Impact of bismuth substitution on the structural and optical properties of lanthanum orthoferrite synthesized via sol-gel method

This study explores the structural, linear, and nonlinear optical properties of lanthanum orthoferrite (LaFeO₃) and the impact of bismuth (Bi) substitution at the A-site in varying concentrations (0.05, 0.1, and 0.15) on these properties. The La_1-xBixFeO₃ samples were synthesized via the sol-gel method using citric acid as a complexing agent. Structural characterization was conducted using X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). XRD analysis confirmed the formation of a perovskite phase with an orthorhombic structure (Pbnm space group), while Rietveld refinement provided precise crystallographic and geometric parameters. FTIR spectra verified the phase purity, revealing no detectable carboxyl or hydroxyl functional groups, indicating the absence of carbonates and minimal hygroscopicity. UV–visible spectroscopy was employed to assess absorbance and reflectance spectra, Which indicated enhanced absorption in the visible region upon doping. The calculated linear optical parameters stated an increase in bandgap energy from 2.045 eV to 2.139 eV with increasing Bi content, accompanied by a reduction in Urbach energy. Furthermore, Bi-doped samples exhibited anomalous dispersion behavior in the refractive index, in contrast to the normal dispersion observed in undoped LaFeO₃. Dielectric properties, optical performance, and electrical conductivity were also improved upon Bi incorporation. The nonlinear optical analysis, including third-order nonlinear susceptibility (χ(3)) and nonlinear refractive index—enables, revealed that pristine LaFeO₃ exhibited superior optical characteristics compared to Bi-doped samples.