Single-Layer T'; Nickelates: Synthesis of the La and Pr Members and Electronic Properties across the Rare-Earth Series

Understanding high-temperature unconventional superconductivity has become a long-lasting problem in which the cuprates stand as central reference materials. Given this impasse, the recent discovery of superconductivity in analogous nickelate thin films represents a fundamental breakthrough calling for the identification of additional materials in this class. In particular, thermodynamically more robust systems are required to "upgrade " nickelate superconductors from thin films to bulk samples. Here, we contribute in this direction by reporting the synthesis of the new single-layer T' Pr2NiO3F compound, assessing this synthesis in relation to the only previous T' nickelate La2NiO3F, and analyzing the electronic properties across the R2NiO3F series (R = La-Lu) via first-principles calculations. We find that these mixed anion systems have a comparatively high degree of stability and their synthesis enables a fine-tuning of their composition as inferred from their characterization. Furthermore, we find that these unprecedented square-planar nickelates hold great promise as prospective superconductors due to their exceptional electronic structure.