Structural investigation of new series of nickel-based pnictide oxide superconductors (Ni2Pn2)(Sr4M2O6) (Pn = P, As; M = Sc, V)

The relationship between crystal structure and physical properties in (Ni2Pn2)(Sr4M2O6)[Pn = P, As; M = Sc, V] was systematically investigated. Rietveld analysis revealed that (Ni2Pn2)(Sr4Sc2O6) have longer a-axis and shorter c-axis than those of (Fe2Pn2)(Sr4Sc2O6), leading to more distorted anti-fluorite layers in (Ni2Pn2)(Sr4Sc2O6). While two dimensionality of crystal structure in these compounds does not contribute to enhancement of Tc, the bond angle of Pn-Ni-Pn is concluded to be a major factor to determine Tc of nickel pnictide superconductors. Structural refinement was also performed for a newly found (Ni2As2)(Sr4V2O6). Interestingly, this compound has exceptionally small As-Ni-As angle which is close to ideal tetrahedron among nickel pnictides, though it does not show superconductivity above 2 K.