First-Principles Investigations of Magnetoelectric Coupling in Anion-Substituted BiCoO3

Multianion materials, where multiple anionic species coexist within the crystal structure, offer a powerful platform for tuning electronic and magnetic properties.

In this study, we investigate the effects of anionic substitution (fluorine and nitrogen) on BiCoO₃, a remarkable room-temperature multiferroic material. BiCoO₃ exhibits unique properties stemming from the stereo chemically active 6s² lone pair of Bi³⁺, which induces significant distortion in the perovskite crystal structure and contributes to a high spontaneous polarization of approximately 179 μC/cm².

Using density functional theory calculations within VASP, we explore the impact of these substitutions on BiCoO₃'s structural, electronic, and magnetic properties. Our results reveal that replacing oxygen with fluorine and/or nitrogen leads to notable modifications in crystal structure and electronic behavior. Fluorine substitution, with its smaller ionic radius (1.33 Å) and higher electronegativity (3.98), enhances lattice distortion and the polar mode. Conversely, nitrogen substitution causes significant in-plane deformation due to its stronger covalent bonding with cobalt (probed by COHP analysis) and larger ionic radius (1.46 Å).

A free energy analysis of the system was conducted to determine the preferential position of the substitution, providing insights into the energetically favorable sites for anionic replacement. Importantly, these substitutions profoundly impact BiCoO₃'s magnetic properties, introducing magnetic frustration and altering spin order which will be detailed in this communication.

We will show that these findings provide critical insights into the interplay between structure, electronic properties, and magnetic order in substituted BiCoO₃. Understanding the mechanisms behind anionic substitution could guide the optimization of its properties

for specific applications, potentially enhancing its multiferroic characteristics for next-generation electronic devices.