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 (Eg), which sets the wavelength range of light absorption. Classic FE perovskite oxides, such as BaTiO3, KNbO3, and PbTiO3, exhibit Eg > 3 eV, which limits their implementation in visible-light-absorbing devices. Furthermore, the tuning of their Eg via a solid solution strategy to a lower Eg 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 Eg. This gives rise to the challenge of decreasing Eg, while maintaining FE distortion. Here, we use first-principles calculations to explore the FE and optical properties of the (KNbO3)x(KTi1/2Mo1/2O3)1-x (KNTM) perovskite oxide solid solution. The introduction of Ti4+ and Mo6+ into the parent KNbO3 decreases the Eg 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.