Crystal structure, dielectric properties, and optical bandgap control in KNbO₃–BiMnO₃ ceramics

(1 − x)KNbO₃–xBiMnO₃ (0 ≤ x ≤ 0.25) ceramics were prepared by the solid-state reaction method. An X-ray diffraction analysis combined with Raman spectroscopy showed the co-solubility of Bi and Mn in the orthorhombic structure to be less than 5% BiMnO₃. Orthorhombic and pseudocubic symmetries coexist in the 0.05 ≤ x ≤ 0.15 region, coinciding with a bimodal grain size distribution. This coexistence of crystal symmetries is further corroborated by several anomalies in the dielectric behavior, which can be ascribed to structural phase transitions. For x ≥ 0.20, only one dielectric anomaly is detected around 100°C, which is commensurate with in situ Raman spectroscopy analysis. This work also shows that Bi/Mn co-doping can be employed to tailor the bandgap of KNbO_3, which narrows continuously with increasing x, resulting in ∼1-eV narrowing for single-phase x = 0.25. This may offer the possibility to employ this ferroic material in photoresponsive technologies.