Effect of the (Ba + Sr)/Ti ratio on the microwave‐tunable properties of Ba 0.6 Sr 0.4 TiO 3 ceramics

The impact of the (Ba + Sr)/Ti (A/B) ratio on the microwave‐tunable characteristics of diffuse phase transition ( DPT ) ferroelectric Ba 0.6 Sr 0.4 TiO 3 (0.6‐ BST ) ceramics was investigated. The reduction in the lattice constant with increasing nonstoichiometry was attributed to introduced partial Schottky defects, i.e.,[V ″ ″Ti− 2 V O ∙ ∙ ] × and[V ″Ba , Sr− V O ∙ ∙ ] × . 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 chargedV ″BaandV ″Srwere 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[V ″ ″Ti− 2 V O ∙ ∙ ] × for larger A/B values was more efficient in generating nucleation sites in the polar nanoregions ( PNR s) than the[V ″Ba , Sr− V O ∙ ∙ ] × 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 .