We report a systematic study of the 5d-electron-doped system Ce(Fe 1−x Ir x ) 2 Al 10 (0 x 0.15). With increasing x, the orthorhombic b axis decreases slightly while accompanying changes in a and c leave the unit cell volume almost unchanged. Inelastic neutron scattering, along with thermal and transport measurements, reveal that for the Kondo semiconductor CeFe 2 Al 10 , the low-temperature energy gap, which is proposed to be a consequence of strong c-f hybridization, is suppressed by a small

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... mount of Ir substitution for Fe and that the system adopts a metallic ground state with an increase in the density of states at the Fermi level. The charge or transport gap collapses (at x = 0.04) faster than the spin gap with Ir substitution. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements demonstrate that the system undergoes long-range antiferromagnetic order below a Néel temperature T N of 3.1(2) K for x = 0.15. The ordered moment is estimated to be smaller than 0.07(1) μ B /Ce, although the trivalent state of Ce is confirmed by Ce L 3 -edge x-ray absorption near edge spectroscopy. It is suggested that the c-f hybridization gap, which plays an important role in the unusually high ordering temperatures observed in CeT 2 Al 10 (T = Ru and Os), may not be necessary for the onset of magnetic order with a low T N seen here in Ce(Fe 1−x Ir x ) 2 Al 10 .