The structural effects of (111) growth of La$_2$CoMnO$_6$ on SrTiO$_3$ and LSAT -- new insights from 3D crystallographic characterisation with 4D-STEM and Digital Dark Field imaging

The 3-dimensional orientation of La atom modulations has been mapped in two thin films of La$_2$CoMnO$_6$ grown on SrTiO$_3$ and LSAT ([La,Sr,Al,Ta] oxide) using a 4D-scanning transmission electron microscopy (4D-STEM) method based on the recently developed Digital Dark Field method. This images the shifts of diffraction spots and the azimuthal intensity distribution in the First Order Laue Zone, and then uses them to reconstruct and map the 3D crystallography. This clearly shows a flip from out-of-plane modulation with tensile strain on SrTiO3 to in-plane modulation with compressive strain on LSAT. This hitherto unobserved crystallographic change had a significant influence on the out-of-plane lattice parameter which left more room for the full incorporation of the larger CoO6 octahedra on LSAT and therefore explained the improved Mn-Co ordering and better properties for this film. Moreover, the method would be applicable to many other systems of epitaxial growth of complex oxides, revealing crystallographic details of crucial importance to properties which are not visible in conventional atomic resolution imaging.