Discovery of n-Type Zintl Phases RbAlSb₄, RbGaSb₄, CsAlSb₄, and CsGaSb₄

Zintl phases are excellent candidates for high-efficiency thermoelectrics (TEs) due to their extremely low lattice thermal conductivity. The manufacturing of an all-Zintl module is particularly attractive for practical applications, as it alleviates concerns regarding the electronic, thermal, and mechanical compatibility of the p- and n-type legs. To date, a large majority of Zintl phases have been realized as p-type TE materials. Our recent discovery of n-type transport in Ba-doped KAlSb4 and KGaSb4 has helped demonstrate the potential of n-type Zintl thermoelectrics. In this paper, we report the experimental discovery of 4 ABX4 Zintl phases: RbAlSb4, RbGaSb4, CsAlSb4, and CsGaSb4. Transport measurements on Ba-doped RbGaSb4 and CsGaSb4 demonstrate near glassy lattice thermal conductivity (<0.5 W m–1 K–1, 350 °C) and lightly doped n-type transport. However, the doping efficiency of Ba in RbGaSb4 and CsGaSb4 is significantly impeded when compared to our prior work on KGaSb4. To investigate the underlying mechanism, we performed first-principles defect calculations and found that the effect of compensating alkali metal vacancies increases in the Rb- and Cs-based analogues. Considering the TE potential of the known ABX4 n-type materials, we have also performed a computational survey over 27 plausible compositions where A = (K, Rb, Cs), B = (Al, Ga, In), and X = (As, Sb, Bi) to investigate the effect of chemistry on potential TE performance.