Ca_1–xMg_xY₂Ge₃O₁₀ ceramics were synthesised using the solid-state reaction method. XRD analysis reveals that the solid solubility limit of Mg²⁺ in Ca_1–xMg_xY₂Ge₃O₁₀ lies between x = 0.15 and 0.20. Multiple characterisation approaches, including Phillips–van Vechten–Levine theory and bond valence analysis, were employed to systematically elucidate the crystal structure and microwave dielectric properties. TEM analysis revealed a distinct superlattice structure in the material, characterised by an increased density of (001) planes along the [100] zone axis. The presence of superlattice in Ca_0.9Mg_0.1Y₂Ge₃O₁₀ gives rise to a higher degree of lattice ordering, thereby reducing the intrinsic loss. The corrected dielectric constant shows a trend opposite to the α_D^T/V_m from Clausius–Mossotti (C–M) equation, primarily because of the rattling effect induced by Mg²⁺ substitution, which limits the applicability of the C–M equation. The intrinsic loss of CaY₂Ge₃O₁₀ was assessed by far-infrared spectroscopy. Key factors governing the microwave dielectric properties include total lattice energy (U_t), ion activation energy (E_a), and packing fraction (P.F.), which collectively determine the variation in Q×f, whereas the average distortion of Y–O bonds (Ave(δY–O)), the rattling effect and proportion of Ge–O bonds in total polarizability (χ(Ge–O)/χ), primarily affect τ_f. The Ca_0.9Mg_0.1Y₂Ge₃O₁₀ ceramic exhibits an enhanced Q×f, with microwave dielectric properties of ε_r = 10.1, Q×f = 106,440 GHz, and τ_f = −27.1 × 10⁻⁶ ℃⁻¹. At x = 0.15, a reduced Ave(δ_Y–O), a maximum χ(Ge–O)/χ, and a stronger rattling effect are observed, leading to a near-zero τ_f of −9.7 × 10⁻⁶ ℃⁻¹, with ε_r = 10.3 and Q×f = 91,920 GHz.

A series of (Sr_0.4Ce_0.4)TiO₃ + 4% (in mass) Sr₂CeO₄ + x% B₂O₃ (SCTO + 4% SCO + x% B₂O₃ for 1≤ x ≤ 5) composites were synthesized via solid-state reaction to investigate the effects of Sr₂CeO₄ and B₂O₃ additives on their structural evolution and microwave dielectric properties. X-ray diffraction (XRD) and Rietveld refinement confirmed the dominant orthorhombic phase (O-phase), with SCO as a secondary phase, indicating a chemically stable composite system. HRTEM and SAED analyses further confirmed the formation of the O-phase through direct observation of the superlattice reflections. Microstructural evolution demonstrated B₂O₃-assisted liquid-phase sintering, reducing porosity (0.017 → 0.006) and increasing grain size (3.34 → 6.01 μm) with increasing x% (in mass). Raman spectroscopy verified octahedral tilting and Ce—O stretching, while B₂O₃ incorporation modified the TiO₆ network via BO₃/BO₄ interactions. The ε_r decreased from 113 (SCTO) to 27 at x = 5%, while τ_f improved from +213 to +12 × 10⁻⁶/℃. The reduction in ε_r arises from a complex interplay of internal factors (ionic polarizability) and external factors (porosity, and density), whereas the variation in τ_f is governed by compensating effects from Sr₂CeO₄ and B₂O₃. Optimal microwave performance was achieved at x = 5%, with Q×f = 43,603 GHz, ε_r = 27, and τ_f of +12 × 10⁻⁶/℃. The study demonstrates that SCO and B₂O₃ act as effective modifiers, enhancing densification and dielectric properties in SCTO-based microwave ceramics.

(Sr_0.4Ce_0.4)_1-xNd_xTi_0.8Mg_0.2O₃ [(SC)NdTMgO₃, x = 0.1–0.4] non-stoichiometric system were obtained through conventional solid-state synthesis technique. All perovskite peaks were attributed to either single (x ≤ 0.3) and/or mixed (x = 0.4) like phases. A solid solution of cubic structure was obtained for x ≤ 0.2, however, for x ≥ 0.3, the (SC)NdTMgO₃ compound, exhibits structure instability and (200)/(310) peaks splitting. The peaks splitting indicates cubic (C-phase) to tetrogonal (T-phase) structure phase transition. All ceramic samples appear nearly dense, presenting a non-uniform grain size distribution. The observed trend of the microwave dielectric properties (ε_r, τ_f, Q × f) is mainly related to the formation of structure phase transition, packing fraction, relative density and ionic polarizability. A key microwave (MW) dielectric properties of ε_r = 53, Q × f = 26,700 GHz and τ_f = +2.8 pmm/℃ could be obtained for composition with x = 0.4.