(Invited) Zintl Phase Thermoelectrics: Engineering Transport through Crystal Chemistry

Intermetallic Zintl phases form complex composite structures characterized by covalently-bonded polyanions stabilized by the surrounding cations. The Zintl polyanions occur in an astounding variety of forms, including isolated 0-dimensional clusters, 1-D chains, and 2-D slabs, making this an ideal class of compounds to study structure-property relationships in complex semiconductors. AM 2 X 2 compounds in the CaAl2Si2 structure type are one of the most successful classes of Zintl thermoelectrics - particularly in light of recent reports of zT ~ 1.5 in n-type MgMg2Sb2. This talk will explore the role played by the defect chemistry of AM 2 X 2 compounds. We have shown both experimentally and using first principles calculations that the low formation energy for cation vacancies in this structure type enables tuning of the electronic properties via either alloying or by directly varying the cation content. In addition, by using high pressure, we have investigated the transport properties of AM 2 X 2 compounds synthesized using high pressure. While AM 2 X 2 compounds in the CaAl2Si2 structure are typically narrow gap semiconductors, those in the related tetragonal BaZn2P2 structure type are exclusively metals or semimetals and are known to exhibit superconductivity. In the first example of a phase transition between these two technologically important structures types, we show that SrAl2Si2 (trigonal CaAl2Si2-type structure at ambient pressure) can be obtained as a metastable phase in the tetragonal BaZn2P2 structure type by employing high-pressure, high-temperature synthesis. Our study shows that the changes in the electronic density of states lead to the emergence of phonon-driven superconductivity in the tetragonal polymorph. By accessing structural motifs that are not thermodynamically stable at ambient conditions, we are thus able to directly investigate the relationship between structure and electronic properties in AM 2 X 2 Zintls.