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The solid‐state synthesis of perovskite BiFeO₃has been a topic of interest for decades. Many studies have reported challenges in the synthesis of BiFeO₃from starting oxides of Bi₂O₃and Fe₂O₃, mainly associated with the development of persistent secondary phases such as Bi₂₅FeO₃₉(sillenite) and Bi₂Fe₄O₉(mullite). These secondary phases are thought to be a consequence of unreacted Fe‐rich and Bi‐rich regions, that is, incomplete interdiffusion. In the present work, in situ high‐temperature X‐ray diffraction is used to demonstrate that Bi₂O₃first reacts with Fe₂O₃to form sillenite Bi₂₅FeO₃₉, which then reacts with the remaining Fe₂O₃to form BiFeO₃. Therefore, the synthesis of perovskite BiFeO₃is shown to occur via a two‐step reaction sequence with Bi₂₅FeO₃₉as an intermediate compound. Because Bi₂₅FeO₃₉and the γ‐Bi₂O₃phase are isostructural, it is difficult to discriminate them solely from X‐ray diffraction. Evidence is presented for the existence of the intermediate sillenite Bi₂₅FeO₃₉using quenching experiments, comparisons between Bi₂O₃behavior by itself and in the presence of Fe₂O₃, and crystal structure examination. With this new information, a proposed reaction pathway from the starting oxides to the product is presented.

Antiferroelectric (AFE) materials are of great interest owing to their scientific richness and their utility in high‐energy density capacitors. Here, the history of AFEs is reviewed, and the characteristics of antiferroelectricity and the phase transition of an AFE material are described. AFEs are energetically close to ferroelectric (FE) phases, and thus both the electric field strength and applied stress (pressure) influence the nature of the transition. With the comparable energetics between the AFE and FE phases, there can be a competition and frustration of these phases, and either incommensurate and/or a glassy (relaxor) structures may be observed. The phase transition in AFEs can also be influenced by the crystal/grain size, particularly at nanometric dimensions, and may be tuned through the formation of solid solutions. There have been extensive studies on the perovskite family of AFE materials, but many other crystal structures host AFE behavior, such as CuBiP₂Se₆. AFE applications include DC‐link capacitors for power electronics, defibrillator capacitors, pulse power devices, and electromechanical actuators. The paper concludes with a perspective on the future needs and opportunities with respect to discovery, science, and applications of AFE.