Giant Thermopower at Low Temperatures in Novel Clathrates Ba8{Cu,Zn}xGe46−x

In the present work we focus on cubic type I Ba-Ge-based clathrates, where Ge in (where □ is a vacancy) is substituted by Cu, Zn, Pd, and Si. Structural investigations in all cases confirm cubic primitive symmetry consistent with the space group type of a typical type I clathrate structure with lattice parameter a ≈ 1.06 nm. Electronic transport has been studied in a broad temperature range from 4.2 K to about 800 K, demonstrating that substitution allows fine-tuning of the charge carrier density, shifting the materials into the proximity of a metal-to-insulator transition. This is evidenced from giant thermopower reaching values of 400 μ V/K in the case of Ba 8 Cu 5.2 Zn 0.8 Ge 40.0 at temperatures well below room temperature ( T max S ≈ 150 K).