Physical properties and superconductivity of skutterudite-related Yb3Co4.3Sn12.7 and Yb3Co4Ge13

Rietveld analysis was performed for the intermetallics Yb3Co4.3Sn12.7 and Y b3Co4Ge13 crystallizing with the closely related structure types, Yb3Rh4Sn1 3 and Yb3Co4Ge13. Below T-c = 3.4 K Yb3Co4.3Sn12.7 crosses over into a type -II superconducting ground state with H-c2(0) similar to 2.5 T. Yb3Co4Ge13 stays in the normal state down to 300 mK. The gamma value of 2.3(2) mJ gat( -1) K-2 and the Debye temperature Theta (D) = 207(5) K deduced from the spe cific heat as well as T, correspond to that of elementary Sn, thus indicati ng conventional BCS superconductivity. Hydrostatic pressure applied to Yb3C o4.3Sn12.7 reveals both an overall decrease of the absolute resistivity val ues, as well as a decrease of T, which vanishes for a critical pressure bel ow 10 kbar. The magnetoresistance of both Yb-based compounds is positive at low temperature but does not exceed 8% in fields of 12 T. The Seebeck coef ficient has a maximum value of about 18 muV K-1 at T similar to 250 K. L-II I and magnetic susceptibility measurements reveal intermediate valence: 2.6 6(3) and 2.18(3) for Yb3Co4Ge13 and Yb3Co4.Sn-3(12.7), respectively.