Structural, Microstructural, and Dielectric Insights into Eu3+-Doped and Eu3+, B3+ Co-Doped Sr2GdTaO6 Double Perovskite Ceramics

The structural, microstructural, and dielectric properties of Eu3+-doped and Eu3+, B3+ co-doped Sr2GdTaO6 double perovskite ceramics were systematically investigated. X-ray diffraction (XRD) analysis confirmed that all samples crystallize in the monoclinic P2₁/n space group with long-range B/B′ ordering, remaining single-phase up to 20 mol% Eu3+ and 70 mol% B3+. The peak-shift analysis revealed a slight shift toward lower angles, attributed to the substitution of Gd3+ by Eu3+ and the indirect structural perturbations induced by boron incorporation through charge compensation and microstrain effects. Crystallite sizes, derived from Scherrer and Williamson–Hall analyses, were found in the 30–45 nm range, with microstrain and defect densities increasing under Eu3+ doping but significantly reduced at moderate B3+ contents, consistent with the fluxing effect of boron. Scanning electron microscopy (SEM) revealed dense, terraced grains of 4–13 µm in the Eu3+ series, while boron addition promoted grain growth up to ~25 µm and faceted morphologies at intermediate concentrations. Excessive boron, however, yielded heterogeneous microstructures and secondary phases, aligning with XRD observations. Dielectric measurements demonstrated frequency-dependent behavior consistent with the Maxwell–Wagner model. For the Eu3+ series, the dielectric constant decreased from 32.4 to 21.8 at 20 Hz with increasing Eu3+, reflecting reduced crystallinity and enhanced microstrain. In contrast, moderate boron addition significantly enhanced the dielectric constant (up to 45.7 at 20 Hz) and suppressed dielectric loss, owing to improved densification, larger grains, and reduced oxygen-vacancy-related conduction. At high boron contents (70 and 100 mol%), both ε′ and tan δ degraded, attributed to strain accumulation and secondary phase formation. These results establish clear correlations between defect chemistry, microstructure, and dielectric response in Sr2GdTaO6 ceramics, highlighting the potential of controlled Eu3+ substitution and optimized boron co-doping for tailoring multifunctional dielectric materials.