Polar phase transitions in heteroepitaxial stabilized La_0.5Y_0.5AlO₃ thin films

We report on the fabrication of epitaxial La_0.5Y_0.5AlO₃ 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 T _c near 500 K, and a polar-polar phase transition at T _a 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 Pmc2 ₁ and Pmn2 ₁ space group, while the high temperature nonpolar structure belongs to the Pbnm space group.