Topochemical Fluorination of Epitaxial Thin Films of Barium-Doped Bismuth Iron Oxyfluoride

Topochemical fluorination of epitaxial transition-metal oxide thin films is one of the superior methods for producing single-crystalline transition-metal oxyfluoride thin films with fascinating physical and chemical properties. In this study, we explore the topochemical fluorinations using polyvinylidene difluoride for epitaxial thin films of Ba-doped bismuth ferrite, a multiferroic related material, at different fluorination temperatures. Epitaxial thin films of perovskite-type bismuth iron oxyfluoride (Bi_0.8Ba_0.2FeO_2.9–xF_2x) were obtained with different F contents through fluorination at 200–300 °C. Increasing the fluorination temperature to 350 °C yielded an Aurivillius-type epitaxial film with partial iron oxide byproducts. Additionally, while BiFeO₃ has been proposed for various optical applications due to its visible optical bandgap (2.2–2.7 eV), we observed a further reduction in the bandgap upon fluorination: 2.0, 2.0, and 1.6 eV for the as-grown oxide, perovskite-type oxyfluoride, and Aurivillius-type oxyfluoride films with partial iron oxide byproducts, respectively. These findings demonstrate the significant influence of fluorination conditions on the crystal structure and electronic states of Ba-doped bismuth ferrite, offering a pathway for tailored tuning of its properties.