SrZn_0.8Mg_0.2Si₃O₈ ceramics capable of rapid densification under atmospheric pressure, while maintaining excellent microwave dielectric properties were investigated. In-situ scanning electron microscopy and high-resolution transmission electron microscopy confirm the formation of a liquid phase during sintering and clarify its chemical composition. Rapid densification originates from intrinsic generation of this liquid phase in SrZn_0.8Mg_0.2Si₃O₈ during the sintering process. Because the liquid phase is produced internally, most of it spontaneously crystallizes into the parent phase during cooling. Therefore, the final microstructure consists predominantly of the SrZn_0.8Mg_0.2Si₃O₈ phase with only a small residual amorphous phase. SrZn_0.8Mg_0.2Si₃O₈ ceramics thus exhibit excellent microwave dielectric properties (ε_r = 6.07 ± 0.02, Q×f = 69,350 ± 650 GHz (f = 16.37 GHz), τ_f = −27.6 ± 1.4 × 10⁻⁶ ℃⁻¹, holding time(H.T.) = 1 min), which are superior to those of conventional composite ceramics that rely on added low-melting-point oxides to induce liquid-phase sintering. In addition, rapid densification enables grain refinement while maintaining high relative density. This microstructural feature improves mechanical strength and electrical breakdown strength and suppresses long range ionic diffusion during heterogeneous cofiring integration.

Excellent microwave dielectric properties and ultra-low thermal expansion are essential for dielectric ceramics in high-frequency substrate applications. However, the inherent constraints among these three key microwave dielectric properties make it challenging to achieve ultra-low relative permittivity (ε_r), high quality factor (Q×f), and near-zero temperature coefficient of the resonance frequency (τ_f) values simultaneously in existing materials. In particular, the coefficient of thermal expansion (CTE) of microwave dielectric ceramics often reaches approximately 10 ppm/°C, indicating limited tunability. In this work, based on novel osumilite-type BaMg₂Al₆Si₉O₃₀ ceramics with high crystal structural symmetry and excellent stability, we designed a strategy involving the substitution of Si⁴⁺ with larger Ge⁴⁺ ions. Replacing Si⁴⁺ with larger Ge⁴⁺ ions directly elongated the Si/Al(1)–O bond length while reducing the Si/Al(1)–O(1)–Si/Al(1) angle (σ₂). This structural modification suppressed the longitudinal vibration of 2-coordinate O(1) along the a-axis, effectively inhibiting negative thermal expansion and yielding a reduced CTE within the operational temperature range. Simultaneously, the elongation of the Si/Al(1)–O bond cooperatively increased the Si/Al(1)–O(2)–Si/Al(1) angle (σ₁) and enhanced the relative covalency of the Si/Al(1)–O bond, synergistically improving the Q×f values. Importantly, Ge⁴⁺ substitution preserved ultra-low ε_r and near-zero τ_f values by maintaining the polarization characteristics and crystal structural stability of BaMg₂Al₆Si₉O₃₀-based ceramics. The optimized BaMg₂Al₆Si_7.75Ge_1.25O₃₀ ceramics achieved excellent microwave dielectric properties: ε_r = 5.84, Q×f = 32,351 GHz, τ_f = −7.27 ppm/°C, and an ultra-low CTE of +1.07 ppm/°C. The successful co-regulation of the Q×f and CTE values was attributed to the polyhedral coupling strategy, which leveraged the structural features of the osumilite-type ceramics to synergistically optimize the tilting and distortion of critical polyhedra.

Recent studies have highlighted the significant influence of external electric fields on the photovoltaic effects of ferroelectric materials. Here, the ferroelectric semiconductor (1−x)Bi_0.5Na_0.4K_0.1TiO₃–xSmCoO₃ (abbreviated as xSmCo, where x ranges from 0.00 to 0.1) was synthesized using the solid-state method, achieving a narrowed optical band gap of 2.20–3.09 eV. Under the combined action of electric and light fields, the short-circuit photocurrent density (J_sc) increases from 44 to 269 nA·cm⁻². When thermal fields are also applied, J_sc further rises to 924 nA·cm⁻². However, the application of additional mechanical or magnetic fields reduces J_sc to 165 and 92 nA·cm⁻², respectively. The mixed valence states of Co²⁺/Co³⁺ introduce a high density of oxygen vacancies, facilitating band gap narrowing. The substitution of Sm³⁺ ions at the A-site preserves the ferroelectric properties. In ferroelectric ceramics, the remnant polarization (P_r) establishes a strong internal electric field, which is crucial for photo-induced charge carrier dynamics. These findings demonstrate that external fields—light, electric, thermal, mechanical, and magnetic—affect spontaneous polarization strength, internal field stability, and charge carrier transitions within the electronic bands. This study provides new insights into the interplay of multiple field interactions and their effects on the photovoltaic properties (J_sc/V_oc) of ferroelectric oxide materials.

Ba_1-xSr_xCuSi₂O₆ compounds with a tetrahedral structure (I41/acd) were prepared through the solid-state reaction method. The phase building process, structural evolution and microwave dielectric properties of Ba_1-xSr_xCuSi₂O₆ were investigated. Single BaCuSi₂O₆ phase can be obtained when calcined at 1050 ℃ for 3 h or 950 ℃ for 10 h. The substitution of Ba₂₊ by Sr₂₊ can effectively promote the sintering process and the maximum solubility of Ba_1-xSr_xCuSi₂O₆ was located between 0.25 and 0.30. Rietveld refinement, Raman-spectra and P-V-L complex chemical bond theory were used to explain the correlations between the crystal structures and microwave dielectric properties. The dielectric constant was dominated by the susceptibility (Σχ^μ) and ionic polarizability. The quality factor (Q × f) was determined by the bond strength, packing fraction and lattice energy, especially the Si-O bond. The susceptibility of Cu-O bond and Si-O bond played an important role in controlling the temperature coefficient of the resonant frequency (τ_f). A near zero τ_f value was obtained at x = 0–0.10 and the optimum microwave dielectric properties for Ba_1-xSr_xCuSi₂O₆ were achieved at x = 0.20 when sintered at 1000 ℃ for 3 h: ε_r–8.25, Q × f = 47616 GHz and τ_f = –9.6 ppm/℃.