Phase-Boundary Mapping to Engineer Defects in Thermoelectric Materials

The optimization of thermoelectric materials for use in various applications, such as spacecraft power generation, waste heat recovery, and Peltier coolers, requires a careful optimization of material properties. This can be achieved via defect engineering in which defects are purposefully added to a material to produce desired properties. In this tutorial, we discuss a defect engineering strategy called phase-boundary mapping. While many compound thermoelectric semiconductors are often called “line compounds” due to their appearance as a line on a binary phase diagram, in reality, due to the laws of thermodynamics, all phases have a finite phase width. The edges of this phase space define the chemical potential of the material. By making small compositional changes across this phase space, appreciable differences in thermoelectric properties are observed due to this change in the chemical potential. Additionally, the phase equilibria of a thermoelectric material impacts alloying and dopability that further impacts material properties. Phase-boundary mapping is a strategy that allows us to explore the limits of a material and ultimately reproducibly optimize thermoelectric performance by considering the effects of off-stoichiometry on chemical potential, and thus defect energies and material properties. This technique can be applied in the optimization of numerous thermoelectric materials as well as extended to other semiconductors with properties controlled by defects.