Synthesis, structure, and transport properties of Ba$_8$Cu$_{16}$ -$_x$Au$_x$ P$_{30}$ clathrate solid solution

Abstract

A new clathrate solid solution Ba$_8$Cu$_{16}$ − $_x$Au$_x$P$_{30}$ (x = 4, 8, 12) was synthesized by a high-temperature solid-state annealing method. The crystal structures of Ba$_8$Cu$_{16}$ − $_x$Au$_x$P$_{30}$ were determined by single crystal x-ray diffraction. This clathrate solid solution crystallizes in the orthorhombic superstructure of clathrate-I type with 23 crystallographically independent framework sites, eight of them are occupied by Au/Cu and 15 are exclusively occupied by P atoms. The distribution of Au and Cu atoms over these eight framework sites is not random with a clear preference for Au to occupy the largest (Au/Cu)P$_4$ tetrahedra in the framework. The thermal stability and thermoelectric properties of the Ba$_8$Cu$_{16}$ − $_x$Au$_x$P$_{30}$ solid solution were evaluated. Low thermal conductivity was achieved for Ba$_8$Cu$_{16}$ − $_x$Au$_x$P$_{30}$ due to the combination of the host–guest crystal structure with rattling Ba atoms with the presence of heavy Au atoms and substitutional Cu/Au disorder in the clathrate framework.