Defect analysis of Y³⁺-doped Ba_0.95Sr_0.05Ti_1-xO_3-δ above Curie temperature

Several possible defect structures in Ba_0.95Sr_0.05TiO₃-doped with 0, 0.2, 0.4 and 0.6% molar of Y³⁺ have been analyzed. Polycrystalline samples used in this study were synthesized via an oxalate coprecipitation technique, subsequently followed by calcination at 700 °C for 2 h and sintering at 1300 °C for 2 h. Structural and microstructural characterizations using XRD and SEM show a slight decrease of lattice tetragonality (c/a) and densification with increasing Y³⁺ content up to 0.4%. Electrical measurements were conducted by applying an alternating electric field of 15 V/cm with the frequency range between 50 Hz and 1 MHz at elevated temperature from room temperature up to 300 °C in air. We have noted three phenomena with respect to the characteristics of dielectric constant exhibited by the samples, namely a structural transition from tetragonal to cubic at Curie temperature, the high frequency as well as low frequency relaxation processes occurring respectively at T ≈ 180 °C and T ≈ 250 °C. Further analysis by the Cole-Cole plot of dielectric constant and complex impedance has suggested the main cause of the related features to be pair defects as V_Ti^″″ - V_O {radical dot} {radical dot}and V^″_Ba - V_O {radical dot} {radical dot}.