Structural modulation and physical properties of cobalt-doped layered La₂M₅As₃O₂ (M= Cu, Ni) compounds

We investigate the structural variation and physical properties of layered La₂M₅As₃O₂ (M=Cu, Ni) compound upon Co doping. It is found that the substitution of Co ion just induces the monotonous change of lattice constants without observing the anomalous kink in superconducting La₂(Cu_1-xNi_x)₅As₃O₂ solid-solutions. Meanwhile, this doping barely changes As-As bond length in M₅As₃^2- subunit (±2%), being significantly smaller than 7% shrinkage of that in La₂(Cu_1-xNi_x)₅As₃O₂. Therefore, the doping dependence of crystal structure exhibits similar trend with Ba_1-xK_xFe₂As₂ without the interference of As1-As2 bonding, implying that the Co substitution for Cu/Ni is hole-doped. In terms of physical property, La₂(Cu_1-xCo_x)₅As₃O₂ turns into itinerant ferromagnetic metal, while La₂(Ni_1-xCo_x)₅As₃O₂ shows paramagnetism and suppressed structural phase transition upon Co-doping. The distinct structural variation and absence of superconductivity provide important clues to understand the effect of As-As bond in M₅As₃^2- subunit.