Topotactically induced oxygen vacancy order in nickelate single crystals

The strong structure-property coupling in rare-earth nickelates has spurred the realization of new quantum phases in rapid succession. Recently, topotactic transformations have provided a new platform for the controlled creation of oxygen vacancies and, therewith, for the exploitation of such coupling in nickelates. Here, we report the emergence of oxygen vacancy ordering in Pr0.92Ca0.08NiO2.75 single crystals obtained via a topotactic reduction of the perovskite phase Pr0.92Ca0.08NiO3, using CaH2 as the reducing agent. We unveil a brownmillerite-like ordering pattern of the vacancies by high-resolution scanning transmission electron microscopy, with Ni ions in alternating square-pyramidal and octahedral coordination along the pseudocubic [100] direction. Furthermore, we find that the crystal structure acquires a high level of internal strain, where wavelike modulations of polyhedral tilts and rotations accommodate the large distortions around the vacancy sites. Our results suggest that atomic-resolution electron microscopy is a powerful method to locally resolve unconventional crystal structures that result from the topotactic transformation of complex oxide materials.

Automated summary

The strong coupling between structure and properties in rare-earth nickelates has led to the discovery of new quantum phases. Topotactic transformations have recently provided a new method for creating and utilizing oxygen vacancies in nickelates. In this study, we observe the emergence of ordered oxygen vacancies in Pr0.92Ca0.08NiO2.75 single crystals obtained through topotactic reduction of Pr0.92Ca0.08NiO3 perovskite phase using CaH2 as the reducing agent. High-resolution scanning transmission electron microscopy reveals a brownmillerite-like ordering pattern of the vacancies, accompanied by a high level of internal strain in the crystal structure.