The interactions of ferroelectric (FE) perovskite oxides (ABO 3) 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 BaTiO 3 , KNbO 3 , and PbTiO 3 , exhibit E g > 3 eV, which limits their implementation in visible-lightabsorbing 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 d 0 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 (KNbO 3) x (KTi 1/2 Mo 1/2 O 3) 1−x (KNTM) perovskite oxide solid solution. The introduction of Ti 4+ and Mo 6+ into the parent KNbO 3 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 firstprinciples-based predictions. These properties make KNTM a promising candidate for further studies and applications as a visible-light-absorbing FE material.

Prediction of properties from composition is a fundamental goal of materials science that is particularly relevant for ferroelectric perovskite oxide solid solutions where compositional variation is a primary tool for material design. Design of ferroelectric oxide solid solutions has been guided by heuristics and first‐principles and Landau–Ginzburg–Devonshire theoretical methods that become increasingly difficult to apply in ternary, quaternary, and quintary solid solutions. To address this problem, a multilevel model is developed for the prediction of the ferroelectric‐to‐paraelectric transition temperature (Tc), coercive field (Ec), and polarization (P) of PbTiO3‐derived ferroelectric solid solutions from composition. The characteristics of the materials at different length scales, starting at the level of the electronic structure and chemical bonding of the constituent ions and ending at the level of collective behavior, are analytically related by using ferroelectric domain walls and cationic off‐center displacements as the key links between the different levels of the model. The obtained composition–structure–property relationships provide a unified quantitatively predictive theory for understanding PbTiO3‐derived solid solutions. Such a multilevel analytical modeling approach is likely to be generally applicable to different classes of ferroelectric perovskite oxides and to other functional properties, and to materials and properties beyond the field of ferroelectrics.

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Using first-principles methods, we investigate the electronic properties of the [Ba(Mo1/2,Mg1/2)O3]x-[BaTiO3]1−x solid solution derived from barium titanate as a potential candidate to be used in photovoltaic devices. Focusing on the bandgap and its origin, we study the effect of different possible Mo and Mg contents, arrangements, and phases of [Ba(Mo1/2,Mg1/2)O3]x-[BaTiO3]1−x. We find that [Ba(Mo1/2,Mg1/2)O3]0.25-[BaTiO3]0.75 is a viable candidate for use in transparent photovoltaics due to its energy bandgap of 2.6 eV in the rhombohedral phase. In all cases, [Ba(Mo1/2,Mg1/2)O3]x-[BaTiO3]1−x materials exhibit spontaneous polarization that allows the exploitation of the bulk photovoltaic effect and in principle may allow high power conversion efficiency exceeding the Shockley-Queisser limit for these materials.

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