Oxygen-Vacancy-Controlled Band-Gap Engineering in Fe/Co Co-doped La-Modified Bismuth Titanate: Integrated Experimental and First-Principles Analysis

Layered Aurivillius ferroelectrics offer a route to multifunctional band-gap engineering, yet the role of oxygen vacancies in band-edge reconstruction in Fe/Co-modified compositions remains poorly resolved. Here, oxygen annealing of Bi3.25La0.75Ti2.60(Fe0.50,Co0.50)0.40O12 (BLFC4) ceramics widens the optical band gap by 63%, from 1.35 eV (as-sintered) to 2.20 eV after 10 h of treatment, accompanied by a 5-fold drop in non-lattice oxygen (22.11% → 4.12%) and a reduction in Ti3+ from 42.68 to 10.47%, as quantified by X-ray photoelectron spectroscopy. Concurrently, X-ray diffraction reveals a decrease in octahedral tilt from 19° to 4°, while Raman spectroscopy shows up to 74% narrowing of B-site vibrational modes, together signaling systematic defect healing accompanied by a visible color change from black to light gray. Density functional theory rationalizes this behavior through a site-dependent vacancy mechanism: vacancies adjacent to Co-centered octahedra collapse the band gap toward a near-metallic state by introducing Co 3d states at the Fermi level, whereas vacancies near Fe leave the wide semiconducting gap intact. These combined results identify oxygen vacancy concentration rather than cation co-doping alone as the dominant lever controlling band-gap width, providing a quantitative, mechanistic framework for engineering Aurivillius ferroelectrics toward optoelectronic and photovoltaic applications.