Electronic and Optical Properties of Transition-Metal-Modified BiFeO₃: A First Principles Study

The structural, electronic, magnetic, and optical properties are explored in the G-type antiferromagnetic BiFeO${}_3$ system by replacing the Fe cation with transition metals to form the BiFe${}_{0.834}$X${}_{0.166}$O${}_3$ compound (where X = Mn, Co, or Ni) by using first-principles DFT+U and TDDFT calculations. All the optimized structures preserve the rhombohedral (R3c) space group, showing moderate changes in the FeO${}_6$ octahedral distortions, lattice parameters, and Fe–O–Fe bond angles. Pristine G-type antiferromagnetic (AFM-G) BiFeO${}_3$ is a typical semiconductor material with a calculated bandgap energy $E_g=1.99$ eV. However, the inclusion of Ni, Co, and Mn at the Fe site introduces additional 3d states near the Fermi level, causing metallic behavior in every case. The local density of states (LDOS), density of states (DOS), and total magnetization results show that the inclusion of Ni, Co, and Mn promotes a transition from antiferromagnetic (AFM) to ferrimagnetic behavior in the modified BiFe${}_{0.834}$X${}_{0.166}$O${}_3$ compositions. On the other hand, in the visible spectral region, the time-dependent density functional theory (TDDFT) revealed that the pristine material has refractive index $n\left(\omega \right)$ values between 2.8 and 3.6, showing that the presence of Co and Ni enhances the extinction and absorption coefficients in both visible and ultraviolet regions, whereas the inclusion of Mn produces less significant effects. These results demonstrate that controlled substitution at the Fe site with transition metals simultaneously modifies the structural, electronic, magnetic, and optical properties of the BiFeO${}_3$ system, offering promising potential for applications in electronic devices with multifunctional properties.