Domain engineering enhanced microwave tunability in nonstoichiometric Ba_0.8Sr_0.2TiO₃

Domain engineering via oxygen vacancy, V^●●_O, loading achieved by A/B modification as well as quenching treatment, was utilized for Ba_0.8Sr_0.2TiO₃ (0.8 BSTs) in an attempt to enhance the microwave tunable characteristics. For similar grain sizes, the domain sizes were notably reduced for all nonstoichiometric BSTs, indicating that the loaded V^●●_O (as a consequence of Ti defects, V^˝˝_Ti) played a role in the nuclei for new domain walls. The tunability T at 100 MHz under a direct current field of 30 V/20 μm increased steadily as the domain size (d.s.) declined for all BSTs, regardless of the A/B ratio, due to the d.s. effect. The tunable characteristics in nonstoichiometric BSTs having a similar d.s. of ˜190 nm were then compared. The tunability and tan δ decreased for A/B = 1.002 (0.2 mol% Ti defects). The introduced V^●●_O formed pinning centers that restricted domain wall motion, leading directly to lower tunability and smaller dielectric loss. However, V^●●_O -overloaded samples (i.e., A/B ≥ 1.005) exhibited increased values for tan δ due to V^●●_O conduction in the domains. The quench treatment of 0.8 BST (with A/B = 1.002) samples resulted in a d.s. reduction from 191 to 170 nm. These quenched specimens showed greater tunability, T_total, originating from the strengthened dipole contribution, T_dipole, as a consequence of the d.s. effect. The tan δ of the quenched specimens was essentially unchanged, indicating a homogenous V^●●_O distribution via the quench, effectively reducing the mobile V^●●_O(which contributes to electrical conduction) in the domains. Consequently, the achieved figure of merit via domain engineering was 2.25 at 100 MHz for the quenched BST with A/B = 1.002, which was 1.54 times larger than that of unmodified BST.