High Thermoelectric Performance of AgSb1-xPbxSe2 Prepared by Fast Nonequilibrium Synthesis

High entropy effect on thermoelectric properties of the nonequilibrium cubic phase of AgBiSe2-2xSxTex with x = 0-0.6

Silver bismuth diselenide (AgBiSe2) has attracted much attention as an efficient thermoelectric material owing to its low thermal conductivity. However, AgBiSe2 exhibits multiple crystal structural transitions with temperature, and high thermoelectric performance was realized only in the high-temperature cubic phase. We previously reported the stabilization of the cubic phase in AgBiSe2-2xSxTex with x = 0.6-0.8 at room temperature using a high-entropy-alloy (HEA) approach. In this paper, we succeeded in stabilizing the cubic phase in AgBiSe2-2xSxTex with x = 0-0.6 using an ice-quenching method and investigated the HE effect on thermoelectric properties below room temperature to avoid the emergence of a hexagonal phase above room temperature. Cubic AgBiSe2-2xSxTex exhibited n-type conductivity at 10-300 K. The increase in electrical conductivity is due to a combined effect of increased charge density and electron mobility, depending on the sample. It is evident that charge carrier density strongly increases in the x = 0.4 sample while the mobility remains roughly the same compared to the x = 0.0 sample. In x = 0.2 and 0.6 samples, electronic conductivity increases primarily due to enhanced mobility. S and Te substitutions induced a variation in the band structure, resulting in carrier mobility enhancement. Furthermore, thermal conductivity showed reduction tendency with increasing amounts of S and Te due to the enhancement of phonon scattering. Simultaneous electronic conductivity increase and thermal conductivity reduction resulted in the systematic improvement of ZT values for HE-type cubic AgBiSe2-2xSxTex.

Thermoelectric Transport Performance in p-Type AgSbTe2-Based Materials through Entropy Engineering

Being a major obstacle, Ag2Te has always been restricted in p-type AgSbTe2-based materials to improve their thermoelectric performance. This work reveals a stabilized AgSbTe2 through Sn/Ge alloying as synthesized by melting, annealing, and hot press. Interestingly, addition of Sn/Ge in AgSbTe2 extended the solubility limit up to ∼30% and hence suppressed Ag2Te in Ag(1-x)SnxSb(1-y)GeyTe2 compounds and led to enhanced electrical transport. Moreover, electrical and thermal transport properties of AgSbTe2 have been greatly affected by the phase transition of Ag2Te near 425 K. However, high-entropy Ag0.85Sn0.15Sb0.85Ge0.15Te2 compound results in a stabilized rock-salt structure and presents a high power factor of ∼10.8 μW cm-1 K-2 at 757 K. Besides, density functional theory reveals that available multivalence bands in Sn/Ge-doped AgSbTe2 lead to reduction in energy offsets. Meanwhile, a variety of defects appear in the Ag0.85Sn0.15Sb0.85Ge0.15Te2 sample due to entropy change, and thus lattice thermal conductivity decreases. Ultimately, a high figure of merit of ∼1.5 is attained at 757 K. This work demonstrates a roadmap for other group IV-VI materials so that the high-entropy approach may inhibit the impurity phases with extended solubility limit and result in high thermoelectric performance.

Long-range ordering and local structural disordering of BiAgSe2 and BiAgSeTe thermoelectrics

Thermoelectric materials are promising for energy harvesting using waste heat. The thermal management of the thermoelectric materials attract scientific and technological interests. The narrow bandgap semiconductor BiAgSe₂ is a good candidate for thermoelectric materials due to its ultralow thermal conductivity. The mother compound BiAgSe₂ crystallizes in hexagonal symmetry at room temperature, but experiences structural transitions to cubic phase at high temperature. By contrast, the daughter compound BiAgSeTe exhibits long range ordering and crystallizes into cubic phase at room temperature. Nevertheless, the local structural disorderings due to the Bi³⁺ and Ag⁺ anti-site defects, as well as local structural distortions, are ubiquitous in both parent BiAgSe₂ and BiAgSeTe. BiAgSeTe exhibits distinct transport properties owing to the disordering-induced drastic changes in the electronic band structure, as well as the scattering dictated by the point defects. It is suggested that BiAgSe₂ and BiAgSeTe could be good candidates for phonon glass and crystal glass (PGEC)-type thermoelectrics.