Multiple Effects Promoting the Thermoelectric Performance of SnTe by Alloying with CuSbTe₂ and CuBiTe₂

Abstract

In a SnTe-based thermoelectric material, the naturally high hole concentration caused by cation vacancies and high total thermal conductivity seriously hinder its thermoelectric performance. A recent work shows that alloying SnTe with other compounds from the I−V−VI 2 family (I = Ag, Na; V = Sb, Bi; VI = Te) can be considered an effective strategy to boost the figure of merit efficiently via the synergy of manipulating hole concentration and lowering lattice thermal conductivity. Herein, we present a markedly enhanced thermoelectric performance in p-type SnTe through CuPnTe 2 (Pn = Sb, Bi) alloying. Moreover, we found that the alloying with both CuSbTe 2 and CuBiTe 2 can facilitate the valence band convergence of SnTe, but their relative influence is different. Interestingly, compared to CuBiTe 2 , alloying with CuSbTe 2 increases the carrier concentration of SnTe, which suppresses the bipolar effect. Ultimately, under the positive effect of valence band convergence, increased vacancy concentration, and decreased lattice thermal conductivity, compounds with a nominal composition of (SnTe) 0.90 (CuSbTe 2 ) 0.10 attains a peak zT of ∼1.26 at 823 K. In contrast, the thermoelectric performance of (SnTe) 1−x (CuBiTe 2 ) x is restricted by the reduced carrier concentration and diminished band gap, showing only a humble maximum zT value of ∼0.91 at 823 K in the sample with a nominal composition of (SnTe) 0.96 (CuBiTe 2 ) 0.04 . These results demonstrate the multiple effects on thermoelectric transport during the formation of complex solid solutions.

When IV−VI Meets I−V−VI2: A Reinvigorating Thermoelectric Strategy for Tin Monochalcogenides

“…It should be noted that, except for SnTe−CuSbTe 2 , all other SnTe− A SbTe 2 alloys show a complete solid‐solution feature with all studied compositions found to be single‐phase, as indicated by XRD measurements [51–54] . This phenomenon suggests the relatively large solubility of AgSbTe 2 , NaSbTe 2 , and LiSbTe 2 in SnTe.…”

“…Furthermore, disorder scattering can be an additional mechanism reducing the lattice thermal conductivity of these alloys as a result of mass fluctuations and strain distortions in their crystal lattice. These three mechanisms jointly impact the phonon propagation, which explains why SnTe− A SbTe 2 has such a low lattice thermal conductivity [51–54] . In addition, the advantage of alloying A SbTe 2 over regular doping or alloying to SnTe can also be inferred here in that the latter is generally aimed at suppressing the intrinsic Sn vacancies exhibited in SnTe, while these vacancies can lead to lattice thermal conductivity reduction, as discussed above.…”

“…For example, in the study by Tan et al ., the zT value of SnTe−AgSbTe 2 was reported to increase monotonically with the increasing molecular fraction of AgSbTe 2 in the alloy throughout the operating temperature range; the peak zT was more than doubled from ∼0.4 for pure SnTe to ∼0.9 for 1SnTe−1/5AgSbTe 2 (the molecular fraction of AgSbTe 2 in the alloy is 1/6) at 800 K before further doping [51] . Likewise, SnTe−NaSbTe 2 , SnTe−CuSbTe 2 , and SnTe−LiSbTe 2 alloys also exhibit dramatic improvement in thermoelectric performance compared to pure SnTe, and these alloys reach their peak zT s of ∼1.2–1.3 at 800–900 K when the molecular fraction of the I−V−VI 2 component is 1/6, 1/10, and 1/9, respectively [52–54] …”

“…Compared to A SbTe 2 , bismuth‐based compounds like AgBiTe 2 , NaBiTe 2 , CuBiTe 2 , and LiBiTe 2 , when alloyed to SnTe, have a less significant, though still positive, impact on the thermoelectric performance, with inferior improvement in zT values. Notably, the molecular fractions for the zT s of their corresponding alloys to reach maxima are also smaller: 1/16 or 3% for AgBiTe 2 ( zT =∼1.1 at 775 K or ∼1.0 at 823 K), 1/25 or 1/51 for NaBiTe 2 ( zT =∼1.1 at 834 K or ∼0.85 at 800–900 K), 1/25 for CuBiTe 2 ( zT =∼0.91 at 823 K), and 1/41 for LiBiTe 2 ( zT =∼1.0 at 873 K) [52–54,65–67] . It is also worth mentioning that all reported A BiTe 2 ‐containing alloys are single‐phase, [52–54,65–67] which negates the possibility that the lower thermoelectric performance of this subgroup of materials originates from intrusive secondary phases due to the alloying content exceeding solubility, as mentioned Section 3.2.…”

“…In fact, there have been previous studies on the thermodynamics of the SnTe−AgSbTe 2 alloy as well as an experimentally established pseudo SnTe−AgSbTe 2 binary phase diagram that can show a solid solution over a very broad compositional range, as shown in Figure 4, which accords with the thermoelectric‐focused study in this case [55,56] . CuSbTe 2 , in contrast, is implied to have an inferior solubility, and that possibly explains why its molecular ratio with respect to SnTe in the alloy's best composition is the lowest among the compositions studied: even though attempts have been made to use larger amounts of CuSbTe 2 than in the best‐performing 1SnTe−1/9CuSbTe 2 composition, some of those alloys could exceed the solubility limit of SnTe such that the resulting undesired secondary phase might have hindered the thermoelectric performance [53] …”