The unit cell of Pb2GeS4. The red atoms are lead (pale red atoms are partially occupied sites). The blue tetrahedra are (GeS4)4- units.
K.M. Poduska, L. Cario,F.J. DiSalvo, K. Min, and P.S. Halasyamani, J. Alloys Comp. 335, 105 (2002).
Mahan has shown that for semiconductors (at optimal doping) the thermoelectric figure of merit can be written as a function of the band gap and a materials parameter B. It turns out that ZT is a monotonically increasing function in each variable. See G. D. Mahan, Solid State Phys. 51, 82 (1998).
We would like to take advantage of the dependence of ZT on NV. For cubic and hexagonal systems, NV can be as large as 48 and 24, respectively, if the band extrema lie at general points in the Brilluoin zone. This situation is more likely to occur in complex cubic and hexagonal cells.
We hope to obtain novel materials with cubic or hexagonal symmetry and with complex unit cells by including tetrahedral molecular anions within a conducting cationic lattice. The conducting network should lead to significant electrical conductivity. In addition, the molecular anions may "rattle" within the lattice which scatters phonons and reduce the thermal conductivity. Some of the anions we are studying:
This has lead to the discovery of materials such as:
A view of the structure of CrPbPS5 emphasizing the isolated PS4 units.