Hidden structural transition in epitaxial Ca0.5Sr0.5IrO3/SrTiO3 thin film

Abstract

A structural transition in an ABO(3) perovskite thin film involving the change of the BO6 octahedral rotation pattern can be hidden under the global lattice symmetry imposed by the substrate and often easily overlooked. We carried out high-resolution x-ray diffraction experiments to investigate the structures of epitaxial Ca0.5Sr0.5IrO3 (CSIO) perovskite iridate films grown on SrTiO3 (STO) and GdScO3 (GSO) substrates in detail. Although the CSIO/STO film layer displays a global tetragonal lattice symmetry evidenced by the reciprocal space mapping, synchrotron x-ray data indicate that its room-temperature structure is monoclinic due to Glazer's a(+)a(-)c(-) -type rotation of the IrO6 octahedra. In order to accommodate the lower-symmetry structure under the global tetragonal symmetry, the film breaks into four twinned domains, resulting in the splitting of the (half-integer, 0, integer) superlattice reflections. Surprisingly, the splitting of these superlattice reflections decrease with increasing temperature, eventually disappearing at T-S = 510(5) K, which signals a structural transition to an orthorhombic phase with a(+)a(-)c(0) octahedral rotation. In contrast, the CSIO/GSO film displays a stable monoclinic symmetry with a(+)b(-)c(-) octahedral rotation, showing no structural instability caused by the substrate up to 520 K. Our study illustrates the importance of the symmetry in addition to the lattice mismatch of the substrate in determining the structure of epitaxial thin films. ©2019 American Physical Society