Stabilities and electronic structures of XYZ₂ thermoelectric materials

A new class of high-performance thermoelectric materials XYZ₂ has been revealed based on high-throughput simulations. But they could also have multiple phase transformations similar to many other high-performance thermoelectric materials like PbTe. Thus, it's important to study the stability and electronic structure of different phases, and how these are related to the chemistry of the compound. The phase stabilities have been assessed by support vector machine methodology. We find compounds with larger X ionic radius have P3m1 space group with X ion in octahedral interstices, while smaller X ionic radius favors a tetrahedral site occupation with a space group of the compound of I-42d. Based on the gradient boosting algorithm, density of states effective mass m_S* and fermi surface complexity factor N_V*K* are identified as important descriptors to enhance zT values. Compounds with larger m_S* have a flat band to enhance Seebeck coefficient if the X-s (Y-d) orbitals have similar energy with Z-p orbital near the VBM. Cu and Ag based compounds with P3m1 space group are benefited from the Y-d orbital contribution to the high N_V*K*. These results demonstrate altering ionic size and molecular orbitals may be used in strategies to improve the thermoelectric performance of XYZ₂ compounds.