Chemical Doping to Ni-Sites in Layered Nickelate Pr4ni3o8 For High-Tc Superconductor Candidate

Layered nickelate R4Ni3O8 (R = La, Pr, Nd, Sm) is a candidate for a high-Tc superconductor because its crystal structure and electronic state are very similar to high-Tc cuprates. Since the recent discovery of superconductivity in infinite-layer nickelate R1-xSrxNiO2 thin films, superconductivity on the NiO2 planes is also expected in R4Ni3O8.Our recent works have pointed out that R4Ni3O8 shows metallic behavior by intercalation-deintercalation treatment with sulfur due to removing additional apical oxygen that impedes metallic conduction. The metallic behavior shows up in the R = Pr system (Pr4Ni3O8) down to low temperature (2 K), suggesting sufficient removal of additional oxygen.In this study, we performed carrier tuning by chemical substitution to Ni sites to bring the electronic states of Ni closer to that of Cu in high-Tc cuprates. We have doped with Co and Cu for the Ni in Pr4Ni3O8. Systematic expansion of the in-plane lattice constant indicates the electron doping to Ni ion, confirmed by the bond valence sum calculation. The Madelung energy calculation suggests that the dopants selectively occupy the specific Ni sites, which is supported by the electrical resistivity measurements showing metallic conduction and no resistivity upturn down to 10 K. However, superconductivity is not observed down to 2 K, suggesting that further carrier tuning is required to optimize the electronic state of Ni to derive the superconductivity.