Structural, Optical, and Magnetic Properties of Brownmillerite KBiFe₂O₅ and KBiFe_1.95Ti_0.05O₅₊ via Reactive Templated Method

Brownmillerite KBiFe₂O₅ (KBFO) and KbiFe_1.95Ti_0.05O₅₊ (KBFTO) ceramics were synthesized using the prereacted nanopowders of KFeO₂ (KFO) and BiFeO₃ (BFO), and KFO and BiFe_0.95Ti_0.05O₃₊ (BFTO), respectively, via the reactive templated method. The powder X-ray diffraction patterns confirmed the monoclinic phase of the KBFO and KBFTO samples. The incorporation of Ti⁴⁺ at Fe³⁺ site prevented the formation of a secondary phase (Bi₂Fe₄O₉) in the KBFTO sample. Our experimentally determined lattice parameters for the orthorhombic P2/c KBFO and KBFTO are consistent with our density functional theory (DFT) computed lattice parameters (within a 5% error). Scanning electron microscopy images revealed an average grain size of ∼1.38 and 0.85 μm for KBFO and KBFTO, respectively. The calculated optical band gaps of the KBFO and KBFTO using the Kubelka-Munk function from their UV data were found to be 1.69 and 1.71 eV, respectively. The measured surface area of KBFTO (1.339 m² g^-1) obtained from BET analysis was double that of KBFO (0.698 m² g^-1). KBFTO's room-temperature magnetization was 0.986 emu g^-1, which was twice that of the KBFO, while its room-temperature conductivity measured using impedance spectroscopy was 5.69 × 10^-3 S m^-1 which was 10 times that of KBFO. Notably, our DFT calculations showed that the small Ti dopant concentration enhanced the partial density of states (PDOS) above the Fermi level. It shifted the Fermi level toward higher energy, and the oxygen PDOS for KBFTO was significantly enhanced relative to KBFO, which ultimately helped improve the conductivity.