Visible-Light-Absorbing Potassium Niobate-Titanate-Molybdate Ferroelectrics

The interactions of ferroelectric (FE) perovskite oxides (ABO₃) with light are increasingly being studied for different applications, such as photovoltaics and optoelectronics. The combination of different cations at the A and B sites to form solid solutions allows tuning of the material’s properties and, most importantly, the band gap (E_g), which sets the wavelength range of light absorption. Classic FE perovskite oxides, such as BaTiO₃, KNbO₃, and PbTiO₃, exhibit E_g > 3 eV, which limits their implementation in visible-light-absorbing devices. Furthermore, the tuning of their E_g via a solid solution strategy to a lower E_g range is limited by the requirement for the presence of a d⁰ metal at the B site, which is necessary for the FE distortion, but leads to a larger E_g. This gives rise to the challenge of decreasing E_g, while maintaining FE distortion. Here, we use first-principles calculations to explore the FE and optical properties of the (KNbO₃)_x(KTi_1/2Mo_1/2O₃)_1−x(KNTM) perovskite oxide solid solution. The introduction of Ti⁴⁺ and Mo⁶⁺ into the parent KNbO₃ decreases the E_g to about 2.2 eV for x = 0.9, while preserving or enhancing polarization. Experimental fabrication and characterization show that the obtained KNTM material at x = 0.9 has an orthorhombic structure at room temperature and a direct gap of <2.2 eV, confirming first-principles-based predictions. These properties make KNTM a promising candidate for further studies and applications as a visible-light-absorbing FE material.