On the Topotactic Phase Transition Achieving Superconducting Infinite-Layer Nickelates.

Area of Science:

• Materials Science
• Solid-State Chemistry
• Superconductivity

Background:

• Topotactic reduction is essential for synthesizing superconducting nickelates, such as Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$, from precursor heterostructures like Nd$_{0.8}$Sr$_{0.2}$NiO$_{3}$/SrTiO$_{3}$.
• Reproducing superconductivity in these materials is challenging due to the sensitivity and potential damage caused by the reduction process.

Purpose of the Study:

• To elucidate the structural mechanisms governing the topotactic reduction pathway in nickelate heterostructures.
• To identify the key factors enabling the successful synthesis of superconducting nickelates and guide future research.

Main Methods:

• In situ synchrotron-based investigations were employed to study the reduction process.
• Analysis focused on the interface between Nd$_{0.8}$Sr$_{0.2}$NiO$_{3}$ and SrTiO$_{3}$ during reduction.

Main Results:

• An ultrathin surface layer on Nd$_{0.8}$Sr$_{0.2}$NiO$_{3}$ mediates hydrogen introduction and apical oxygen removal at the interface.
• This mediation allows for the stabilization and propagation of the square-planar/perovskite interface without defects.
• No significant geometric rotations or hydrogen incorporation within the films were observed.

Conclusions:

• The formation of an intermediate surface layer is critical for defect-free topotactic reduction in nickelates.
• Understanding this pathway is key to reliably achieving superconductivity in reduced nickelate systems.