Synthesis and some properties of Ba24-x(Ga,Sn)(136)(x similar to 4) type-II clathrates

We obtained Ba-Ga-Sn type-II clathrate crystals by centrifuging melted Na-Ba-Ga-Sn ingots. This clathrate had Ba guest atoms in both the 20-atom and 28-atom cages, unlike a previously reported Ba-Ga-Sn type-II clathrate, Ba16Ga32Sn104, which had no guest atoms in the 28-atom cages. The chemical composition and lattice constant of this clathrate crystals were Ba20Ga47Sn89 and 1.6918 nm, respectively. We sintered the crystals to measure the thermoelectric properties of the clathrate. The sintered samples exhibited p-type properties at low temperatures. The room-temperature (RT) thermal conductivities were as low as 5-6 mW cm(-1) K-1. However, the RT carrier mobilities were only similar to 5 cm(2) V-1 s(-1), and these are very low for type-II clathrates with Ga-Sn frameworks. It is partly because the sintered samples suffered from porosity and grain boundary scattering. Such low mobilities may also be associated with the band structure of this clathrate.