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Polar phase transitions in heteroepitaxial stabilized La0.5Y0.5AlO3 thin films

We report on the fabrication of epitaxial La0.5Y0.5AlO3 ultrathin films on (001) LaAlO3 substrates. Structural characterizations by scanning transmission electron microscopy and x-ray diffraction confirm the high quality of the film with a−b+c− AlO6 octahedral tilt pattern. Unlike either of the nonpolar parent compound, LaAlO3 and YAlO3, second harmonic generation measurements on the thin films suggest a nonpolar–polar phase transition at Tc near 500 K, and a polar–polar phase transition at Ta near 160 K. By fitting the angular dependence of the second harmonic intensities, we further propose that the two polar structures can be assigned to the Pmc21 and Pmn21 space group, while the high temperature nonpolar structure belongs to the Pbnm space group.