Determining the oxygen stoichiometry of cobaltite thin films*

Transition metal oxides (TMOs) are promising materials to realize low-power neuromorphic devices. Their physical properties critically depend on their oxygen vacancy concentrations, whose experimental determination remains a challenging task. Here we focus on cobaltites, in particular La_1-xSr_xCoO_3-δ (LSCO) and present a strategy to identify fingerprints of oxygen vacancies in X-ray absorption (XA) spectra. Using a combination of experiment and theory, we show that the variation of the oxygen vacancy concentration in the perovskite phase of LSCO is correlated with the change of the relative peak positions of the O K-edge XA spectra. We also identify an additional geometrical fingerprint that captures both the changes of the Co-O bond length and Co-O-Co bond angle in the material due to the presence of oxygen vacancies. Finally, we predict the oxygen vacancy concentration of experimental samples and show how the resistivity of the oxide material may be tuned as a function of the defect concentration, in the absence of any structural transformation. Our study [1] shows that, in order to predict the complex transport properties of Mott materials, it is crucial to gain a detailed understanding of their oxygen defect density.