Composition dependent polymorphism and superconductivity in Y 3+ x {Rh,Ir} 4 Ge 13- x .

Polymorphism is observed in the Y 3+ x Rh 4 Ge 13- x series. The decrease of Y-content leads to the transformation of the primitive cubic Y 3.6 Rh 4 Ge 12.4 [ x = 0.6, space group Pm 3̄ n , a = 8.96095(9) Å], revealing a strongly disordered structure of the Yb 3 Rh 4 Sn 13 Remeika prototype, into a body-centred cubic structure [La 3 Rh 4 Sn 13 structure type, space group I 4 1 32, a = 17.90876(6) Å] for x = 0.4 and further into a tetragonal arrangement (Lu 3 Ir 4 Ge 13 structure type, space group I 4 1 / amd , a = 17.86453(4) Å, a = 17.91076(6) Å) for the stoichiometric ( i.e. x = 0) Y 3 Rh 4 Ge 13 . Analogous symmetry lowering is found within the Y 3+ x Ir 4 Ge 13- x series, where the compound with Y-content x = 0.6 is crystallizing with La 3 Rh 4 Sn 13 structure type [ a = 17.90833(8) Å] and the stoichiometric Y 3 Ir 4 Ge 13 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 Y 3+ x Rh 4 Ge 13- 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. Y 3 Rh 4 Ge 13 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 Yb 3 Rh 4 Sn 13 -like structural arrangements for Y 3 Rh 4 Ge 13 and Y 3 Ir 4 Ge 13 .