  We report the synthesis and superconducting properties of a layered cage
compound Ba3Rh4Ge16. Similar to Ba3Ir4Ge16, the compound is composed of 2D
networks of cage units, formed by noncubic Rh-Ge building blocks, in marked
contrast to the reported rattling compounds. The electrical resistivity,
magnetization, specific heat capacity, and muSR measurements unveiled
moderately coupled s-wave superconductivity with a critical temperature Tc =
7.0 K, the upper critical field ~ 2.5 T, the electron-phonon coupling strength
~ 0.80, and the Ginzburg-Landau parameter ~ 7.89. The mass reduction by the
substitution of Ir by Rh is believed to be responsible for the enhancement of
Tc and coupling between the cage and guest atoms. Our results highlight the
importance of the atomic weight of the framework in cage compounds in
controlling the electron-phonon coupling strength and Tc.
