The authors review the use of in situ optical absorption to probe point defect equilibria and kinetics in oxides, with particular attention to thin films and high temperature measurements evaluating changes in oxygen content. The introduction notes the impact of point defects on oxide behavior, including differences in thin films vs. bulk materials, and describes methods to evaluate point defect equilibria and kinetics; advantages of optical absorption among these methods are highlighted. Section 2 provides an overview of how optical absorption and point defect concentrations are related, including various possible structural and electronic origins of defect-mediated absorption changes corresponding to oxygen content changes. Section 3 traces the development of historical understanding of optical absorption in nonstoichiometric oxides and emergence of thin film applications and measurement approaches that resulted. Section 4 details the procedure for determining surface exchange coefficients from optical transmission relaxation measurements and provides specific examples of how this approach uniquely has enabled new insights into thin film surface exchange behavior with and without current collectors. Section 5 provides examples of optically-derived diffusivity measurements and comparisons of bulk and thin This article is protected by copyright. All rights reserved. 2 film defect equilibria. Section 6 outlines potential opportunities and future developments in this area, and Section 7 concludes the manuscript.

The coupled Maxwell and time-dependent Kohn-Sham equations are solved using the Riemann-Silberstein vectors to represent the electromagnetic fields. Momentum-space time propagation of the Riemann-Silberstein vectors are proposed and test calculations are presented to show the accuracy of the approach.

Crystallization of a perovskite mixed conductor is coupled to oxidation, which drives defect chemistry, ion coordination, polyhedra alignment, hierarchical microstructure, and property evolution – including a boost to electrical conductivity.

Rapid oxygen exchange kinetics and fast ionic and electronic transport are central to the performance of materials in solid-state electrochemical devices, such as metal-air batteries, solid oxide fuel/electrolysis cells, and gas sensors [1]. When fabricated at high temperatures, these materials are plagued by surface segregation of large cations, which leads to sluggish oxygen exchange [2]. When further operated at high temperatures, ongoing surface degradation can occur, while operation at low temperatures yields higher electrical resistances and even slower oxygen surface exchange kinetics due to their thermally-activated nature. By contrast, recent work in our group has demonstrated a low-thermal-budget route to fabricate SrTi0.65Fe0.35O3-x (STF35) thin films with enhanced low-temperature oxygen exchange kinetics by crystallizing amorphous-grown STF35 thin films, thereby maintaining a low Sr surface concentration [3,4]. While this result is promising, the crystallization-induced structural evolution and its influence on the charge transport behavior have yet to be studied. In this work, amorphous STF35 thin films grown by room temperature pulsed laser deposition (PLD) were used to study the relationship between structure and conductivity during crystallization of a mixed conductor. The degree of crystallinity and local ion environment were evaluated by X-ray diffraction (XRD) and synchrotron X-ray absorption spectroscopy (XAS). In situ AC impedance spectroscopy was used to monitor the conductivity of the film during crystallization over a 400 °C isothermal hold in a controlled gas environment. The dependence of the conductivity on the volume percent crystalline showed two distinct behaviors: a small, fast increase in conductivity after a short time at temperature and a larger, percolation-type increase later in the anneal. The percolation behavior suggests a microstructural influence on the transport behavior. From the XAS results, we observed an increase in the oxidation number of Fe within the first 10 minutes of the anneal, indicating rapid oxidation. Additionally, there was an increase in symmetry and coordination number in the Fe coordination unit over the first 30 minutes of the anneal, which is correlated to an increase in the orbital overlap of the O 2p and Fe 3d orbitals and thus the hole mobility. Combined with the conductivity and XRD results, the difference in annealing-time dependence of the Fe oxidation state and Fe coordination unit symmetry enable us to separate the relative contributions of changing orbital overlap, hole concentration, and microstructure to the evolving charge transport behavior. [1] Hong, W.T., Risch, M., Stoerzinger, K.A., Grimaud, A., Suntivich, J. and Shao-Horn, Y., 2015. Energy & Environmental Science, 8(5), pp.1404-1427. [2] Perry, N.H. and Ishihara, T., 2016. Materials, 9(10), p.858. [3] Chen, T., Harrington, G.F., Sasaki, K. and Perry, N.H., 2017. Journal of Materials Chemistry A, 5(44), pp.23006-23019. [4] Chen, T., Harrington, G., Masood, J., Sasaki, K. and Perry, N.H., 2019. ACS applied materials & interfaces.