would be to use a local probe such as in situ heating in the transmission electron microscope (TEM) which has the potential to illuminate determining mechanistic factors on the atomic scale, and to sample a large phase of temperature, pressure, crystal size, particle morphology, and composition. Here we propose using in situ TEM to determine the key mechanistic details of the solidstate reaction of Bi2O3 and Fe2O3 to form multiferroic BiFeO3.