We have performed an isovalent substitution study in a layered titanium oxypnictide system BaTi${}_2$(Sb${}_{1-x}$Bi${}_x$)${}_2$O ($0⩽x⩽0.40$) by measurements of x-ray diffraction, electrical resistivity, and magnetic susceptibility. The parent compound BaTi${}_2$Sb${}_2$O 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 Ti${}_2$Sb${}_2$O 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 $\sim$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.

• Received 10 November 2012

DOI:https://doi.org/10.1103/PhysRevB.87.100502