Beyond Periodic Order

In Chapter 3 we learned that complex crystal structures, having low symmetry lattices, generally lead to low lattice thermal conductivi- ties, which favors large figure of merit (FOM) values. Afterward, in Chapter 4 we saw that the presence of highly degenerate, relatively flat bands close to the Fermi level is a very convenient feature in order to obtain large Seebeck coefficient and electrical conductivity values, naturally leading to high power factors as well. Nevertheless, these degenerate bands usually occur in materials crystallizing into high symmetry lattice structures. Therefore, we face an apparent structural incompatibility, demanding a low enough crystal sym- metry to guarantee the lattice complexity requirement along with a high enough lattice symmetry in order to satisfy optimal power factor conditions.