Decoupling the factors governing diffuse phase transition and relaxor–ferroelectric evolution in high-entropy tetragonal tungsten bronze ceramics

Abstract

High-entropy engineering has emerged as an effective strategy for tailoring structural disorder and functional responses in tetragonal tungsten bronze (TTB) ferroelectrics. However, how configurational entropy, crystal structure, modulation characteristics, and relaxor dynamics govern the macroscopic electrical response remains insufficiently understood. Here, K(NaPr)1-x(SrBa)xNb5O15 ceramics with continuously varying configurational entropy were designed to investigate the origin of relaxor-ferroelectric evolution. With increasing Sr/Ba substitution, the electrical response evolves from an ergodic-relaxor state with slim hysteresis loops and four current peaks to a ferroelectric-like state with double current peaks, fatter loops, and increased remanent and maximum polarizations. This evolution is accompanied by increased lattice parameter c and tetragonality (c/a), indicating a structural evolution toward a more stable polar state. The dielectric anomalies and Vogel-Fulcher analysis further reveal an increase in freezing temperature and a decrease in activation energy, indicating strengthened coupling among polar nanoregions and a gradual evolution from the relaxor state toward a non-ergodic relaxor or ferroelectric state. Notably, this trend is opposite to the commonly expected enhancement of relaxor behavior in high-entropy systems, suggesting that configurational entropy does not simply strengthen relaxor characteristics through increased compositional disorder. Instead, configurational entropy modulates the order-disorder state of A-site occupation, thereby changing the octahedral tilting patterns and the modulation structure, which affects the diffuseness of the phase transition. These results demonstrate that the macroscopic electrical response in the high-entropy K(NaPr)1-x(SrBa)xNb5O15 system is regulated by crystal structure, modulation structure, and polar nanoregion dynamics, providing new insight into the relaxor behavior of TTB ferroelectrics.