Breaking the Tunability–Loss Trade-off in Bi₆Ti₅WO₂₂-Based Relaxors via Nominal Compositional Tuning

Achieving high dielectric tunability with minimal dissipation remains a critical challenge for next-generation microwave devices and is often elusive in conventional perovskites due to intrinsic losses. Here, we investigate a non-perovskite Bi₆Ti₅WO₂₂ (BTWO)-based relaxor system synthesized via a sol–gel route. Unlike conventional solid-state methods, this approach ensures superior precursor homogeneity, enabling controlled access to a narrow phase stability window. We demonstrate that precisely targeting the optimal BTWO-49Bi composition eliminates detrimental secondary phases while simultaneously minimizing local lattice disparity. Consequently, the optimized ceramics deliver a high figure-of-merit (FOM ≈ 1500), characterized by a large tunability (60% at 30 kV cm⁻¹) and an ultralow dielectric loss (tan δ in the ∼ 10⁻⁴ range). By correlating macroscopic electrical responses with local structural probes, we reveal that this enhanced microscopic uniformity effectively minimizes random pinning fields. As a result, the intrinsic polar nanoregions (PNRs) retain high rotational freedom and respond to external fields highly cooperatively. Ultimately, this work breaks the conventional tunability-loss trade-off, confirming an effective strategy to explore the intrinsic structural limits of the BTWO system.