Octahedral rotation patterns in strained EuFeO3 and other Pbnm perovskite films: Implications for hybrid improper ferroelectricity

Here, we report the relationship between epitaxial strain and the crystallographic orientation of the in-phase rotation axis and A-site displacements in Pbnm-type perovskite films. Synchrotron diffraction measurements of EuFeO₃ films under strain states ranging from 2% compressive to 0.9% tensile on cubic or rhombohedral substrates exhibit a combination of a^-a⁺c^- and a⁺a^-c^- rotational patterns. We compare the EuFeO₃ behavior with previously reported experimental and theoretical work on strained Pbnm-type films on nonorthorhombic substrates, as well as additional measurements from LaGaO₃, LaFeO₃, and Eu_0.7Sr_0.3MnO₃ films on SrTiO₃ . Compiling the results from various material systems reveals a general strain dependence in which compressive strain strongly favors a^-a⁺c^- and a⁺a^-c^- rotation patterns and tensile strain weakly favors a^-a^-c⁺ structures. In contrast, EuFeO₃ films grown on Pbnm-type GdScO₃ under 2.3% tensile strain take on a uniform a^-a⁺c^- rotation pattern imprinted from the substrate, despite strain considerations that favor the a^-a^-c⁺ pattern. Our results point to the use of substrate imprinting as a more robust route than strain for tuning the crystallographic orientations of the octahedral rotations and A-site displacements needed to realize rotation-induced hybrid improper ferroelectricity in oxide heterostructures.