  We have performed an isovalent substitution study in a layered titanium
oxypnictide system BaTi$_{2}$(Sb$_{1-x}$Bi$_{x}$)$_{2}$O (0$\leq x\leq$ 0.40)
by the 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 negative
chemical pressure and lattice distortion on the (super)conducting
Ti$_2$Sb$_2$O-layers. The 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 additional Bi doping. A metal-to-nonmetal transition takes place
around $x$=0.3, and superconductivity at $\sim$1K exists at the nonmetal side.
The CDW/SDW anomaly, in comparison, is rapidly suppressed by the Bi doping, and
vanishes for $x\geq$0.17. The results are discussed in terms of negative
chemical pressure and disorder effect.
