Investigations on the Origin of Topotactic Phase Transition of LaCoO₃ Thin Films with In Situ XRD and Ambient Pressure Hard X-ray Photoelectron Spectroscopy

With the applications of in situ X-ray diffraction (XRD), electrical I-V measurement, and ambient pressure hard X-ray photoelectron spectroscopy (AP-HAXPES), the characteristics of the topotactic phase transition of LaCoO₃ (LCO) thin films are examined. XRD measurements show clear evidence of structural phase transition (SPT) of the LCO thin films from the perovskite (PV) LaCoO₃ to the brownmillerite (BM) La₂Co₂O₅ phases through the intermediate La₃Co₃O₈ phase at a temperature of 350 °C under high-vacuum conditions, ∼10^-5 mbar. The reverse SPT from BM to PV phases is also found under ambient pressure (>100 mbar) of air near 100 °C. Both observed SPTs in XRD are also identified in the electrical I-V measurements, i.e., the metallic PV phase to the insulating BM phase and vice versa. During the onset of SPTs, the bulk chemical and electronic states of LCO thin films are monitored with AP-HAXPES. The oxidation states in Co 2p spectra indicate that the oxygen vacancies are closely related to the SPT of LCO thin films. Also, the presence of enlarged band gap is observed as the SPT from PV to BM phases takes place, revealing the modified electronic properties of LCO due to the creation of oxygen vacancies. The analysis of valence band structures is further compared to the I-V measurements.