Investigation of the surface chemical state of SrFeO_2.5 films during topotactic transition by ambient pressure X-ray photoelectron spectroscopy

Topotactic transition between a metallic perovskite SrFeO₃ (SFO) and an insulating brownmillerite SrFeO_2.5 has been extensively studied due to its potential applications, such as synaptic components for neuromorphic computing and memory devices. Only ordered oxygen vacancies (OVCs) are formed and removed during the transition while maintaining the crystallographic framework. The formation of OVCs is known to be strongly influenced by the strain and surface energy that can affect the capability to dissociate gas molecules of a material. However, there are still few studies on the effect of the strain on the surface chemistry and the topotactic transition.

In this study, we investigated the correlation between the surface chemistry and the topotactic transition of SFO films with different strain states. SFO films were grown on SrTiO₃(001) and (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7(001), LSAT substrates using the pulsed laser deposition. Structural analysis based on X-ray diffraction confirmed that both films have high crystallinity, and the SFO film grown on STO(001) showed weaker in-plane compressive strain than the film grown on LSAT(001) due to the small lattice mismatch between the film and STO substrate. We investigate the change in the surface chemical state and the corresponding electrical resistance of the films during the topotactic transition using Ambient-pressure X-ray photoelectron spectroscopy. We observed a larger surface SrO state in the film grown on the STO(001) substrate, where the phase transition occurs more slowly than the film grown on the LSAT(001) substrate. We will discuss the correlation between strain state, surface SrO, and resistance on the topotactic transition.