New n-type Zintl Phases for Thermoelectrics: Discovery, Structural Characterization and Band Engineering of the Compounds A2CdP2 (A = Sr, Ba, Eu)

Zintl phases, owing to their complex crystal structures and intricate chemical bonding, have recently been recognized as promising candidates for thermoelectric (TE) applications. Band engineering, including band convergence has been shown to be an effective way to enhance the thermoelectric performance of such materials. In this work, a series of emerging TE materials, the isostructural Zintl phases with the general formula A₂CdP₂ (A = Sr, Ba, Eu) are presented for the first time. Their structures, established from single-crystal X-ray diffraction methods, show them to crystallize with the orthorhombic Yb₂CdSb₂ structure type, with first-principles calculations on phase stability confirming that Ba₂CdP₂ and Sr₂CdP₂ are thermodynamically stable. Computationally, it was found that both Ba₂CdP₂ and Sr₂CdP₂ have the potential to exhibit high n-type TE performance (0.6 and 0.7 relative to the n-type PbTe, a reference TE material). To optimize the TE performance, band engineering strategies, including isovalent substitution and cation mutations, were investigated. From the band engineering of Ba₂CdP₂ via isovalent substitution of Sr on a single Ba site, leading to the quaternary composition SrBaCdP₂, it can be suggested that increasing the conduction band valley degeneracy is an effective way to improve the n-type TE performance by three-fold. Moreover, first-principles defect calculations reveal that both Ba₂CdP₂ and SrBaCdP₂ are n-type dopable, adding these compounds to a small list of rare n-type dopable Zintl phases. The band engineering strategies used in this work are equally applicable to other TE materials, either for optimization of existing TE materials or designing new materials with desired properties.