Structural Basis for Topotactic Transformation toward Superconducting Infinite-Layer Nickelates

The meticulous experimental verification of the infinite-layer superconducting nickelates heralds a new chapter of superconductivity after a long time extensive pursuit in rare-earth nickelate compounds for achieving cuprate-like unconventional superconductors. Despite great attention to<br/>explore the new forefront, the thermodynamic fragility of the parent precursor phase and infinite-layer phase adopting an unfavorably low nickle valence has posed a formidable experimental challenge in the thin film synthesis and the subsequent chemical reduction to attain superconductivity in infinite-layer nickelate heterostructures. Therefore, significant effort to obtain the metastable nickelate precursor phase thin films (i.e. RE0.8Sr0.2NiO3, RE = La, Nd, Pr...) with minimized extended defects has been undertaken to circumvent the main obstacles due to the chemical instability of high Ni valence and the concomitant Ruddelsden-Popper or other competing phases, for instance optimizing the non-equilibrium deposition energetics and the selection of close-matched substrate for epitaxy. However, the succeeding chemical reduction process to attain the infinite-layer structure hosting superconductivity remains serendipitous in experimental practice, and the holistic prospect of individual key steps of the evolving non-equilibrium reduction process has not yet been clearly revealed. The decisive understanding of the intricate topotactic reduction remains elusive due to its non-equilibrium dynamics and the lacking of real-time structural insights during key transformation steps.<br/><br/>Here, in this talk, we will demonstrate our <i>in situ</i> synchrotron surface X-ray scattering study combined with element-specific spectroscopies to probe at individual key steps of the topotactic reduction of the prototypical epitaxial Nd0.8Sr0.2NiO3 thin films into Nd0.8Sr0.2NiO2. The reduction occurs through a low temperature reaction with CaH2. The relationships between lattice structure, reaction temperature, and time within the strongly reducing environment are discussed. Our experimental observations provide much needed structural and chemical insights into formation of the square-planar structure key to the development of superconductivity in nickelate heterostructures, including clarifying the actual transformation pathway apart from other possible scenario. In particular, we uncovered that the infinite-layer phase initiates at the heterointerface and propagates toward the film surface. Notably, a dynamic surface boundary layer is present introducing hydrogen to the infinite-layer phase while removing and transferring apical oxygen ions to the reducing environment. Moreover, the measurements sensitive to the hydrogen distribution elucidate its intervening role in the intermediate reduction step, and further discount any significant contribution of hydrogen to stabilize superconductivity in the completely converted infinite-layer phase with fully suppressed oxygen octahedral rotations. These results timely address the pressing question whether superconductivity can still be realized in these nickelate compounds without hydrogen. Our study unveils the structural basis underlying the transformation pathway and provides precise experimental guidance to improve the effective reduction for obtaining intrinsic superconductivity behaviors.