Microstructural Effects of Topotactic Reduction in Nickelate Single Crystals

Rare-earth nickel oxides, known for their intricate interplay between structure and properties, serve as a pivotal foundation for the exploration of novel quantum phases and advanced applications. Recent topotactic transformations of perovskite nickelates have enabled precise control over oxygen vacancies, thereby harnessing the consequential coupling effects in these materials. Gaining in-depth insights into the atomic-scale lattice and electrical structure during topotactic reduction is imperative for unraveling the potential of these phenomena. In this study, we employ atomic-resolution scanning transmission electron microscopy (STEM) imaging and electron energy-loss spectroscopy (EELS) to examine two distinct nickelate single crystals variants, namely <i>R</i>_1-xCa_xNiO_3-δ (where <i>R</i> represents either La or Pr). These single crystals are synthesized through topotactic reduction of the perovskite phase, employing CaH₂ as a reducing agent. The presentation will provide a comprehensive analysis of the oxygen vacancies, hole doping into the material, and the influence of cation structure.<br/><br/>We primarily focus on Pr_1-xCa_xNiO_3-δ single crystals, revealing an oxygen-deficient phase with δ ~ 0.25 occurring during topotactic reduction. A novel arrangement of oxygen vacancies within the brownmillerite structure, diverging from previously observed in reduced rare-earth nickelates. Precise quantification of polyhedral tilting and bond angles shows that a significant amount of internal strain drives wave-like variations in polyhedral tilting and rotations to accommodate the local lattice structure [1]. Subsequently, we studied La_1-xCa_xNiO_3-δ single crystals subjected to topotactic reduction, resulting in the formation of an infinite-layer phase with a composition of δ ~ 1 [2]. Additionally, we unraveled the microstructural effects of topotactic reduction on the undoped LaNiO₂ single crystals [3]. Our attention is devoted to a precise examination of the detailed lattice and chemical structures of these crystals, where our measurements of Ni–O bonding conditions and electronic structures affirm the existence of the infinite-layer structure following the reduction process. The removal of apical oxygen atoms forming NiO₂ planes within the infinite-layer phase results in metallic behavior reminiscent of weakly doped thin films. These discoveries establish a critical connection between the observable characteristics of nickelates and their underlying microscopic origins, paving the way for further exploration of nickelates with distinct crystal structures achievable only through topotactic reduction. [4]<br/><br/><b>References:</b><br/>[1] Yu-Mi Wu <i>et al</i>., “Topotactically induced oxygen vacancy order in nickelate single crystals” <i>Physical Review Materials </i><b>7</b>, 053609 (2023).<br/>[2] Pascal Puphal <i>et al</i>., "Topotactic transformation of single crystals: From perovskite to infinite-layer nickelates." <i>Science Advances</i> <b>7</b>, eabl8091 (2021).<br/>[3] Yu-Mi Wu <i>et al</i>., <i>unpublished</i>.<br/>[4] This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 823717 – ESTEEM3.