Improved Solubility in Metavalently Bonded Solid Leads to Band Alignment, Ultralow Thermal Conductivity, and High Thermoelectric Performance in SnTe

Abstract

SnTe is an emerging Pb‐free thermoelectric compound that has drawn significant attention for clean energy conversion. Chemical doping is routinely used to tailor its charge carrier concentration and electronic band structures. However, the efficacy of dopants is often limited by their small solubility. For example, only 0.5% Ag can be incorporated into the SnTe matrix. Yet, significantly more Ag (>7%) can be dissolved if SnTe is alloyed with AgSbTe2. This large enhancement of solubility can be understood from a chemical bonding perspective. Both SnTe and AgSbTe2 employ metavalent bonding as identified by an unusual bond‐rupture in atom probe tomography. Density functional theory calculations show that upon Ag doping the energy offset of the upmost two valence bands decreases significantly. This induces band alignment in SnTe, which results in an enhanced power factor over a broad temperature range. Moreover, the increased concentration of point defects and associated lattice strain lead to strong phonon scattering and softening, contributing to an extremely low κL of 0.30 Wm−1K−1. These synergistic effects contribute to a peak ZT of 1.8 at 873 K and a record‐high average ZT of ≈1.0 between 400 and 873 K in Sn0.87Mn0.08Sb0.08Te–5%AgSbTe2 alloy.

Interstitial Defects Facilitate Dense Dislocations and Band Convergence for High Thermoelectric Performance in SnTe

“…52 Both SnTe and AgSbTe 2 employ metavalent bonding as identified by an unusual bond rupture in atom probe tomography. 52 The solid solubility limit of Ag in SnTe has been significantly enhanced due to the coexistence of two S6). The line scanning EDS results (Figure S7) clearly indicate that the V elements are rich around the nanoprecipitates.…”

“…We previously found that the solubility limit can be significantly enhanced by alloying SnTe with compounds employing the same chemical bonding mechanism . Both SnTe and AgSbTe 2 employ metavalent bonding as identified by an unusual bond rupture in atom probe tomography . The solid solubility limit of Ag in SnTe has been significantly enhanced due to the coexistence of two metavalent bonding systems.…”

“…This large enhancement of solubility in this work can be understood from a chemical bonding perspective. We previously found that the solubility limit can be significantly enhanced by alloying SnTe with compounds employing the same chemical bonding mechanism . Both SnTe and AgSbTe 2 employ metavalent bonding as identified by an unusual bond rupture in atom probe tomography .…”

SnTe thermoelectric materials with low lattice thermal conductivity synthesized by a self-propagating method under a high-gravity field

“…[8][9][10] However, the stoichiometric SnTe has a poor thermoelectric performance. Recently, the thermoelectric performance of SnTe has been improved due to a deeper theoretical understanding 11,12 and imaginative experimental methods. [13][14][15][16][17][18][19][20][21] Compared to thermoelectric performance (zT and efficiency), the economic viability of the fabrication process is equally important for the large-scale industrial application.…”

“…8–10 However, the stoichiometric SnTe has a poor thermoelectric performance. Recently, the thermoelectric performance of SnTe has been improved due to a deeper theoretical understanding 11,12 and imaginative experimental methods. 13–21…”