Effect of the (Ba + Sr)/Ti ratio on the microwave-tunable properties of Ba_0.6Sr_0.4TiO₃ ceramics

The impact of the (Ba + Sr)/Ti (A/B) ratio on the microwave-tunable characteristics of diffuse phase transition (DPT) ferroelectric Ba_0.6Sr_0.4TiO₃ (0.6-BST) ceramics was investigated. The reduction in the lattice constant with increasing nonstoichiometry was attributed to introduced partial Schottky defects, i.e., (Formula presented.) and (Formula presented.). The magnitude of the dielectric constant, ε′, at room temperature in the absence of an applied electric field was governed by the shift in the dielectric maximum temperature, T_m, because T_m was close to room temperature for the 0.6-BST. The dielectric loss, tanδ, diminished as the ε′ decreased for 0.98≤A/B≤1.05, while the tanδ was much higher for A/B=0.95 having the greatest A-site vacancy loading. The negatively charged Vˮ_Ba and Vˮ_sr were mainly compensated by oxygen vacancies and likely partly compensated by holes, h^•, which contributed to the electrical conduction. The tunability, T, at 100 MHz was almost constant at 20%–25% for A/B≥1.00 despite the reduction of the ε′, whereas T decreased for A/B<1.00 to ca. 10% for A/B=0.95 having the greatest A-site vacancy loading. The results implied that the (Formula presented.) for larger A/B values was more efficient in generating nucleation sites in the polar nanoregions (PNRs) than the (Formula presented.) for smaller A/B values, thereby providing greater dipole polarization. Consequently, the figure of merit, FOM, reached its maximum of 250 at A/B=0.9875, which was ca. 155% higher than that of the stoichiometric BST.