Correlating Crystallization-Induced Structural and Electrical Evolutions in SrTi_0.65Fe_0.35O_3-X Thin Films

Abstract

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.