Topotactic oxidation of Ruddlesden-Popper nickelates reveals new structural family: oxygen-intercalated layered perovskites

TL;DR

This study introduces a new class of oxygen-intercalated layered perovskites derived from Ruddlesden-Popper nickelates, revealing structural, electronic, and hybridization changes that could impact high-temperature superconductivity research.

Contribution

The paper demonstrates topotactic oxygen intercalation as a novel method to synthesize and characterize a new structural family of layered perovskites with enhanced properties.

Findings

Large c-axis expansion upon oxygen intercalation

Intercalation of ~0.7 oxygen atoms per formula unit

Enhanced nickel-oxygen hybridization linked to metallicity

Abstract

Layered perovskites such as the Dion-Jacobson, Ruddlesden-Popper, and Aurivillius families host a wide range of correlated electron phenomena, from high-temperature superconductivity to multiferroicity. Here we report a new family of layered perovskites, realized through topotactic oxygen intercalation of La_{n+1}Ni_{n}O_{3n+1} (n=1-4) Ruddlesden-Popper nickelate thin films grown by ozone-assisted molecular-beam epitaxy. Post-growth ozone annealing induces a large c-axis expansion - 17.8% for La_{2}NiO_{4} (n=1) - that monotonically decreases with increasing n. Surface X-ray diffraction coupled with Coherent Bragg Rod Analysis reveals that this structural expansion arises from the intercalation of approximately 0.7 oxygen atoms per formula unit into interstitial sites within the rock salt spacer layers. The resulting structures exhibit a spacer layer composition intermediate between that of the Ruddlesden-Popper and Aurivillius phases, defining a new class of layered perovskites. Oxygen-intercalated nickelates exhibit metallicity and significantly enhanced nickel-oxygen hybridization, a feature linked to high-temperature superconductivity. Our work establishes topotactic oxidation as a powerful synthetic approach to accessing highly oxidized, metastable phases across a broad range of layered oxide systems, offering new platforms to tune properties via spacer-layer chemistry.