Composition dependent polymorphism and superconductivity in Y3+x{Rh,Ir}(4)Ge13-x

Polymorphism is observed in the Y3+xRh4Ge13-x series. The decrease of Y-content leads to the transformation of the primitive cubic Y3.6Rh4Ge12.4 [x = 0.6, space group Pm3n, a = 8.96095(9) angstrom], revealing a strongly disordered structure of the Yb3Rh4Sn13 Remeika prototype, into a body-centred cubic structure [La3Rh4Sn13 structure type, space group I4(1)32, a = 17.90876(6) angstrom] for x = 0.4 and further into a tetragonal arrangement (Lu3Ir4Ge13 structure type, space group I4(1)/amd, a = 17.86453(4) angstrom, a = 17.91076(6) angstrom) for the stoichiometric (i.e. x = 0) Y3Rh4Ge13. Analogous symmetry lowering is found within the Y3+xIr4Ge13-x series, where the compound with Y-content x = 0.6 is crystallizing with La3Rh4Sn13 structure type [a = 17.90833(8) angstrom] and the stoichiometric Y3Ir4Ge13 is isostructural with the Rh-analogue [a = 17.89411(9) angstrom, a = 17.9353(1) angstrom]. The structural relationships of these derivatives of the Remeika prototype are discussed. Compounds from the Y3+xRh4Ge13-x series are found to be weakly-coupled BCS-like superconductors with T-c = 1.25, 0.43 and 0.6, for x = 0.6, 0.4 and 0, respectively. They also reveal low thermal conductivity (<1.5 W K-1 m(-1) in the temperature range 1.8-350 K) and small Seebeck coefficients. The latter are common for metallic systems. Y3Rh4Ge13 undergoes a first-order phase transition at T-f = 177 K, with signatures compatible to a charge density wave scenario. The electronic structure calculations confirm the instability of the idealized Yb3Rh4Sn13-like structural arrangements for Y3Rh4Ge13 and Y3Ir4Ge13.