The effects of Sr-substitution in Ba₂SmTi₂Nb₃O₁₅ ceramics: structural study, optical properties, and complex...

Ceramics of (Ba_1-Sr)₂SmTi₂Nb₃O₁₅, denoted as BSSTN (x = 0, 0.25 and 0.5), were synthesized by the conventional solid-state reactions route. The impact of Sr-substitution in Ba₂SmTi₂Nb₃O₁₅ ceramics on their structural, optical, and dielectric properties are investigated. The Rietveld method was employed to confirm the formation of tetragonal tungsten bronze with the P4bm space group, using X-ray diffraction data. The substitution of Ba by Sr resulted in a decrease in cell parameters, density, and the average crystallite size as determined by Scherrer's formula ranged from 29.4 to 32 nm. The compounds frequency-dependent dielectric properties were studied using complex impedance spectroscopy over a temperature range of 50 to 420 °C at different frequencies. Dielectric measurements revealed a high dielectric constant, and the compounds exhibited characteristics of diffuse ferroelectric behavior. As the Sr content increased, optical gap energy increases from 3.29 to 3.59 eV, diffusivity increased from 1.19 to 1.52, Curie temperature (T_c) decreased from 269 to 213 °C, and the dielectric loss at room temperature and 1 kHz significantly decreased from 3 × 10^-3 to 7 × 10^-4. The correlation between (T_c) and the off-center cationic displacement of Ti/Nb in the octahedral Ti/NbO₆ was analyzed. Cole-Cole plots for each sample displayed a single semicircular arc, indicating the presence of a single relaxation process.