"Complex thermoelectric materials" - The widespread use of thermoelectric generators has been limited by the low material efficiency of the thermoelectric material. A number of strategies for Complex Thermoelectric Materials  with higher Thermoelectric figure of merit, zT, are being actively studied at Caltech. Complex electronic band structures provide mechanisms to achieve high zT in thermoelectric materials through band structure engineering. High zT is obtained p-type PbTe and PbSe which contains both light and heavy valence bands that can be engineered to achieve
high valley degeneracy which leads to an extraordinary peak zT of nearly 1.8 at 750K . Complex crystal structures that enable relatively low thermal conductivity have led to several new classes of thermoelectric materials.
Ca3AlSb3, Ca3AlSb3 and Yb14AlSb11 are complex Zintl compounds containing differently connected AlSb4 tetrahedra that obtain zT near 1 at high temperatures. Fast diffusing or ‘liquid-like’ elements in the complex materials Zn4Sb3  and Cu2Se  provide additional mechanisms to scatter and otherwise inhibit phonon heat conductivity. The principles of Zintl chemistry facilitates the search for new complex materials and the tuning of known thermoelectric materials with earth abundant, non-toxic elements . Finally, the incorporation of nanometer sized particles reduces thermal conductivity from long mean-free-path phonons. This principle has been successfully demonstrated in PbTe with large nanoscale precipitates (>100nm) that can be
independently doped with La (n-type) or Na (p-type). The synthesis of nanoscale composites can be controlled with the aid of equilibrium phase diagrams (experimental or theoretically determined) to produce microstructure
of varying composition and length scale .