Continuously controllable optical band gap in orthorhombic ferroelectric KNbO₃-BiFeO₃ ceramics

The optical bandgap of orthorhombic ferroelectric KNbO₃ is shown to be continuously controllable via Bi and Fe co-substitution according to a K_1-xBi_xNb_1-xFe_xO₃ doping mechanism. The room temperature X-ray diffraction data combined with Raman spectroscopy analysis show the polar orthorhombic crystal structure to persist up to x = 0.25, while the bandgap narrows monotonically by 1 eV (∼33%). In-situ Raman spectroscopy corroborates the polar nature of all compositions in the temperature range of -100 to 200 °C. The ability to control the bandgap while maintaining the spontaneous polarisation makes the K_1-xBi_xNb_1-xFe_xO₃ system interesting for photoinduced processes in a wide temperature range.