Effect of target nickel stoichiometry on the synthesis of infinite-layer nickelates

The discovery of superconductivity in infinite-layer nickelate films marks a groundbreaking addition to the family of unconventional superconductors, providing new insights into mechanism of unconventional high temperature superconductivity. However, synthesizing these superconducting nickelates presents significant challenges: they cannot be grown directly and instead require a two-step synthesis protocol involving initial deposition of a perovskite precursor phase (e.g., Nd0.8Sr0.2NiO3) followed by topotactic reduction to the infinite-layer structure (Nd0.8Sr0.2NiO2). This process is further complicated by the extreme sensitivity of both steps to synthesis conditions, necessitating stringent control over the crystallinity and stoichiometry of the parent phase. In this study, we uncover nickel deficiency during pulsed laser deposition (PLD) of the parent-phase Nd0.8Sr0.2NiO3. By incorporating 15% excess nickel into the PLD target, we mitigate this loss, suppress secondary phase formation in the Nd0.8Sr0.2NiO3 parent film, and ultimately obtain a phase-pure Nd0.8Sr0.2NiO2 film exhibiting superconductivity after following reduction. Notably, we observe a doping-dependent insulator-to-superconductor transition in films synthesized from targets with varying nickel content after reduction. X-ray photoelectron spectroscopy (XPS) confirms that the Nd/Ni ratio in films derived from nickel-over-doped targets (15% excess) aligns closely with the ideal stoichiometry. These findings underscore the indispensable role of stoichiometric precision in stabilizing infinite-layer nickelates and establish a practical synthesis strategy for optimizing their superconducting performance.