Crystal structure and physical properties of quaternary clathrates Ba8ZnxGe46-x-ySiy, Ba8(Zn,Cu)xGe46-x and Ba8(Zn,Pd)xGe46-x

Three series of vacancy-free quaternary clathrates of type I, Ba8ZnxGe46-x-y, Ba-8(Zn,Cu)(x)Ge46-x, and Ba-8(Zn,Pd)(x)Ge46-x, have been prepared by reactions of elemental ingots in vacuum sealed quartz at 800 degrees C. In all cases cubic primitive symmetry (space group Pm (3) over barn, a similar to 1.1 nm) was confirmed for the clathrate phase by X-ray powder diffraction and X-ray single crystal analyses. The lattice parameters show a linear increase with increase in Ge for Ba8ZnGe46-x-ySiy. M atoms (Zn, Pd, Cu) preferably occupy the 6d site in random mixtures. No defects were observed for the 6d site. Site preference of Ge and Si in Ba8ZnxGe46-x-ySiy has been elucidated from X-ray refinement: Ge atoms linearly substitute Si in the 24k site whilst a significant deviation from linearity is observed for occupation of the 16i site. A connectivity scheme for the phase equilibria in the Ba8Ge46 corner at 800 degrees C has been derived and a three-dimensional isothermal section at 800 degrees C is presented for the Ba-Pd-Zn-Ge system. Studies of transport properties carried out for Ba-8{Cu,Pd,Zn}(x)Ge46-x and Ba8ZnxSiyGe46-x-y evidenced predominantly electrons as charge carriers and the closeness of the systems to a metal-to-insulator transition, fine-tuned by substitution and mechanical processing of starting material Ba8Ge43. A promising figure of merit, ZT similar to 0.45 at 750 K, has been derived for Ba8Zn7.4Ge19.8Si18.(8), where pricey germanium is exchanged by reasonably cheap silicon. (C) 2010 Elsevier Inc. All rights reserved.