Superconductivity in the Layered Cage Compound Ba_3Rh_4Ge_16

We report the synthesis and superconducting properties of a layered cage compound Ba_3Rh_4Ge_16. Similar to Ba_3Ir_4Ge_16, 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 μSR measurements unveiled moderately coupled s-wave superconductivity with a critical temperature T_\rm c=7.0 K, the upper critical field \mu_0H_\rm c2(0) \sim 2.5 T, the electron-phonon coupling strength \lambda_\rm e-ph \sim 0.80, and the Ginzburg–Landau parameter \kappa \sim 7.89. The mass reduction with the substitution of Ir by Rh is believed to be responsible for the enhancement of T_\rm c and coupling between the cage and guest atoms. Our results highlight the importance of atomic weight of framework in cage compounds in controlling the \lambda_\rm e-ph strength and T_\rm c.