Decomposition of La_2−xSr_xCuO₄ into several La₂O₃ phases at elevated temperatures in ultrahigh vacuum inside a transmission electron microscope

We report the decomposition of La_2-xSr_xCuO₄ into La₂O₃ and Cu nanoparticles in ultrahigh vacuum, observed by in situ heating experiments in a transmission electron microscope. The analysis of electron diffraction data reveals that the phase decomposition process starts at about 150 °C and is considerably expedited in the temperature range of 350 °C-450 °C. Two major resultant solid phases are identified as metallic Cu and La₂O₃ by electron diffraction, simulation, and electron energy-loss spectroscopy (EELS) analyses. With the aid of calculations, La₂O₃ phases are further identified to be derivatives of a fluorite structure - fluorite, pyrochlore, and (distorted) bixbyite - characterized by different oxygen-vacancy order. Additionally, the bulk plasmon energy and the fine structures of the O K and LaM₄,5 EELS edges are reported for these structures, along with simulated O K x-ray absorption near-edge structure. The resultant Cu nanoparticles and La₂O₃ phases remain unchanged after cooling to room temperature.