X-Ray Absorption Spectroscopy of SrTi0.65Fe0.35O3-Δ : Understanding Local Structure Evolution during Crystallization

Abstract

Mixed ionic and electronic conductors (MIECs) serve as active materials and electrodes in a variety of electrochemical energy conversion, storage, processing, and sensing devices. Compared to composites of an ionic conductor and electronic conductor, single phase perovskite-structured MIECs that conduct both oxide ions and electronic species extend the reaction site for surface oxygen exchange beyond the triple phase boundaries and can exhibit high efficiencies. There has been increasing interest in lower temperature processing and application of MIECs. Therefore, there is a significant need to understand the local structure of amorphous and crystallizing MIECs so that structure-property relationships pertaining to electrode function (surface exchange kinetics, conductivity) can be established. To this end, we have studied the model MIEC SrTi0.65Fe0.35O3-d (STF). To better understand the role of crystallization on local structure evolution of STF films, X-ray absorption spectroscopy (XAS) studies at Argonne National Laboratory’s DND-CAT beamline were carried out on films at various stages of crystallization. Three films were deposited using PLD at several temperatures to control crystallinity: 1) room temperature (amorphous), 2) room temperature and subsequently annealed for 2h at 550 °C (semi-crystalline), and 3) at a substrate temperature of 700 °C (highly crystalline). Our experiments observed the Sr, Fe, and Ti K-edges using XAS to compare the local structure (valence states, coordination environment, and bond lengths) of amorphous, semi-crystalline, and crystalline STF thin films. Our results to date suggest significant differences in the first shell mean bond length and coordination environment of Fe and Ti polyhedra. Although it is widely assumed that the local environment of amorphous and crystalline materials is similar, with the main distinction being in the long-range ordering, our results show that the amorphous samples are significantly undercoordinated relative to their crystalline counterparts. Furthermore, we observe that the Ti-O and Fe-O coordination number increases with increasing crystallinity. However, it appears that even in the fully crystalline samples, there remain tetrahedrally coordinated cation centers with Fe atoms having, across all samples, more oxygen vacancies than Ti atoms within the same film.