Oxygen vacancy effects on double perovskite Bi₂FeMnO₆: A first-principles study

Double perovskite Bi₂FeMnO₆ (BFMO) is a potential candidate for the highly sought single-phase multiferroic system. The large orbital radius of the Bi 6s₂ lone pairs is responsible for BFMO to exhibit low symmetries and spontaneous polarization, whereas B-site ordering of Mn and Fe contributes to its magnetic properties. In this work, we study both electronic correlation and oxygen vacancy effects on magnetic, electronic and optical properties of BFMO by performing first-principles simulations using density functional theory within the local spin-density approximation (LSDA) and the LSDA+U method. We have numerically demonstrated that a strong on-site Hubbard interaction is critical for the gap opening in a pristine BFMO. We have performed calculations on a supercell constructed with eight chemical formula units of BFMO, from which oxygen atoms were removed incrementally. We showed that the average magnetization decreases with the increase of oxygen vacancy concentration. From the calculated band structure and optical conductivity, an insulator-metal transition or crossover was identified with oxygen in BFMO.