Determining the Oxygen Stoichiometry of Cobaltite Thin Films

Abstract

Transition metal oxides (TMOs) are promising materials to realize low-power neuromorphic devices. Their physical properties critically depend on their oxygen vacancy concentration, whose experimental determination remains a challenging task. Here, we focus on cobaltites, in particular La 1−x Sr x CoO 3−δ (LSCO), and present a strategy to identify fingerprints of oxygen vacancies in X-ray absorption (XA) spectra. Using a combination of experiments and theory, we show that the variation of the oxygen vacancy concentration in the perovskite phase of LSCO is correlated with the change in the relative peak positions of the O K-edge XA spectra. We also identify an additional geometrical fingerprint that captures both the changes in 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 shows that, in order to predict the complex transport properties of TMOs, it is crucial to gain a detailed understanding of their oxygen defect density.

Metallic Interface between Two Insulating Phases of La1–xSrxCoO3−δ

“…We also predicted that the threshold voltage to trigger an MIT in LSCO is close to that of a VO 2 -based device . Interestingly, multiple resistive states can be achieved by fine-tuning δ, which is a desirable feature for neuromorphic devices …”

“…Previous experimental and computational studies suggested I 2 mb and Pnma for the SrCoO 2.5 BM structure; , however, disordered arrangements of the CoO 4 tetrahedra leading to the Imma space group have also been proposed for several TMOs. , In all symmetry groups, the tilting of the CoO x chain is different from that of the perovskite LaCoO 3 (a – a – a – ): the Pnma has a + b – b – tilting, the I 2 mb (nonstandard setting of Ima 2) has a 0 a 0 c – , and the Imma has a 0 b – b – . This difference in tilting is important to understand the physical properties of the interface, as pointed out in the case of several other TMO thin films. − Without losing generality, in our study, we adopted the Pnma group symmetry for the La 1– x Sr x CoO 2.5 BM phase, consistent with our previous studies. − In addition, we approximated the rhombohedral perovskite LaCoO 3 structure with a pseudocubic one and represented it with an orthorhombic supercell (see Figure S1), which could then be easily interfaced with the orthorhombic BM structure. Importantly, the octahedral tilting pattern of LaCoO 3 is preserved in our heterostructure model.…”

“…One promising category of TMOs is the family of Sr-doped lanthanum cobaltites (La 1– x Sr x CoO 3−δ , or LSCO), for which we have recently carried out a series of studies as a function of oxygen vacancy concentration δ. − In particular, we showed that by increasing δ from 0 to 0.5 at a Sr doping concentration x ≈ 0.33, the material undergoes a topotactic transition from a ferromagnetic metallic perovskite (P, δ ≈ 0) to an antiferromagnetic (AFM) insulating brownmillerite phase (BM, δ = 0.5); without any Sr doping ( x = 0), a similar transition occurs from a diamagnetic insulating P phase (band gap measured in the range 0.6–1.1 eV − ) to an AFM insulating BM phase (calculated band gap 1.9 eV), with the band gap increasing with δ . We also predicted that the threshold voltage to trigger an MIT in LSCO is close to that of a VO 2 -based device .…”

“…18 Interestingly, multiple resistive states can be achieved by fine-tuning δ, which is a desirable feature for neuromorphic devices. 19 Experimentally, it has been found that in LSCO, the BM and P phases may coexist during the topotactic transition, 25 and the impact of this coexistence on the MIT is not well known. In fact, the spontaneous formation of P/BM interfaces was observed in several TMOs under various conditions, e.g, in SrCoO 3−δ /SrCoO 2.5 fabricated with pulsed laser epitaxy technique, 26 in SrFeO x during a topotactic transition by annealing, 27 and in La 2/3 Sr 1/3 MnO 3 during topotactic transitions controlled by voltage.…”