Atomic-scale observation of geometric frustration in a fluorine-intercalated infinite layer nickelate superlattice

TL;DR

This study uses advanced microscopy to observe how fluorine intercalates in a nickelate superlattice, causing atomic-level structural distortions and domain formations, which enhances understanding of precise anion doping in complex materials.

Contribution

It provides the first atomic-scale visualization of fluorine intercalation effects in a nickelate superlattice, revealing structural distortions and domain formations due to anion doping.

Findings

Fluorine intercalates mainly at apical sites, with some basal occupation.

Structural distortion leads to two distinct domains within the nickelate layer.

Disordered fluorine distribution causes diverse local structural distortions.

Abstract

Anion doping offers immense potential for tailoring material properties, but achieving precise control over anion incorporation remains a challenge due to complex synthesis processes and limitations in local dopant detection. Here, we investigate the F-ion intercalation within an infinite layer NdNiO2+x/SrTiO3 superlattice film using a two-step synthesis approach. We employ advanced four-dimensional scanning transmission electron microscopy (4D-STEM) coupled with electron energy loss spectroscopy to map the F distribution and its impact on the atomic and electronic structure. Our observations reveal a striking geometric reconstruction of the infinite layer structure upon fluorination, resulting in a more distorted orthorhombic phase compared to the pristine perovskite. Notably, F-ion intercalation occurs primarily at the apical sites of the polyhedron, with some occupation of basal sites in localized regions. This process leads to the formation of two distinct domains within the nickelate layer, reflecting a competition between polyhedral distortion and geometric frustration-induced neodymium (Nd) displacement near domain interfaces. Interestingly, we observe an anomalous structural distortion where basal site anions are displaced in the same direction as Nd atoms, potentially linked to the partial basal site F-ion occupation. This coexistence of diverse structural distortions signifies a locally disordered F-ion distribution within the infinite layer structure with distinct F-ion configurations. These findings provide crucial insights into understanding and manipulating anion doping at the atomic level, paving the way for the development of novel materials with precisely controlled functionalities.