Octahedral rotation patterns in strained EuFeO_3 and other Pbnm perovskite films: Implications for hybrid improper ferroelectricity

摘要

We report the relationship between epitaxial strain and the crystallographic orientation of the in-phase rotation axis and A-site displacements in Pimm-type perovskite films. Synchrotron diffraction measurements of EuFeO_3 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 EuFeO_3 behavior with previously reported experimental and theoretical work on strained Pbnm-type films on nonorthorhombic substrates, as well as additional measurements from LaGaO_3, LaFeO_3, and Eu_(0.7)Sr_(0.3)MnO_3 films on SrTiO_3. 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, EuFeO_3 films grown on Pbnm-type GdScO_3 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. These 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.