Tailoring polarity in a layered nickelate with single atomic layer control

Many of the 3d transition metal oxides share a common structural MO$_{\mathrm{6}}$ building unit---a central transition metal (TM) cation octahedrally coordinated with oxygen nearest neighbors. The electronic states in these materials can be modified by tailoring the M-O bonds, which typically include the application of epitaxial strain in thin films, or pressure and isovalent cation substitution in bulk samples. Here, we present a new route to tailor the M-O bonds without changes to the strain state or stoichiometry in two-dimensional perovskite nickelate (n$=$1 in the Ruddlesden Popper series). We do this by tailoring the dipolar electrostatic interactions at the unit cell level in nominally non-polar LaSrNiO$_{\mathrm{4}}$ via single atomic layer-by-layer synthesis using oxide-MBE. We reconstruct the response of the crystal lattice to the induced polarity using a x-ray phase retrieval technique (COBRA). We find that the response of the O anions to the resulting local electric fields distorts the M-O bonds, being largest for the apical oxygens (O$_{\mathrm{ap}})$. It also alters the Ni valence.