X-ray Photon Correlation Spectroscopy Studies of Oxygen Vacancy Dynamics in SrCoO3-δ Heterostructures

Start Date
20-06-2018 16:00
End Date
20-06-2018 17:00
Auditorium, Central Building
Speaker's name
Dr. Dillon Fong
Speaker's institute
Materials Science Division, Argonne National Laboratory, Argonne, USA
Contact name
Anaïs Fernandes
Host name
Paul Evans
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Strontium cobaltite or SrCoO3-δ is an end member of La1-xSrxCoO3-δ, a well-known correlated electron material and oxide catalyst [1]. In reducing conditions, oxygen exits from the surface, δ approaches 0.5, and SrCoO3-δ forms the brownmillerite phase, creating a superlattice peak from oxygen vacancy ordering. As oxygen is re-incorporated into SrCoO3-δ via a surface redox reaction, the superlattice peak disappears as the perovskite phase is re-formed. The speed at which δ is varied depends on the sample temperature and the surrounding oxygen partial pressure (pO2). While the redox behavior of such oxides has been the subject of recent interest [2-6], much concerning the kinetics and dynamics of these materials remain unknown.

Utilizing in situ coherent X-ray scattering at the Advanced Photon Source, we monitored speckle from epitaxial SrCoO3-δ thin films grown on both SrTiO3(001) and (LaAlO3)0.3(Sr2TaAlO6)0.7(001) as oxygen was incorporated and evolved (switching the environment from O2 to N2) to gain insight into the dynamics of oxygen-induced phase evolution in complex oxide materials. We found that the kinetics of the brownmillerite to the perovskite phase transition could be varied from tens of minutes to several hours over a small temperature range (300°C to 350°C), observing pronounced differences between the oxidation and reduction behaviors, the latter involving substantial incubation times to re-nucleate the brownmillerite phase. From X-ray photon correlation spectroscopy performed at the brownmillerite superlattice reflection, we find that the two-time correlation function differs greatly between the two different substrates. We will discuss the kinetics and dynamics of the vacancy-ordering phase transition and the methods used to distinguish the different atomic and electronic mechanisms taking place. In addition, we will discuss in situ growth studies of SrCoO3-δ thin films by pulsed laser deposition and methods by which improved coherence at the upgraded synchrotron can benefit understanding of the deposition process.

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