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(Cu1–xNix)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(Cu1–xNix)5As3O2. Therefore, the doping dependence of crystal structure exhibits similar trend with Ba1–xKxFe2As2 without the interference of As1–As2 bonding, implying that the Co substitution for Cu/Ni is hole-doped. In terms of physical property, La2(Cu1–xCox)5As3O2 turns into itinerant ferromagnetic metal, while La2(Ni1–xCox)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. [ABSTRACT FROM AUTHOR]