Large orbital polarization in a metallic square-planar nickelate

High-temperature cuprate superconductivity remains a defining problem in condensed-matter physics. Among myriad approaches to addressing this problem has been the study of alternative transition metal oxides with similar structures and 3d electron count that are suggested as proxies for cuprate physics. None of these analogues has been superconducting, and few are even metallic. Here, we report that the low-valent, quasi-two-dimensional trilayer compound Pr4Ni3O8 avoids a charge-stripe-ordered phase previously reported for La4Ni3O8, leading to a metallic ground state. X-ray absorption spectroscopy shows that metallic Pr4Ni3O8 exhibits a low-spin configuration with significant orbital polarization and pronounced d(x2-y2) character in the unoccupied states above the Fermi energy, a hallmark of the cuprate superconductors. Density functional theory calculations corroborate this finding, and reveal that the d(x2-y2) orbital dominates the near-E-f occupied states as well. Belonging to a regime of 3d electron count found for hole-doped cuprates, Pr4Ni3O8 thus represents one of the closest analogues to cuprates yet reported and a singularly promising candidate for high-T-c superconductivity if electron doping could be achieved.