Ab initio studies of dopant-defect complexes in KNbO3

In this work, we explore the effect of various dopant candidates in multiple defect centers in the nonlinear optical material potassium niobate (KNbO3) by means of ab initio calculations. We employ Density Functional Theory (DFT) to model different transition metal defect complexes within KNbO3 and explore trends in the band structures and projected densities of states to make qualitative predictions about the optical and infrared response of these systems. We also perform a detailed study of Fe as a substitutional impurity on both K and Nb sites in different local environments and ionization states and make detailed assignments of defect center levels reported in the experimental literature. Using DFT, we can explore a rather large number of dopant candidates and defect complexes identifying promising systems for deeper study. We have begun exploring the application of the GW approximation to some of these systems, using results for the pure phases of KNbO3 as a baseline. While limitations in computational time and resources prevent the extensive use of GW on doped systems, we discuss these limitations and present viable alternative routes of study for extending this methodology.