Superconductivity, charge- or spin-density wave, and metal-nonmetal transition in BaTi2(Sb1-xBix)2O

We have performed an isovalent substitution study in a layered titanium oxypnictide system BaTi2(Sb1-xBix)(2)O (0 <= x <= 0.40) by measurements of x-ray diffraction, electrical resistivity, and magnetic susceptibility. The parent compound BaTi2Sb2O is confirmed to exhibit superconductivity at 1.5 K as well as charge-or spin-density wave (CDW/SDW) ordering below 55 K. With the partial substitution of Sb by Bi, the lattice parameters a, c, and c/a all increase monotonically, indicating a negative chemical pressure and lattice distortion for the (super) conducting Ti2Sb2O layers. Bi doping elevates the superconducting transition temperature to its maximum T-c = 3.7 K at x = 0.17, and then T-c decreases gradually with further Bi doping. A metal-to-nonmetal transition takes place around x = 0.3, and superconductivity at similar to 1 K survives on the nonmetal side. The CDW/SDW anomaly, in comparison, is rapidly suppressed by Bi doping, and vanishes for x >= 0.17. The results are discussed in terms of negative chemical pressure and the disorder effect.