Structure and thermoelectric properties of Ba₆Ge(25-x), Ba₆Ge₂₃Sn₂, and Ba₆Ge₂₂In₃: Zintl phases with a chiral clathrate structure

A single phase of Ba₆Ge(25-x) (x ~ 1) was obtained from a direct element combination reaction in a sealed graphite tube at 830°C. Its structure was determined by single-crystal X-ray diffraction methods. Ba₆Ge(25-x) adopts the chiral cubic space group P4₁32 (No. 213) with dimensions of a = 14.5483(2) Å and Z = 4. The structure consists of helical chains of Ba atom-centered pentagonal dodecahedra, Ba@Ge₂₀, and zeolite-like cages formed by those chains. There are vacancies distributed over the Ge sites giving a composition of Ba₆Ge₂₄, in which charge balance according to the Zintl formalism is retained. The compound therefore satisfies the classical Zintl concept. Polycrystalline ingots of Ba₆Ge(25-x) showed an electrical conductivity of 2000 S/cm and a Seebeck coefficient of - 18 μV/K at room temperature. An anomalous broad transition in the electrical conductivity and Seebeck coefficient was observed at Å 100 K. Ba₆Ge_23.32Sn_1.68 and Ba₆Ge_21.93In_3.07 as ternary derivatives of Ba₆Ge(25-x) were also synthesized and their structural and physical properties compared with those of Ba₆Ge(25-x). (C) 2000 Academic Press.