In situ investigation of the redox behaviour of strained La0.5Sr0.5Mn0.5Co0.5O3-¿ thin films

Abstract

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.