Crystal structure, dielectric properties, and optical bandgap control in KNbO ₃ –BiMnO ₃ ceramics

Abstract (1 − x )KNbO 3 – x BiMnO 3 (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% BiMnO 3 . 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.