Composition dependent polymorphism and superconductivity in Y₃₊{Rh,Ir}₄Ge_13-

Polymorphism is observed in the Y₃₊Rh₄Ge_13- series. The decrease of Y-content leads to the transformation of the primitive cubic Y_3.6Rh₄Ge_12.4 [x = 0.6, space group Pm3̄n, a = 8.96095(9) Å], revealing a strongly disordered structure of the Yb₃Rh₄Sn₁₃ Remeika prototype, into a body-centred cubic structure [La₃Rh₄Sn₁₃ structure type, space group I4₁32, a = 17.90876(6) Å] for x = 0.4 and further into a tetragonal arrangement (Lu₃Ir₄Ge₁₃ structure type, space group I4₁/amd, a = 17.86453(4) Å, a = 17.91076(6) Å) for the stoichiometric (i.e. x = 0) Y₃Rh₄Ge₁₃. Analogous symmetry lowering is found within the Y₃₊Ir₄Ge_13- series, where the compound with Y-content x = 0.6 is crystallizing with La₃Rh₄Sn₁₃ structure type [a = 17.90833(8) Å] and the stoichiometric Y₃Ir₄Ge₁₃ is isostructural with the Rh-analogue [a = 17.89411(9) Å, a = 17.9353(1) Å]. The structural relationships of these derivatives of the Remeika prototype are discussed. Compounds from the Y₃₊Rh₄Ge_13- 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.