The σ-phase superconductors Nb20.4Rh5.7Ge3.9 and Nb20.4Rh5.7Si3.9

We show that the previously unreported ternary sigma phases Nb20.4Rh5.7Ge3.9 and Nb20.4Rh5.7Si3.9 are both superconductors with T-c values of approximately 1.9 K. The superconducting transitions were characterized through temperature-dependent magnetic susceptibility, electrical resistance, and specific heat measurements. The Sommerfeld constants, gamma, for Nb20.4Rh5.7Ge3.9 and Nb20.4Rh5.7Si3.9 are 89(1) mJ mol-f.u.(-1) K-2 and 86(1) mJ mol-f.u.(-1) K-2 (about 3.0 and 2.9 mJ mol-atom(-1) K-2), respectively. The normalized specific heat jumps at T-c, Delta C/gamma T-c, are approximately 1.39 for Nb20.4Rh5.7Ge3.9 and Nb20.4Rh5.7Si3.9, consistent with both materials being weakly coupled BCS type superconductors.