(1-x%)(K_0.5Na_0.5NbO₃)-x%(Sr_0.7Bi_0.2TiO₃) (x = 1, 4, 7, 10, and 15 molar fraction) lead-free ferroelectric ceramics, denoted as xSBT, were synthesized by a conventional solid-state reaction method and a subsequent pressureless sintering. The effect of SBT incorporation on the microstructure, phase evolution, optical properties, and energy storage performance was investigated. The introduction of SBT effectively refined the grain size and promoted densification, resulting in a high optical transmittance of 72% and 58% in the visible region for the 7SBT and 10SBT ceramics, respectively. SBT doping induced a transition from a normal ferroelectric to a relaxor ferroelectric state in the KNN-based matrix, with the 7SBT and 10SBT compositions exhibiting pronounced relaxor characteristics. The results of ferroelectric performance tests revealed recoverable energy storage densities of approximately 1.05 J/cm³ at 150 kV/cm for 7SBT and 3.61 J/cm³ at 300 kV/cm for 10SBT, accompanied by an energy storage efficiency of 52%. This outstanding comprehensive performance (energy storage density and optical transmittance) demonstrated significant advantages over other reported lead-free ferroelectric ceramic systems, thereby providing both a material system and theoretical foundation for developing high-performance lead-free ferroelectric transparent dielectrics for optoelectronic integrated applications.

(1-x%)(K_0.5Na_0.5NbO₃) -x%(Sr_0.7Bi_0.2TiO₃) (x = 1, 4, 7, 10, and 15) ceramics, denoted as xSBT, were fabricated with high-purity oxides and carbonates as starting materials by a conventional solid-state reaction method and a subsequnt pressureless sintering. The phase structure was analyzed by X-ray diffraction (XRD). The microstructure and elemental distribution were examined by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS). The structural evolution was determined by Raman spectroscopy. The optical transmittance was measured by ultraviolet-visible-near infrared spectrophotometry. The temperature-dependent relative dielectric permittivity (ε_r) was evaluated by an impedance analyzer and the polarization-electric field (P-E) hysteresis loops were measured by a ferroelectric test system to assess energy storage performance. Results and discussion The SEM images demonstrate that the introduction of xSBT effectively refines the grain size and promotes the microstructural densification. The grain size is significantly reduced from approximately 290 nm (as x = 4) to below 150 nm (as x = 7 and 10), effectively minimizing a light scattering. Consequently, the 7SBT and 10SBT ceramics achieve a high optical transmittance of approximately 72% and 58% at 800 nm in the visible light region, respectively.The XRD patterns and XRD Rietveld refinement results reveal a continuous phase structural evolution with increasing x content, i.e., from an orthorhombic phase (x = 1) to a tetragonal phase (x = 4), then to a coexistence of tetragonal and cubic phases (x = 7, 10), and finally to a pure cubic phase (x = 15). This progression signifies a structural transition from low-symmetry orthorhombic to high-symmetry cubic phases. The enhanced structural symmetry, along with chemical disorder induced by heterovalent substitutions of Sr²⁺/Bi³⁺/Ti⁴⁺, effectively can disrupte the long-range ferroelectric order, facilitating the formation of polar nanoregions (PNRs) and leading to a pronounced relaxor behavior. This is quantitatively supported by the significant increase in the diffuseness coefficient (γ) to 1.49 and 1.56 for x = 7 and 10, respectively. Correspondingly, the P-E loops evolve from a saturated square shape for 1SBT to a slim shape for 10SBT. As a result, the 7SBT and 10SBT compositions deliver recoverable energy storage densities (W_rec) of 1.05 J/cm³ in 150 kV/cm and 3.61 J/cm³ in 300 kV/cm, respectively, with an efficiency (η) of 52%. The combination of high optical transmittance and superior energy storage performance makes the xSBT ceramics a promising lead-free material system for integrated optoelectronic applications.

This study achieved the grain refinement and structural densification in KNN-based ceramics via doping SBT components, resulting in lead-free ceramics with a high optical transparency. The optical transmittance of the 7SBT and 10SBT ceramics in the visible light region reached 72% and 58%, respectively. Furthermore, the increased crystal symmetry and introduced chemical disorder by SBT doping disrupted the long-range ferroelectric order of the KNN matrix, thereby inducing pronounced relaxor characteristics. These modifications contributed to excellent ferroelectric energy storage properties in the 7SBT and 10SBT transparent ceramics. The 7SBT and 10SBT ceramics maintained an energy storage efficiency of 52% in electric fields of 150 kV/cm and 300 kV/cm, respectively, while achieving recoverable energy storage densities of approximately 1.05 J/cm³ and 3.61 J/cm³. Their outstanding overall performance (i.e., energy storage density and optical transparency) demonstrated significant advantages among reported lead-free ferroelectric transparent ceramics. This work could provide important theoretical and experimental foundations for advancing key materials in next-generation optoelectronic integrated systems and open pathways for designing novel multifunctional lead-free ferroelectric ceramics. The main limitation could lie in the substantial room for improvement in the energy storage efficiency of the current system. Future work could involve more precise composition control and interfacial engineering to reduce dielectric loss and leakage. In addition, the stability of electrical performance across wide temperature ranges and high frequencies could also require a systematic evaluation.