The effects of Co substitution on Ba(Ni1-xCox)2As2 (0 ≤ x ≤ 0.251) single crystals grown out of Pb flux are investigated via transport, magnetic, and thermodynamic measurements. BaNi2As2 exhibits a first-order tetragonal to triclinic structural phase transition at Ts = 137 K upon cooling, and enters a superconducting phase below Tc = 0.7 K. The structural phase transition is sensitive to cobalt content and is suppressed completely by x ≥ 0.133. The superconducting critical temperature, Tc, increases continuously with x, reaching a maximum of Tc = 2.3 K at x = 0.083 and then decreases monotonically until superconductivity is no longer observable well into the tetragonal phase. In contrast to similar BaNi2As2 substitutional studies, which show an abrupt change in Tc at the triclinic-tetragonal boundary that extends far into the tetragonal phase, Ba(Ni1-xCox)2As2 exhibits a domelike phase diagram centered around the zero-temperature tetragonal-triclinic boundary. Together with an anomalously large heat capacity jump ΔCe/γT ∼ 2.2 near optimal doping, the smooth evolution of Tc in the Ba(Ni1-xCox)2As2 system suggests a mechanism for pairing enhancement other than phonon softening
