Cation Ordering in Layered Nickelates

The single layer Ruddlesden-Popper nickelates present a model system to understand how the effects of digital dopant cation ordering may affect the properties of 2-dimensional conducting sheets. We investigate the effects of aliovalent A-site cation order on LaSrNiO$_{4}$ films. Using molecular beam epitaxy, we interleave full layers of SrO and LaO in a series of chemically equivalent films, varying the pattern of SrO and LaO layers relative to the NiO$_{2}$ layers. Through synchrotron surface x-ray diffraction and Coherant Bragg Rod Analysis (COBRA), we directly investigate the A-site cation order and the resulting atomic displacements for each ordering pattern. We correlate these results with theoretical calculations and transport measurements of the layered nickelate films.