Exploring the Natural Superlattices of High-pressure Superconductor La₃Ni₂O₇ and Nano-scale Effects of Topotactic Reduction in LaNiO₂ crystals

Rare-earth nickel oxides, known for their complex interplay between structure and properties, serve as a key platform for novel quantum phases and advanced applications. 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). In addition, the discovery of high-temperature superconductivity – with a superconducting transition of ~80 K – in La₃Ni₂O₇ at elevated pressures (>14 GPa) has stimulated extensive research efforts (2). The fundamental properties of the superconducting phase have been 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.

Recently, we reported the unconventional structure of optical floating zone-grown high-pressure superconducting La₃Ni₂O₇ single crystals (3) and the microstructural effects of topotactic reduction on the undoped LaNiO₃ single crystals (4). Focusing on high-resolution scanning transmission electron microscopy (STEM) imaging and electron energy-loss spectroscopy (EELS) we reveal the multiple crystallographic phases in the La₃Ni₂O₇ crystals, where the main phase is dominated by alternating monolayers and trilayers of NiO₆ octahedra (3,5) different from the La₃Ni₂O₇ systems composed of bilayer structures. Additionally, we demonstrate that the reduction process leads to different types of structural deformations in topotactically reduced LaNiO₃ crystals (4).

References:

1. D. Li et al., Nature. 572, 624–627 (2019).

2. H. Sun et al., Nature. 621, 493–498 (2023).

3. P. Puphal et al., Phys. Rev. Lett. 133, 146002 (2024).

4. Y.-M. Wu et al., APL Materials. 12, 091119 (2024).

5. X. Chen et al., J. Am. Chem. Soc. 146, 3640–3645 (2024).