Dielectric response modulation in relaxor (Sr,Ba)Nb2O6 via engineered defect

Defect engineering provides an effective pathway to tune the dielectric response of relaxor ferroelectrics, yet the roles of individual defect species in tungsten bronze (TTB) systems remain unclear. Here, we investigated how engineered Ti–VO•• defect dipoles and native oxygen vacancies introduced through reduction annealing modify the dielectric properties of relaxor (Sr0.5,Ba0.5)Nb2O6 (0.5-SBN). Ti loading forms stable defect dipoles that generate smaller polar nanoregions (PNRs), suppress their overgrowth, and increase the number of dynamically active PNRs at room temperature, leading to enhanced permittivity ε′ while preserving DC-bias stability and breakdown strength (EB). Reduction annealing generates Nb4+-related local polarizations aligned with the external electric field, which strengthen dipole fluctuations and slightly increase ε′. Electron localization further relaxes the TTB-A2 site disorder–induced random fields, yielding a sharper dielectric peak and promoting more uniform PNR growth. Microwave dielectric spectroscopy combined with electromagnetic simulations revealed that the increases in ε′ for both Ti-loaded and reduced samples (0.5-SBNT and Reduced 0.5-SBN, respectively) originate predominantly from enhanced dipole polarization. Despite grain coarsening in 0.5-SBNT, the EB remained unchanged, indicating that defect dipoles act as pinning centers that inhibit field-induced PNR aggregation. In reduced 0.5-SBN, EB increased up to pO0 = 4.3 × 10−3 atm owing to relaxed random fields, whereas excessive reduction reduced EB through carrier-driven space-charge effects. These results demonstrate that precise control of defect species enables simultaneous optimization of ε′, DC-bias stability, and EB in TTB-type relaxor ferroelectrics.