Investigations\non\nthe Origin of Topotactic Phase Transition\nof LaCoO<sub>3</sub> Thin Films with In Situ XRD and Ambient Pressure\nHard X‑ray Photoelectron Spectroscopy

With the applications of in situ\nX-ray diffraction (XRD),\nelectrical <i>I</i>–<i>V</i> measurement,\nand ambient pressure\nhard X-ray photoelectron spectroscopy (AP-HAXPES), the characteristics\nof the topotactic phase transition of LaCoO<sub>3</sub> (LCO) thin\nfilms are examined. XRD measurements show clear evidence of structural\nphase transition (SPT) of the LCO thin films from the perovskite (PV)\nLaCoO<sub>3</sub> to the brownmillerite (BM) La<sub>2</sub>Co<sub>2</sub>O<sub>5</sub> phases through the intermediate La<sub>3</sub>Co<sub>3</sub>O<sub>8</sub> phase at a temperature of 350 °C\nunder high-vacuum conditions, ∼10<sup>–5</sup> mbar.\nThe reverse SPT from BM to PV phases is also found under ambient pressure\n(>100 mbar) of air near 100 °C. Both observed SPTs in XRD\nare\nalso identified in the electrical <i>I</i>–<i>V</i> measurements, i.e., the metallic PV phase to the insulating\nBM phase and vice versa. During the onset of SPTs, the bulk chemical\nand electronic states of LCO thin films are monitored with AP-HAXPES.\nThe oxidation states in Co 2p spectra indicate that the oxygen vacancies\nare closely related to the SPT of LCO thin films. Also, the presence\nof enlarged band gap is observed as the SPT from PV to BM phases takes\nplace, revealing the modified electronic properties of LCO due to\nthe creation of oxygen vacancies. The analysis of valence band structures\nis further compared to the <i>I</i>–<i>V</i> measurements.