cience

Thermoelectric (TE) materials convert heat into electric energy and vice versa and as such may reduce our dependence on fossil fuels. Thermoelectric materials can be used for a wide range of applications such as Freon-free refrigerators and AC units, waste heat and direct solar thermal energy converters. The performance of a TE material is expressed by the dimensionless figure-of-merit ZT, ZT = TS²/kp, where S is thermopower, T is the absolute temperature, p is electrical resistivity, and k is thermal conductivity. Thus, a TE material should be a good electrical conductor (low p) and induce a high voltage in response to a temperature gradient (high S), but should be a poor heat conductor to maintain the applied temperature gradient (low k). State-of-the-art TE materials exhibit ZT values in the range of 1-2 depending on the application temperature. Further increases in ZT are required for TE materials to become competitive with conventional heat exchange systems. The main issue with engineering more efficient TEs arises from the fundamental properties of solids: the parameters S, k, and p are strongly coupled and cannot be optimized independently. Metals are inefficient TEs due to their high thermal conductivity, while insulators exhibit very high electrical resistivity, which decreases their ZT drastically.

The development of novel materials where charge and heat transport are partially de-coupled is a key factor for the next generation of TEs. The "phonon glass-electron crystal (PGEC)" concept, suggests the use of perfectly crystalline compounds containing loosely bonded atoms inside oversized cages. These materials are expected to have low thermal conductivity and low electrical resistivity. A widely studied class of PGEC compounds are clathrates, a class of inclusion compounds containing a three-dimensional framework with large cavities in which guest atoms are situated. For the last 45 years about 200 tetrel clathrates were reported. The term "tetrel" signifies that the frameworks of almost all clathrates are based on Si, Ge, and Sn. Only a few exceptions were reported where a clathrate framework was composed of late transition metals of groups 11-12 and pnicogens, group 15 elements. We believe that clathrates containing a high concentration of transition metals in the framework will exhibit structural chemistry and transport properties that are different from conventional tetrel-based clathrates.

Our search for new thermoelectric materials has resulted in several compounds with unique crystal structures and thermoelectric properties. We are continuing our work in this direction...