Defect-driven structural, optical, and dielectric properties of hydrothermally synthesized Ba0.8Co0.2-xGdxTiO3 nanoparticles

Ba 0.8 Co 0.2-x Gd x TiO 3 (x = 0.01–0.04) nanoparticles were synthesized via a hydrothermal method to investigate the role of rare-earth Gd³ ⁺ substitution on structural, optical, and dielectric properties. X-ray diffraction confirmed the formation of a predominantly single-phase perovskite structure with enhanced crystallinity, while a minor secondary phase at x = 0.04 indicated the solubility limit of Gd 3 + . The crystallite size and lattice strain exhibited non-linear variation, with optimized structural ordering at x ≈ 0.03. Microstructural analysis revealed nanoscale particles with reduced grain size due to dopant-induced lattice distortion and grain boundary pinning.Optical studies revealed a non-monotonic variation in optical band gap energies, with values of 2.57, 2.67, 2.49, and 2.75 eV corresponding to x = 0.01, 0.02, 0.03, and 0.04, respectively. The observed behavior is governed by defect states, lattice distortion, and particle size effects. Photoluminescence spectra confirmed defect-mediated emission dominated by oxygen vacancies, with reduced intensity at higher doping levels. Dielectric measurements demonstrated strong frequency dispersion with significantly enhanced dielectric constant (ε′ ≈ 2684 at 100 Hz), attributed to Maxwell–Wagner interfacial polarization and defect-induced charge carrier dynamics. Impedance and modulus spectroscopy revealed non-Debye relaxation behavior and confirmed that electrical transport is governed by defect-mediated hopping conduction. The optimized composition (x ≈ 0.03) exhibited improved crystallinity, reduced defect density, and enhanced dielectric performance, making these materials promising for energy storage and electronic applications.