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

Abstract

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 KAlSb 4 and KGaSb 4 has helped demonstrate the potential of n-type Zintl thermoelectrics. In this paper, we report the experimental discovery of 4 ABX 4 Zintl phases: RbAlSb 4 , RbGaSb 4 , CsAlSb 4 , and CsGaSb 4 . Transport measurements on Ba-doped RbGaSb 4 and CsGaSb 4 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 RbGaSb 4 and CsGaSb 4 is significantly impeded when compared to our prior work on KGaSb 4 . 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 ABX 4 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.

Doping by design: finding new n-type dopable ABX4 Zintl phases for thermoelectrics

“…Figure 4(a) presents the summary of the computational search of new ABX 4 Zintl phases in the prototypical KGaSb 4 structure. Out of the 36 phases considered in this study, only 6 are known/reported, [11][12][13] indicating that the chemical phase space of ABX 4 compound is under-explored. From electronic structure calculations, we find that 25 out of 36 ABX 4 phases (in the assumed KGaSb 4 structure) have non-zero DFT band gap ( Figure 4a).…”

“…However, only a few n-type ones have been reported in experiments. [11][12][13][14] The inherent features of Zintl phases, such as small band gaps, dispersive electronic bands, and complex structures, make them suitable as TE materials. Zintl phases such as Yb 14 MnSb 11 , 15 in mid-to high-temperature regime.…”

“…11 However, the discovery of new n-type Zintl phases so far has been driven by chemical intuition and serendipity. The recent discoveries of new n-type Zintl phases, including KAlSb 4 , 11 KGaSb 4 , 12 RbGaSb 4 , 13 CsGaSb 4 , 13 and Ba 3 Cd 2 P 4 , 14 seem to suggest that (1) there are Zintl phases that may overcome the doping asymmetry, and (2) the chemical and structural phase space of n-type Zintl phases is largely underexplored. One plausible route for finding new n-type dopable Zintl phases with high TE performance is by chemical replacements in structure prototype (CRISP).…”

“…KGaSb 4 serves as an appropriate prototype for two reasons: (1) it has good n-type TE performance, with zT ⇠1, and (2) can be doped n-type to >10 19 cm 3 electron concentrations. 12 Out of the 36 plausible Zintl phases, only 2 are reported in the Inorganic Crystal Structure Database (ICSD) 26 and four of them have been recently discovered, 13 indicating that the phase space of KGaSb 4 type Zintl phases is under-explored. Importantly, the 6 known ABX 4 phases are all n-type dopable.…”

“…Importantly, the 6 known ABX 4 phases are all n-type dopable. 12,13 Using firstprinciples calculations, we theoretically evaluate the thermodynamic and dynamic stability of the 36 ABX 4 Zintl phases and predict that 13 of them should be stable, including the 6 known phases (KGaSb 4 , KAlSb 4 , CsGaSb 4 , CsAlSb 4 , RbGaSb 4 , RbAlSb 4 ) and 7 new phases. Of the 7 undiscovered phases, we choose the top 3 candidates (NaAlSb 4 , NaGaSb 4 and CsInSb 4 ) that are predicted to have the highest n-type TE performance; first-principles defect calculations reveal that they are all n-type dopable.…”