Simultaneous Electrical, Electrochemical, and Optical Relaxation Measurements of Oxygen Surface Exchange Coefficients: Sr(Ti,Fe)O_3−d Film Crystallization Case Study

The oxygen surface exchange coefficient (k) dictates the efficiency and response time of many mixed conductors, so its accurate, continuous measurement in realistic conditions, enabling rational tailoring, is necessary. However, recent results showed that k values determined by a thin-film optical transmission relaxation (OTR) method were orders of magnitude lower than those extracted from the cross-cell AC-impedance spectroscopy (AC-IS) approach, and similar discrepancies among methods exist in the literature. OTR has also detected dramatic increases in k in situ during crystallization. Therefore, in this work, we sought to establish whether k values from OTR are reliable, and to gain further insight into crystallization-induced changes, via comparison to the electrical conductivity relaxation (ECR) method. We performed simultaneous OTR and ECR measurements on the same region of an as-grown amorphous SrTi0.65Fe0.35O2.825+δ (STF) film, prepared by pulsed laser deposition and characterized by Rutherford backscattering spectrometry, during thermal treatment to induce crystallization and a large increase in k. We also compared cross-cell AC-IS vs OTR on an as-grown amorphous film during crystallization and OTR vs ECR on a crystalline-grown film. Simultaneous measurements eliminate variability in k between samples or due to different thermal/gas history. OTR and ECR methods yielded the same k values, and the same crystallization temperature, within error. Both isothermal optical absorption and electrical conductivity changes are proportional to the hole and oxygen concentration changes under the conditions of this study. However, while OTR was able to measure optical absorption changes under all of the conditions tested, ECR was not viable in the high-resistance regime. Cross-cell AC-IS k values were elevated vs OTR values, were less stable over time, and were only accessible in limited conditions. We discuss the potential impacts of current collectors and oxygen exchange driving force on k values determined by cross-cell AC-IS vs ECR vs OTR.