Solvent-mediated hydrothermal synthesis of K/Bi-doped [Ba(1−x)Kx][Bi(1−y)Tiy]O3 perovskites: structure–property relationships and bandgap engineering

Abstract

K/Bi co-doped BaTiO₃ perovskites with composition [Ba_(1−x)K_x][Bi_(1−y)Ti_y]O₃ were synthesized using a hydrothermal method at 220 °C, employing either ethylene glycol (EG) or water as solvent. X-ray diffraction analysis with Rietveld refinement revealed a hexagonal perovskite structure (P6₃/m, a = b = 9.925 ± 0.008 Å, c = 7.285 ± 0.006 Å). Minor impurity phases (<5%) were identified as K₂CO₃ and Bi₂Ti₂O₇. FTIR spectra confirmed the formation of metal–oxygen bonds such as Ba–O, Ti–O, and Bi–O. SEM images showed regular hexagonal morphologies for EG-derived samples and flake-like structures for water-derived ones. EDS analyses, complemented by ICP-OES elemental analysis, confirmed the elemental homogeneity. Thermo-gravimetric analysis indicated that EG-derived samples exhibited greater mass loss compared to water-based ones, attributed to endothermal processes and residual organic content (confirmed by CHN analysis). UV-vis spectroscopy showed absorption in the UV region with direct bandgaps ranging from 4.25 ± 0.06 eV to 4.52 ± 0.08 eV, calculated using Tauc plots. The structural characteristics, including perovskite symmetry with stereo chemically active Bi³⁺ ions and potential morphotropic phase boundary features, suggest these materials may be promising candidates for UV optoelectronic applications and potentially for ferroelectric devices, pending direct polarization measurements. This study presents an eco-friendly synthesis route for perovskite materials with tunable properties and offers insights into solvent effects on microstructure and stability.