Quasi-Parallel Ferroelectric Composites Decoupling the Permittivity-Tunability Trade-Off

The development of frequency-agile devices is constrained by the permittivity–tunability trade-off in ferroelectric materials, where high tunability is typically accompanied by excessive permittivity and dielectric loss. Conventional ceramic/polymer composites can alleviate impedance mismatch but suffer from severe tunability degradation due to depolarization effects. Here, we report a quasi-parallel ferroelectric composite that enables improved decoupling between permittivity, loss, and tunability. This is achieved via a cross-scale dual-correction strategy, where fine sol–gel-derived Bi₆Ti₅WO₂₂ particles enhance wall densification while controlled freezing reduces structural defects and improves alignment. Furthermore, real-structure finite-element modeling demonstrates that mitigating these microstructural deviations is essential for maximizing local electric field utilization. The resulting 15 vol. % composite exhibits a high dielectric tunability of 50% at 30 kV cm⁻¹, retaining ∼ 83% of the bulk ceramic tunability, together with an ultralow dielectric loss (tan δ < 0.001) at room temperature. A high figure of merit (FOM) of 633 is achieved at a permittivity more favorable for impedance matching than that of conventional ferroelectric ceramics. These results demonstrate that cross-scale structural and material optimization provides an effective strategy for high-performance tunable dielectric composites.