  We have investigated the superconducting critical temperature Tc of solid
solution Nb5Ir3-xPtxO with the Ti5Ga4-type structure. The both end-members of
Nb5Ir3O and Nb5Pt3O are reported to be superconductors with Tc of 10.5 K and
3.8 K, respectively. Particularly Nb5Ir3O is considered as a two-gap
superconductor. The entire series of alloy hold the Ti5Ga4-type structure and
show the linear x dependence of lattice parameters. On the other hand, a
nonlinear Tc vs x plot was obtained, suggesting that Tc is not determined by
only lattice parameters. The experimental result is discussed based on the
Matthias rule and compared with those of the other two-gap superconductors such
as MgB2, Mo8Ga41 and Nb3Sn.
