On the Topotactic Phase Transition Achieving Superconducting Infinite-Layer Nickelates

Topotactic reduction is critical to a wealth of phase transitions of current interest, including synthesis of the superconducting nickelate Nd_0.8Sr_0.2NiO₂, reduced from the initial Nd_0.8Sr_0.2NiO₃/SrTiO₃ heterostructure. Due to the highly sensitive and often damaging nature of the topotactic reduction, however, only a handful of research groups have been able to reproduce the superconductivity results. A series of in situ synchrotron-based investigations reveal that this is due to the necessary formation of an initial, ultrathin layer at the Nd_0.8Sr_0.2NiO₃ surface that helps to mediate the introduction of hydrogen into the film such that apical oxygens are first removed from the Nd_0.8Sr_0.2NiO₃ / SrTiO₃ (001) interface and delivered into the reducing environment. This allows the square-planar / perovskite interface to stabilize and propagate from the bottom to the top of the film without the formation of interphase defects. Importantly, neither geometric rotations in the square planar structure nor significant incorporation of hydrogen within the films is detected, obviating its need for superconductivity. These findings unveil the structural basis underlying the transformation pathway and provide important guidance on achieving the superconducting phase in reduced nickelate systems.