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−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, a∼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 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 ∼0.45 at 750 K, has been derived for Ba8Zn7.4Ge19.8Si18.8, where pricey germanium is exchanged by reasonably cheap silicon.

Quaternary phase diagram of Ba–Pd–Zn–Ge system at 800 °C. Figure optionsDownload as PowerPoint slide