Antiferroelectric to Relaxor Ferroelectric Transition in Na-Doped K₂PrNb₅O₁₅ Tetragonal Tungsten Bronzes

Tetragonal tungsten bronze (TTB)-structured ceramics exhibit extraordinary and fascinating dielectric properties due to their unique structural characteristics and abundant compositional tunability. The dielectric and ferroelectric properties are affected by commensurate or incommensurate modulated structures. Herein, the antiferroelectric-relaxor ferroelectric (AFE-RFE) transition in K_{2- x}Na_xPrNb₅O₁₅ ceramics with a TTB structure is successfully achieved by constructing incommensurate modulated structures, which are attributed to cation disorder and lattice distortion arising from Na⁺ substitution. Compositions with x ≤ 0.2 exhibit antiferroelectric phases at ambient temperature, with two dielectric anomalies corresponding to ferroelectric-antiferroelectric and antiferroelectric-paraelectric transitions in the dielectric spectra. When x ≥ 0.4, the samples transition into relaxor ferroelectrics with one dielectric anomaly representing the ferroelectric phase transition. Excessive Na⁺ substitution eliminates the AFE temperature plateau and enhances relaxation, as evidenced by the increased incommensurability parameter δ. Notably, the composition with x = 1.0, which has the maximum δ value of 0.255, achieves a high recoverable energy density of 4.65 J cm^{- 3} and maintains a stable energy storage efficiency exceeding 90%. This work proposes an intriguing strategy for achieving the AFE-RFE transition and provides critical experimental support for the comprehensive investigation of the antiferroelectric with TTB structures.