(Na0.85K0.15)0.5Bi0.5Ti1–xSnxO3 ceramics (NKBTS_100x, where x = 0, 0.05, 0.15 and 0.25) were synthesized by the conventional solid-state reaction sintering route to elucidate the coupling among average structure, local lattice dynamics, microstructure and optical response in NKBT-based perovskites, promoted by the isovalent Sn4+ substitution. The coexistence of both rhombohedral (R3c) and tetragonal (P4mm) perovskite-type phases was confirmed from X-ray diffraction in all compositions, revealing a Sn-driven phase-stability shift characterized by a progressive stabilization of the R3c component and strong suppression of the P4mm phase at higher Sn contents. Raman spectroscopy corroborated the successful formation of the perovskite framework across the entire compositional range, results also revealed a systematic Sn-dependent phonon renormalization, including softening of A-site/tilt-related and octahedral bending modes together with a non-linear evolution of the BO6 stretching band, consistent with modified Ti/Sn–O bonding and local heterogeneity. The ferroelectric character of the studied system was further supported by complementary electrical measurements, thought the P–E hysteresis loops and dielectric response. Scanning electron microscopy showed homogeneous microstructures and well-defined grains, with a decreasing trend up to NKBTS_15, followed by a grain coarsening at NKBTS_25, suggesting that grain-boundary mobility and transport-related effects may contribute to the observed microstructural evolution as the Sn content increases. The optical characterization has shown enhanced visible/near-IR absorbance with the inclusion of the Sn cation, together with a low-energy optical absorption onset around 1.47 eV and a slight minimum for the NKBTS_15 sample. These results highlight this ceramic composition as a particularly promising candidate for visible/near-IR light harvesting and potential photovoltaic exploration in polar oxides.
The demand for sustainable ceramic processing has stimulated interest in waste-derived fluxing materials that can reduce firing requirements. This study investigates the direct incorporation of uncalcined green mussel shell powder into stoneware bodies, thereby eliminating an additional precalcination step. Three shell particle-size fractions were incorporated at 10–20 wt% and fired at 800–1250 °C. Rietveld refinement showed that all fractions were aragonite-rich, indicating that differences in thermal behavior were primarily associated with particle size rather than CaCO3 polymorph composition. Medium-sized shell particles at 10 wt% (MGS10) exhibited the most favorable processing behavior, with concentrated carbonate decomposition occurring over approximately 650–800 °C, allowing CO2 to evacuate before advanced matrix vitrification. Shell-derived Ca2+ subsequently acted as a network modifier within the aluminosilicate glass, promoting liquid-phase sintering and densification at 1150 °C, while further reaction at 1250 °C resulted in anorthite crystallization. MGS10 achieved a bulk density of approximately 2.38 g cm-3 and a water absorption value of approximately 0.8% at 1250 °C, comparable to that of the unmodified stoneware body, while maintaining structural integrity at 1150 °C. These results demonstrate that particle-size control is critical for integrating uncalcined biogenic CaCO3 into stoneware and provides a practical route for shell-waste valorization and reduced-temperature ceramic processing.
