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

(1 - x)KNbO3-xBiMnO(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% 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 degrees 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 KNbO3, which narrows continuously with increasing x, resulting in similar to 1-eV narrowing for single-phase x = 0.25. This may offer the possibility to employ this ferroic material in photoresponsive technologies.

Automated summary

(1 - x)KNbO3-xBiMnO(3) (0 <= x <= 0.25) ceramics were prepared using the solid-state reaction method. Bi and Mn were found to have low co-solubility in the orthorhombic structure. Both orthorhombic and pseudocubic crystal structures coexist within a certain range, and the grain size distribution shows bimodal behavior. Several anomalies in the dielectric behavior and phase transitions confirm the coexistence of crystal symmetries. Furthermore, it was discovered that the bandgap of KNbO3 can be tailored by co-doping Bi and Mn, resulting in a narrowing bandgap with increasing x.