Hidden structural transition in epitaxial Ca$_{0.5}$Sr$_{0.5}$IrO$_{3}$/SrTiO$_{3}$ thin film

TL;DR

This study reveals a hidden temperature-dependent structural transition in epitaxial Ca$_{0.5}$Sr$_{0.5}$IrO$_{3}$ thin films, showing how substrate symmetry influences octahedral rotations and phase stability.

Contribution

It uncovers a temperature-induced transition from monoclinic to orthorhombic symmetry in Ca$_{0.5}$Sr$_{0.5}$IrO$_{3}$ films on SrTiO$_{3}$, highlighting the role of substrate symmetry in thin film structures.

Findings

The CSIO/STO film exhibits a monoclinic structure at room temperature.

A structural transition occurs at T$_{S}$ = 510 K to an orthorhombic phase.

The CSIO/GSO film remains monoclinic up to 520 K.

Abstract

A structural transition in an ABO$_{3}$ perovskite thin film involving the change of the BO$_{6}$ 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 Ca$_{0.5}$Sr$_{0.5}$IrO$_{3}$ (CSIO) perovskite iridate films grown on the SrTiO$_{3}$ (STO) and GdScO$_{3}$ (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 indicates that its room temperature structure is monoclinic due to Glazer's a$^{+}$a$^{-}$c$^{-}$-type rotation of the IrO$_{6}$ 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.