Solid state synthesis of BiFeO3 occurs through the intermediate Bi25FeO39 compound

Abstract The solid‐state synthesis of perovskite BiFeO 3 has been a topic of interest for decades. Many studies have reported challenges in the synthesis of BiFeO 3 from starting oxides of Bi 2 O 3 and Fe 2 O 3 , mainly associated with the development of persistent secondary phases such as Bi 25 FeO 39 (sillenite) and Bi 2 Fe 4 O 9 (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 Bi 2 O 3 first reacts with Fe 2 O 3 to form sillenite Bi 25 FeO 39 , which then reacts with the remaining Fe 2 O 3 to form BiFeO 3 . Therefore, the synthesis of perovskite BiFeO 3 is shown to occur via a two‐step reaction sequence with Bi 25 FeO 39 as an intermediate compound. Because Bi 25 FeO 39 and the γ‐Bi 2 O 3 phase are isostructural, it is difficult to discriminate them solely from X‐ray diffraction. Evidence is presented for the existence of the intermediate sillenite Bi 25 FeO 39 using quenching experiments, comparisons between Bi 2 O 3 behavior by itself and in the presence of Fe 2 O 3 , and crystal structure examination. With this new information, a proposed reaction pathway from the starting oxides to the product is presented.