Off-stoichiometric silver antimony telluride: An experimental study of transport properties with intrinsic and extrinsic doping

Abstract

AgSbTe2 is a thermoelectric semiconductor with an intrinsically low thermal conductivity and a valence band structure that is favorable to obtaining a high thermoelectric figure of merit zT. It also has a very small energy gap Eg ∼ 7.6 ± 3 meV. As this gap is less than the thermal excitation energy at room temperature, near-intrinsic AgSbTe2 is a two carrier system having both holes (concentration p) and electrons (n). Good thermoelectric performance requires heavy p-type doping (p > > n). This can be achieved with native defects or with extrinsic doping, e.g. with transition metal element. The use of defect doping is complicated by the fact that many of the ternary Ag-Sb-Te and pseudo-binary Sb2Te3-Ag2Te phase diagrams are contradictory. This paper determines the compositional region most favorable to creating a single phase material. Through a combination of intrinsic and extrinsic doping, values of zT > 1 are achieved, though not on single-phased material. Additionally, we show that thermal conductivity is not affected by defects, further demonstrating that the low lattice thermal conductivity of I-V-VI2 materials is due to an intrinsic mechanism, insensitive to changes in defect structure.

Thermal Stability and Tuning of Thermoelectric Properties of Ag1−xSb1+xTe2+x (0 ≤ x ≤ 0.4) Alloys

“…However, the λ e calculated from the Wiedemann-Franz law seems to be overestimated for these samples, resulting in very low lattice contribution (Figure 7b). The other alloys exhibit twice lower λ(T) values, in the range of 0.4-0.7 W·m −1 ·K −1 , similar to literature reports [7,26]. Lattice part is the main contribution to total thermal conductivity, independent of synthesis procedure (Figure 7b).…”

“…Singlephase alloy with the nominal composition Ag0.875Sb1.125Te2 (not included in the pseudobinary Sb2Te3-Ag2Te system) was characterized by PF = 2 × 10 −5 W•m −2 •K −2 at RT [24]. High TE performance of Ag1−xSb1+xTe2+x alloys (ZT = 1.2-1.4 at 600 K) with x >0.25 was reported in [25,26]. There is also a strong conviction in the literature that a small change in Ag/Sb ratio effectively enhances electrical properties, due to the formation of vacancies at the Ag/Sb site, without altering thermal conductivity [25,27,28].…”

Direct observation of vast off-stoichiometric defects in single crystalline SnSe

“…For Na doping, Na would spontaneously substitute Sn, as evidenced by the negative formation energy of Na Sn . This strongly off-stoichiometric feature was also observed in Mg 2 (Si, Sn) [27,28] and AgSbTe 2 [29,30] systems, without severely altering the carrier concentration via the possible self-doping mechanism. In what follows, the individual role each point defect plays in phonon scattering processes will be evaluated and analyzed.…”

Synthesis of small Ag–Sb–Te nanocrystals with composition control

“…This composition study highlights that it is possible to make Ag x Sb 1-x Te 1.5-x alloys with x between 0.3 and 0.6 at the nanoscale, which represents a larger range for solid solutions than in bulk where solid solutions have only been reported only between Ag 0.41 Sb 0.59 Te 1.09 and AgSbTe 2 (Figure 3c). [14][15][16]47 This larger accessible composition range in nanocrystals compared to the bulk can be associated with the soft crystal boundaries (i.e., the small physical dimension of nanocrystals) or with the cation-exchange formation mechanism (i.e., facilitating metastable crystal structures) 48,49 .…”

“…This phase has a characteristic octahedral rock-salt-type arrangement of atoms, tolerating small non-stoichiometry (e.g., Sb-rich Ag x Sb 1-x Te 1.5-x with x up to 0.41 14,15 ). Understanding the formation and compositional range of rock-salt-type Ag 2 Te-Sb 2 Te 3 solid solution in the Ag-Sb-Te material system is important for designing thermoelectric devices due to compositiondependent effects such as nanoscale domain ordering, atomic vacancy concentration, and inclusions of secondary phase 11,[16][17][18] . In general, colloidal nanocrystals promise new opportunities for device engineering.…”