Neutron diffraction study of layered nickelates Pr₄Ni_3-xCo_xO₈ for high-temperature superconductor candidate

Layered nickelate R₄Ni₃O₈ (R = La, Pr, Nd, Sm) is a candidate for a high-temperature superconductor because its crystal structure and electronic state are very similar to high-temperature superconducting cuprates. We have performed carrier tuning by chemical substitution to Ni sites to bring the electronic states of Ni closer to that of Cu in superconducting cuprates. We aim to substitute the specific Ni sites for carrier tuning without disturbing the NiO₂ planes. In this study, we perform neutron powder diffraction experiments for the Pr₄Ni_3-xCo_xO₈ samples with doping Co for Ni sites to investigate the dopant distribution among the Ni sites. Rietveld analysis showed a tendency to preferentially occupy specific Ni sites when the doping level was low, but this is not the case for higher doping levels. We also analyzed the oxygen deficiency and found no oxygen deficiency in the NiO₂ planes with lower chemical substitutions, indicating that the relatively undisturbed conduction planes were maintained. Finally, we discuss the effects of Co doping into NiO₂ planes for the physical properties of R₄Ni₃O₈. (author)