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LaCuO3 is one of the most fundamental cuprate perovskite blocks and has a similar structure to the cuprate high-temperature superconductor. Therefore, some consider that a study on LaCuO3 is one of the clues to clarify the mechanism of cuprate high-temperature superconductivity. However, since the synthesis of bulk LaCuO3 is difficult due to the need for the high-pressure and high-temperature (HPHT) method, not enough research on LaCuO3 has been done so far. In this paper, I report the successful synthesis of rhombohedral LaCuO3 thin films using the oxidation effect of NaClO solution without using the HPHT method. The nanobeam electron diffraction pattern of the cross-section of LaCuO3−δ thin film oxidized in NaClO solution at 80 °C for 72 hours demonstrates the formation of rhombohedral LaCuO3. The amount of oxygen defects in the rhombohedral LaCuO3 thin films has not been measured directly because it is difficult to do so. However, considering the relationship between the amount of oxygen defects and the symmetry of bulk LaCuO3−δ, these rhombohedral LaCuO3 thin films are considered to be very close to the stoichiometric composition. The temperature dependence of the resistance of the thin films indicates that rhombohedral LaCuO3 is an insulator. Considering the previous studies on the first-principles calculation of stoichiometric LaCuO3 and this study, the rhombohedral LaCuO3 close to the stoichiometric composition is presumed to be a Mott insulator.
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