Origins and Control of Optical Absorption in a Nondilute Oxide Solid Solution: Sr(Ti,Fe)O _{3- x} Perovskite Case Study

Understanding and rationally tailoring defect-mediated optical absorption of nondilute oxide solid solutions represents a complex challenge. In this work, we investigate compositions in the SrTiO ₃ -SrFeO _2.5 solid solution, departing from the simpler dilute Fe-substituted SrTiO ₃ case. Through ex situ and in situ optical absorption measurements of mixed conducting thin films prepared by pulsed laser deposition and through density functional theory simulations, we demonstrate understanding and rational tailoring of the optical absorption behavior. Experimentally, broad subgap absorption peaks, centered around 2.1 and 2.8-3 eV, increase in intensity with increasing Fe and/or O concentrations and decrease with increasing La donor dopant concentration. Consistent with these observations, the absorption is found to be proportional to the hole concentration and to the Fe concentration under fully oxidized conditions. This behavior is similar to the dilute case; however, the solid solution electronic structure and optical absorption behavior cannot be represented simply by Fermi level shifts in a rigid-band model. Simulations confirm these trends and identify transitions responsible for absorption as occurring from within the valence band to empty states at the hybrid (O 2p/Fe 3d) valence band maximum and to empty states at the hybrid (Ti/Fe 3d) conduction band minimum. Adding oxygen or iron, or removing La, increases the density of empty states at the top of the valence band and bottom of the conduction band, increasing the intensity of these transitions. This approach for studying solid solution behavior can be extended to other systems in the future, and the fundamental understanding of the origins of absorption also enables its in situ use as a quantitative probe of thin film point defect concentrations and kinetics.