Atomic-Scale Characterization of the High-Pressure Superconductor La₃Ni₂O₇ and Topotactically Reduced LaNiO₂ Single Crystals

Rare-earth nickel oxides, known for their complex interplay between structure and properties, serve as a pivotal base for novel quantum phases and advanced applications. Recent topotactic transformations of perovskite nickelates have allowed precise control of oxygen vacancies, leading to the discovery of superconductivity in thin films of the infinite-layer (IL) nickelate Nd_0.8Sr_0.2NiO₂. [1] To unravel the potential of these phenomena, it is crucial to gain in-depth insights into the atomic-scale lattice and electronic structure during topotactic reduction. [2] Recently, the discovery of high-temperature superconductivity in La₃Ni₂O₇ at high pressures (&gt;14 GPa) has stimulated considerable research efforts. [3] However, the fundamental properties of the superconducting phase are currently the subject of controversial debates, including the interpretation of the possible filamentary character, whereas early investigations consistently postulated a crystal structure consisting of NiO₆ octahedral bilayers stacked along the c-axis on La₃Ni2O₇. In this talk, I will discuss our atomic-scale investigations that link the observed properties of IL and Ruddlesden-Popper nickelates to their underlying microscopic origins.<br/>To investigate the atomic-scale properties of two different infinite-layer nickelate single crystal variants, <i>i.e.</i>, Pr_1-xCa_xNiO_3-δ and undoped LaNiO₂, synthesized by topotactic reduction of the perovskite phase, and to reveal the unconventional structure of optically floating zone-grown high-pressure superconducting La₃Ni₂O₇ single crystals, we employed high-resolution scanning transmission electron microscopy (STEM) imaging and electron energy-loss spectroscopy (EELS). We first studied Pr_1-xCa_xNiO_3-δ crystals, revealing an oxygen-deficient phase with δ ~ 0.25 during topotactic reduction. The novel arrangement of oxygen vacancies within the brownmillerite-like structure differs from previously observed reduced rare-earth nickelates. [4] Next, we investigated the microstructural effects of topotactic reduction on the undoped LaNiO₂ single crystals and showed that the reduction process leads to different types of structural deformations. [5] More recently, we focused on the structural and electronic properties of high-pressure superconducting La₃Ni₂O₇ single crystals using high-resolution STEM imaging and STEM-EELS. Although we observed multiple crystallographic phases in the La₃Ni₂O₇ crystals, the main matrix is dominated by alternating monolayers and trilayers of NiO6 octahedra [6,7] demonstrating a profound deviation from the previously proposed bilayer structures.<br/><br/><b>References</b><br/>[1] D. Li <i>et al.</i>, <i>Nature</i> <b>572</b>, 7771 (2019).<br/>[2] P. Puphal <i>et al.</i>, <i>Science Advances</i> <b>7</b>, eabl8091 (2021).<br/>[3] H. Sun <i>et al.</i>, <i>Nature</i> <b>621</b>, 7979 (2023).<br/>[4] Y.-M. Wu <i>et al.</i>, <i>Phys. </i><i>Rev. Mater</i>. <b>7</b>, 053609 (2023).<br/>[5] Y.-M. Wu <i>et al.</i>, <i>unpublished</i>.<br/>[6] P. Puphal <i>et al.</i>, <i>arXiv</i>:2312.07341, (2023).<br/>[7] X. Chen <i>et al.</i>, <i>J. Am. </i><i>Chem. Soc</i>. <b>146</b>, 3640 (2024).