Topochemical Fluorination\nof Epitaxial Thin Films\nof Barium-Doped Bismuth Iron Oxyfluoride

Topochemical fluorination of epitaxial transition-metal\noxide thin\nfilms is one of the superior methods for producing single-crystalline\ntransition-metal oxyfluoride thin films with fascinating physical\nand chemical properties. In this study, we explore the topochemical\nfluorinations using polyvinylidene difluoride for epitaxial thin films\nof Ba-doped bismuth ferrite, a multiferroic related material, at different\nfluorination temperatures. Epitaxial thin films of perovskite-type\nbismuth iron oxyfluoride (Bi<sub>0.8</sub>Ba<sub>0.2</sub>FeO<sub>2.9–<i>x</i></sub>F<sub>2<i>x</i></sub>) were obtained with different F contents through fluorination at\n200–300 °C. Increasing the fluorination temperature to\n350 °C yielded an Aurivillius-type epitaxial film with partial\niron oxide byproducts. Additionally, while BiFeO<sub>3</sub> has been\nproposed for various optical applications due to its visible optical\nbandgap (2.2–2.7 eV), we observed a further reduction in the\nbandgap upon fluorination: 2.0, 2.0, and 1.6 eV for the as-grown oxide,\nperovskite-type oxyfluoride, and Aurivillius-type oxyfluoride films\nwith partial iron oxide byproducts, respectively. These findings demonstrate\nthe significant influence of fluorination conditions on the crystal\nstructure and electronic states of Ba-doped bismuth ferrite, offering\na pathway for tailored tuning of its properties.