Low Lattice Thermal Conductivities of Sintered Type-II Clathrates (K,Ba)24(Ga,Sn)136 with Anharmonic and Disordered Guest Potentials Arising from Large Guest-Site Splitting and Ga–Sn Alloyed Frameworks

Abstract

The type-I clathrate Ba8Ga16Sn30 is known to exhibit very low lattice thermal conductivity due to the off-center rattling effect. The present type-II clathrates exhibited lower lattice thermal conductivities than the type-I clathrate at 400–700 K. The lowest value of 3.3 mW cm–1K–1 at 640 K is comparable to the theoretical minimum value for Sn. We performed thermoelectric property measurements, crystal structure analyses, and electronic structure calculations for the type-II clathrates to discuss such low values. As a result, we found that the following factors contributed to the reduced thermal conductivities: the large number of atoms (160) in the unit cell, large guest-site splitting and associated off-center rattling in the large cages, and a unique guest potential. The uniqueness of the guest potential includes the energy depth at the split sites, the anharmonic shape, and the site-to-site variation. This variation is due to the Ga–Sn alloy arrangement at the framework site. Due to their low lattice thermal conductivity, these clathrates achieved a maximum dimensionless figure of merit ZT of 1.32 at 640 K.