Substitute effects of Ga on Tc in superconducting Zr2(Co1−xGax)

► A new pseudo-binary series Zr2(Co1−xGax)(0≦x≦0.3) has been synthesized. ► A prominent maximum in the unit cell volume v versus x curve appears around x=0.2. ► Experimental results show that there is an explicit maximum Tc close to x=0.05. ► The superconducting Tc in the series is not a monotonic function of a,c,c/a, or v. ► The Tc value in Zr2(Co1−xGax) may mainly relate to the spin density fluctuations.