Semiconductor clathrates: A PGEC system with potential for thermoelectric applications

An investigation into the structural, chemical, electronic, and thermal properties of polycrystalline semiconductor clathrate compounds with different "guest" ions in the voids of these structures is presently underway by the author and his colleges. Despite the well-defined crystalline structure of these semiconductor materials, the thermal conductivity shows a magnitude and temperature dependence more typical of amorphous materials than crystalline materials. The localized low frequency vibrations of these "guest" ions is believed to interact with the acoustic phonons of the host lattice resulting in a very low thermal conductivity. Indeed in the case of Ge-clathrates a "glass-like" thermal conductivity was observed. The markedly low thermal conductivity displayed by these semiconductor materials, along with the high power factors, demonstrates the potential of this material system for thermoelectric applications.