Structural changes in transition metal oxides are often considered to be synonymous
with changes in the oxygen defect structure of a material. Understanding, and tuning,
structural and chemical changes in perovskite oxides is critical for the optimisation of
oxygen evolution and reduction reactions, and studying these processes in situ is essential
for improving the catalytic performance of perovskites.
La0.5Sr0.5Mn0.5Co0.5O3-d (LSMC) has been studied as a model system as it is one of the
fewreported perovskite oxides that is stable with d > 0.5, accommodating a fully oxidised
phase (d = 0) and reduced phase (d = 0.62) with a reversible, topotactic transition between
these two phases in bulk form. In this work, thin films of LSMC were deposited using
pulsed laser deposition on single crystal substrates resulting in compressive strain on
LaAlO3 (LAO), and tensile strain on (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) and SrTiO3 (STO).
Complementary X-ray diffraction (XRD), for determining structural changes, and X-ray
absorption near edge spectroscopy (XANES), to determine oxygen stoichiometry, were
used to investigate the effects of mechanical strain. The LSMC unit cell volumewas found
to be linearly dependant on the strain, however the same relationshipwas not observed for
the oxygen stoichiometry. Both the compressively strained LSMC/LAOand LSMC/LSAT
were oxidised, while the LSMC/STO was more reduced due to a change in Mn valence.
In situ XRD, conducted at elevated temperatures (400 to 750C) with varying oxygen
partial pressures (pO2 = 1x10^1 to 2.2x10^5 ppm), demonstrate that the unit cell volume of
LSMC responds to changes in oxygen partial pressure. Films grown on LAO show a five
times greater cell parameter change between oxidising and reducing conditions compared
to those grown on STO, with changes of approximately 3x10^−3 Å and 6.5x10^−4 Å
at 700C, respectively. In situ XANES, at temperatures up to 500C under reducing
conditions (pO2 < 1x10^−5 ppm), provided evidence for a greater change in the Mn
oxidation state. Further, the tensile strain of the LSMC/STO samples was shown to
stabilise a LSMC thin film with a lower oxidation stoichiometry, indicating that strain is
a successful method for tuning the defect structure under operating conditions.
Electrical characterisation was performed by applying bias through LSMC/Nb:STO
thin films to change the effective oxygen stoichiometry. These results showed promising
resistive switching behaviour and initial in situ measurements with isotopically labelled
oxygen provide evidence for an interface based switching mechanism.
These results deepen the understanding of methods to tune the defect structure
of perovskites and can be used to guide the optimisation of perovskite properties for
electrochemical devices including energy storage and memristors.