  The physical properties of the previously reported superconductor IrGe and
the Rh1-xIrxGe solid solution are investigated. IrGe has an exceptionally high
superconducting transition temperature (Tc = 4.7 K) among the isostructural 1:1
late-metal germanides MGe (M = Rh, Pd, Ir and Pt). Specific-heat measurements
reveal that IrGe has an anomalously low Debye temperature, originating from a
low-lying phonon, compared to the other MGe phases. A large jump at Tc in the
specific-heat data clearly indicates that IrGe is a strong coupling
superconductor. In the Rh1-xIrxGe solid solution, a relationship between an
anomalous change in lattice constants and the Debye temperature is observed. We
conclude that the unusually high Tc for IrGe is likely due to strong
electron-phonon coupling derived from the presence of a low-lying phonon.
