  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 analogs 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. By combining x-ray absorption
spectroscopy and density functional theory calculations, we further find that
metallic Pr4Ni3O8 exhibits a low-spin configuration and significant orbital
polarization of the unoccupied eg states with pronounced dx2-y2 character near
the Fermi energy, both hallmarks of the cuprate superconductors. Belonging to a
regime of 3d electron count found for hole-doped cuprates, Pr4Ni3O8 thus
represents one of the closest analogies to cuprates yet reported and a
singularly promising candidate for high-Tc superconductivity if appropriately
doped.
