Rapid\nCharacterization of Point Defects in Solid-State\nIon Conductors Using Raman Spectroscopy, Machine-Learning Force Fields,\nand Atomic Raman Tensors

The successful design\nof solid-state photo- and electrochemical\ndevices depends on the careful engineering of point defects in solid-state\nion conductors. Characterization of point defects is critical to these\nefforts, but the best-developed techniques are difficult and time-consuming.\nRaman spectroscopywith its exceptional speed, flexibility,\nand accessibilityis a promising alternative. Raman signatures\narise from point defects due to local symmetry breaking and structural\ndistortions. Unfortunately, the assignment of these signatures is\noften hampered by a shortage of reference compounds and corresponding\nreference spectra. This issue can be circumvented by calculation of\ndefect-induced Raman signatures from first principles, but this is\ncomputationally demanding. Here, we introduce an efficient computational\nprocedure for the prediction of point defect Raman signatures in solid-state\nion conductors. Our method leverages machine-learning force fields\nand “atomic Raman tensors”, i.e., polarizability fluctuations\ndue to motions of individual atoms. We find that our procedure reduces\ncomputational cost by up to 80% compared to existing first-principles\nfrozen-phonon approaches. These efficiency gains enable synergistic\ncomputational–experimental investigations, in our case allowing\nus to precisely interpret the Raman spectra of Sr(Ti<sub>0.94</sub>Ni<sub>0.06</sub>)O<sub>3‑δ</sub>, a model oxygen ion\nconductor. By predicting Raman signatures of specific point defects,\nwe determine the nature of dominant defects and unravel impacts of\ntemperature and quenching on <i>in situ</i> and <i>ex situ</i> Raman spectra. Specifically, our findings reveal\nthe temperature-dependent distribution and association behavior of\noxygen vacancies and nickel substitutional defects. Overall, our approach\nenables rapid Raman-based characterization of point defects to support\ndefect engineering in novel solid-state ion conductors.