Ultrafast photo-induced O2− channel-opening in oxygen vacancy ordered SrCoO2.5 film

The recent development of electric-field controlled brownmillerite SrCoO2.5 (BM-SCO) to SrCoO3-δ phase transformation greatly enriches the controlling diversity of functional materials. However, the required potential is much larger than that for the standard electrolysis of H2O and the detailed mechanisms for the corresponding oxygen insertion are still unclear. In this study, we mimic such electric-field control step with optical pulse excitation. In specific, by exciting BM-SCO thin film with femtosecond 400 or 800 nm pulses, and monitoring the lattice dynamics using ultrafast x-ray diffraction, we find that 400 nm photo-excitation can induce a distinctive transient BM-SCO state containing both Co2+ and Co4+, which is more suitable for O2− invasion. This transient BM-SCO state is suggested to originate from the redistribution of electrons on CoT (tetragonal layer) and CoO (octahedral layer) 3d orbitals, which is further confirmed by femtosecond transient reflectance measurements. We suggest that this distinctive transient BM-SCO state, which is critical for the phase transition, is also induced during the electric-field controlled BM-SCO to SrCoO3-δ phase transformation. This study intends to contribute an intriguing research thought for the inherent mechanism that might be powerless with traditional means and a special phase control method as well.