Tunable energy bandgap of Fe-doped (Bi, Li) co-substituted barium titanate

Abstract

In this work, a polycrystalline Ba0.96(½ Bi, ½ Li)0.04Ti(1-x) FexO3; (0 ≤ x ≤ 0.08) ceramics have been synthesised using a solid-state reaction method. The prepared systems were explored to detect the impact of Fe substitution on the energy bandgap of the ceramics. XRD patterns confirmed that there is a structural phase transition from tetragonal (P4 mm) to hexagonal (P63/mmc) phase as the concentration of Fe increases. Rietveld refinement was performed to obtain the lattice information. Furthermore, Raman spectroscopic analysis confirmed the structural information obtained from XRD study. The average bond length variations, strain evolutions, crystallite size, and theoretical density have been calculated from the structural analysis. It is found that the lower Fe concentration with the tetragonal phase showed a strong Jahn-Teller effect. Meanwhile, the higher concentration of Fe led to phase transition to hexagonal phase with fewer structural distortions. The optical band gap species were investigated through UV-Vis. Following the onset of defects induced by acceptor ions, an exciting band gap reduction up to 2.09 eV for the sample with x = 0.08 was attained. ESR and PL spectroscopies analyses showed that in the hexagonal phase region more defects are formed giving rise to promoting band gap narrowing. Furthermore, the ac conductivity analysis indicates the appearance of defect levels due to the formation of oxygen vacancies. This study demonstrates that the right choice of Fe content in the host material can tune the energy band gap significantly in the BLBTF system and may be exploited in photovoltaics in the visible region.