Structure and thermoelectric properties of Ba6Ge25-x, Ba6Ge23Sn2, and Ba6Ge22In3:: Zintl phases with a chiral clathrate structure

A single phase of Ba6Ge25-x (x similar to 1) was obtained from a direct element combination reaction in a sealed graphite tube at 830 degrees C, Its structure was determined by single-crystal X-ray diffraction methods, Ba6Ge25-x adopts the chiral cubic space group P4(1)32 (No, 213) with dimensions of a = 14.5483(2) Angstrom and Z = 4, The structure consists of helical chains of Ba atom-centered pentagonal dodecahedra, Ba@Ge-20, 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 Ba6Ge25-x showed an electrical conductivity of 2000 S/cm and a Seebeck coefficient of -18 mu V/K at room temperature. An anomalous broad transition in the electrical conductivity and Seebeck coefficient was observed at similar to 100 K, Ba6Ge23.32Sn1.68 and Ba6Ge21.93In3.07 as ternary derivatives of Ba6Ge25-x were also synthesized and their structural and physical properties compared with those of Ba6Ge25-x. (C) 2000 Academic Press.