Defect‐Gradient Engineering for Enhanced Dielectric and Nonlinear Properties in K ⁺ ‐Doped Bi _2/3 Cu ₃ Ti ₄ O ₁₂ Multilayers

ABSTRACT K + ‐doped Bi 2/3 Cu 3 Ti 4 O 12 multilayer ceramics with homogeneous, symmetric, and gradient architectures were fabricated via a conventional solid‐state reaction method combined with layer‐by‐layer dry pressing. All samples showed no obvious delamination and mainly formed dense cubic‐like perovskite primary phases. Dielectric measurements revealed that the multilayer structure effectively enhanced the electrical performance, with BKB10 showing a favorable dielectric response ( ε r ≈ 4587, tan δ ≈ 0.069) and BKB05 exhibiting improved nonlinear properties ( α = 6.236, E b = 8.907 kV/cm). Complex impedance spectra revealed pronounced electrical heterogeneity, and the intermediate‐frequency dielectric response mainly originated from the internal barrier layer capacitance (IBLC) effect between grains and grain boundaries. Energy‐dispersive X‐ray spectroscopy (EDS), electron probe microanalysis (EPMA), and X‐ray photoelectron spectroscopy (XPS) analyses revealed limited interlayer diffusion of K + , local K enrichment near well‐bonded interfaces, and spatially nonuniform oxygen‐related defects. These interface‐dependent compositional and defect features jointly modulate local conductivity mismatch and internal barriers, thereby regulating space‐charge accumulation, interfacial polarization, and carrier transport, which play a critical role in achieving low dielectric loss, enhanced nonlinearity, and relatively high breakdown strength.