Optimization of the Thermoelectric Power Factor: Coupling between Chemical Order and Transport Properties

Many thermoelectric materials are
multicomponent systems that exhibit
chemical ordering, which can affect both thermodynamic and transport
properties. Here, we address the coupling between order and thermoelectric
performance in the case of a prototypical inorganic clathrate (Ba8Ga16Ge30) using a combination of density
functional and Boltzmann transport theory as well as alloy cluster
expansions and Monte Carlo simulations. The calculations describe
the experimentally observed site occupancy factors and reproduce experimental
data for the transport coefficients. By inverting the cluster expansion,
we demonstrate that the power factor can be increased by more than
60% for certain chemical ordering patterns that involve reducing the
number of the trivalent species on the 6c Wyckoff
site. This enhancement is traced to specific features of the electronic
band structure. The approach taken in the present work can be readily
adapted to other materials and enables a very general form of band
structure engineering. In this fashion, it can guide the computational
design of compounds with optimal transport properties.