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N. Nasir, A. Grytsiv, N. Melnychenko-Koblyuk, P. Rogl, I. Bednar, E. Bauer:
"Crystal structure and physical properties of quaternary clathrates Ba8ZnxGe46-x-ySiy, Ba-8(Zn,Cu)(x)Ge46-x and Ba-8(Zn,Pd)(x)Ge46-x";
Journal of Solid State Chemistry, 183 (2010), S. 2329 - 2342.

Three series of vacancy-free quaternary clathrates of type I, Ba8ZnxGe46−x−ySiy, Ba8(Zn,Cu)xGe46−x, and Ba8(Zn,Pd)xGe46−x, have been prepared by reactions of elemental ingots in vacuum sealed quartz at 800 °C. In all cases cubic primitive symmetry (space group Pm3̄n, anot, vert, similar1.1 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 Ba8ZnxGe46−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 °C has been derived and a three-dimensional isothermal section at 800 °C is presented for the Ba-Pd-Zn-Ge system. Studies of transport properties carried out for Ba8{Cu,Pd,Zn}xGe46−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 not, vert, similar0.45 at 750 K, has been derived for Ba8Zn7.4Ge19.8Si18.8, where pricey germanium is exchanged by reasonably cheap silicon.

http://dx.doi.org/10.1016/j.jssc.2010.07.047