Crystallographic Insight and Functional Properties of Zn-Modified Sr₅CaTi₂Nb₈O₃₀ Tungsten Bronze Ceramics

This work reports a comprehensive investigation of the structural, dielectric, electrical, optical, and energy storage properties of Sr₅CaTi₂₋ₓZnₓNb₈O₃₀ (x = 0–0.08) tetragonal tungsten bronze ceramics synthesized via the solid-state reaction method. Rietveld refinement of X-ray diffraction data confirms the formation of a single-phase tetragonal tungsten bronze structure with Im2a space group, highlighting the successful incorporation of Zn ions into the lattice and the resulting modifications in crystallographic parameters. Microstructural analysis reveals dense ceramics with low porosity and uniformly distributed grains, with average grain sizes ranging from 15.01 to 17.16 µm. Dielectric measurements performed over a temperature range of 30–400 °C and frequencies from 1 kHz to 1 MHz exhibit two distinct anomalies corresponding to the ferroelectric phase transition (Tc) and a relaxation process (Ts), indicating diffuse phase transition behavior. Impedance spectroscopy, Nyquist plots, and electric modulus analysis demonstrate the significant contribution of grain and grain boundary effects to the electrical response, while activation energy calculations confirm thermally activated conduction mechanisms. The polarization–electric field hysteresis loops exhibit slim shapes, characteristic of relaxor ferroelectrics, which are favorable for energy storage applications. Compared to previous studies, this work provides deeper insight into the relationship between crystallographic structure and multifunctional properties, emphasizing the role of Zn substitution in tuning dielectric relaxation and enhancing energy storage performance in tungsten bronze ceramics.