The Phase Evolution and Lattice Dynamics of Dy³⁺/Sr²⁺ Co-Doped KNN Ferroelectric Ceramics

The phase structure serves as the fundamental determinant of the macroscopic optoelectronic response in ferroelectric ceramics, and therefore its evolutionary behavior constitutes the physical basis for understanding the functional properties. In this work, Dy³⁺/Sr²⁺ co-doped (K_0.5Na_0.5)NbO₃ ceramics are comprehensively investigated via condensed matter spectroscopy. Doping-induced ionic radius mismatch produces lattice distortion, tunes A-site cation ordering, and promotes O-T phase coexistence at room temperature, while inducing composition-dependent evolution of the optical band gap and luminescence properties. Temperature-dependent Raman and PL spectra confirm successive phase transitions and nonlinear thermal drift of luminescence and Raman peaks governed by electron-phonon coupling. A composition-temperature phase diagram was built to verify that doping modulates phase transition temperatures, yielding optimal optical properties and relaxor ferroelectric features. These findings elucidate the mutual coupling among lattice symmetry, electronic structure, and optoelectronic properties, providing theoretical guidance for the design of high-performance lead-free ferroelectric ceramics.