Cationic clathrate of type-III Ge(172-x)P(x)Te(y) (y ≈ 21.5, x ≈ 2y): synthesis, crystal structure and thermoelectric properties.

A first germanium-based cationic clathrate of type-III, Ge(129.3)P(42.7)Te(21.53), was synthesized and structurally characterized (space group P4(2)/mnm, a = 19.948(3) Å, c = 10.440(2) Å, Z = 1). In its crystal structure, germanium and phosphorus atoms form three types of polyhedral cages centered with Te atoms. The polyhedra share pentagonal and hexagonal faces to form a 3D framework. Despite the complexity of the crystal structure, the Ge(129.3)P(42.7)Te(21.53) composition corresponds to the Zintl counting scheme with a good accuracy. Ge(129.3)P(42.7)Te(21.53) demonstrates semiconducting/insulating behavior of electric resistivity, high positive Seebeck coefficient (500 μV K(-1) at 300 K), and low thermal conductivity (<0.92 W m(-1) K(-1)) within the measured temperature range.