Phase Boundary Mapping for Exploring New Thermoelectric Zintl Compounds

Understanding and controlling the defect chemistry of bulk materials can vastly increase the opportunities for discovering highly efficient thermoelectrics. Good thermoelectrics are degenerate semiconductors and there are two types: n-type thermoelectrics, whose charge carriers are electrons, and p-type thermoelectrics conducting holes as carriers. Although normally one type can attain superior thermoelectric properties to the other depending on the electronic band structure of a material, a formation of the unfavorable defects sometimes prevents a material from obtaining the desired type. Similarly, even if the desired carrier type is realized, the Fermi level, which is a measure of the carrier density, could be kept from the optimum due to the formation of compensating defects. It has been known from growing binary semiconductors for electronics and optoelectronics such as GaAs and GaN that a growth condition can substantially alter the defect concentration of resulting samples. This is primarily due to the change in the reference atomic chemical potentials, but such defect engineering has not been utilized for the bulk thermoelectric research, resulting in overlooking the promising candidate materials. In this work, we established an experimental methodology to fully explore all the accessible variations in chemical potentials of a target phase and demonstrated its implementation. Although a pursuance of purity of samples to be measured is a common experimental concept in solid-state chemistry, in practice, a small single-phase region of semiconductors allows samples to have a certain amount of impurities. Since different multi-phase equilibria have discrete chemical potentials, only when all the boundaries of the multi-phase equilibria around target phase are mapped out in nominal composition space are the measured transport properties of resulting samples properly correlated with their atomic chemical potential. Utilizing this experimental concept we call "phase boundary mapping", we have identified the mechanism of obtaining the superior n-type conduction in Mg3Sb2-based compounds. To achieve their exceptionally high thermoelectric figure-of-merit (zT = 1.5 at 750 K), the formation energy of Mg-vacancy needs to be suppressed with excess Mg but this condition had been missing for over 80 years due to the absence of experimental concept to fully investigate properties of a material. Implementing phase boundary mapping has also allowed an inexpensive thermoelectric Zintl compound Ca9Zn4+xSb9 to be one of the best thermoelectrics in the intermediate temperature range (zT = 1.1). We have also successfully reduced the carrier concentration of Yb9Zn4+xSb9, which was originally thought to be impossible, leading to zT increased by a factor of five.