Insights into the design of thermoelectric Mg3Sb2 and its analogs by combining theory and experiment

Abstract

Over the past two decades, we have witnessed a strong interest in developing Mg 3 Sb 2 and related CaAl 2 Si 2 -type materials for lowand intermediate-temperature thermoelectric applications. In this review, we discuss how computations coupled with experiments provide insights for understanding chemical bonding, electronic transport, point defects, thermal transport, and transport anisotropy in these materials. Based on the underlying insights, we examine design strategies to guide the further optimization and development of thermoelectric Mg 3 Sb 2 -based materials and their analogs. We begin with a general introduction of the Zintl concept for understanding bonding and properties and then reveal the breakdown of this concept in AMg 2 X 2 with a nearly isotropic three-dimensional chemical bonding network. For electronic transport, we start from a simple yet powerful atomic orbital scheme of tuning orbital degeneracy for optimizing p-type electrical properties, then discuss the complex Fermi surface aided by high valley degeneracy, carrier pocket anisotropy, and light conductivity effective mass responsible for the exceptional n-type transport properties, and finally address the defect-controlled carrier density in relation to the electronegativity and bonding character. Regarding thermal transport, we discuss the insight into the origin of the intrinsically low lattice thermal conductivity in Mg 3 Sb 2 . Furthermore, the anisotropies in electronic and thermal transport properties are discussed in relation to crystal orbitals and chemical bonding. Finally, some specific challenges and perspectives on how to make further developments are presented.npj Computational Materials (2019) 5:76 ; https://doi.

Rapid One‐Step Synthesis and Compaction of High‐Performance n‐Type Mg3Sb2 Thermoelectrics

“…Most of the experimental data of n‐type (Sc, Te)‐doped Mg 3 Sb 2 samples show reasonably good agreements with the previous theoretical results simulated based on the full ab initio band structure. Some of the experimental data deviating from theory may be attributed to factors such as the minor impurity phases and doping on the cation sites …”

“…All n‐type Mg 3 Sb 2 samples generally show reasonably low lattice thermal conductivity values, which, to a large extent, are due to the intrinsic low κ L . The intrinsic low κ L in Mg 3 Sb 2 is mainly related to the large anharmonic scattering rate induced by the soft transverse acoustic phonon modes . Comparing with the pure Mg 3 Sb 2 prepared using the same synthesis method, the Te doping in Mg 3 Sb 2 slightly reduces the lattice thermal conductivity.…”

“…Recently n‐type Mg 3 Sb 2 ‐based compounds have been intensively studied because of their outstanding TE performance as well as the low‐cost and abundantly available constituent elements . Exceptional n‐type TE properties were first reported in Te‐doped Mg 3+ δ Sb 1.5 Bi 0.5 with a high zT value of about 1.6 at 725 K by Tamaki et al .…”

Metallic n‐Type Mg3Sb2 Single Crystals Demonstrate the Absence of Ionized Impurity Scattering and Enhanced Thermoelectric Performance

“…n‐Type Mg 3 Sb 2 –Mg 3 Bi 2 alloys are one of the most potent thermoelectric materials for low‐ (near room temperature) to mid‐temperature range [ 1–9 ] because of their highly degenerate conduction band structure [ 1,4 ] and extremely low phonon thermal conductivity. [ 10–11 ] Since the discovery of n‐type Mg 3 Sb 1.5 Bi 0.5 with zT of 1.6 at 700 K in 2016, [ 1 ] extensive research have been conducted to improve their thermoelectric performance via the engineering of the electronic band structure, [ 5,12 ] chemical doping, [ 13–16 ] bonding [ 17–19 ] and modulation of microstructure. [ 20–22 ] The studies of carrier and phonon transport mechanisms have also been carried out in parallel to understand the underlying reasons behind the high thermoelectric performance.…”

Rapid One‐Step Synthesis and Compaction of High‐Performance n‐Type Mg3Sb2 Thermoelectrics

“…Recently n‐type Mg 3 Sb 2 ‐based compounds have been intensively studied because of their outstanding TE performance as well as the low‐cost and abundantly available constituent elements . Exceptional n‐type TE properties were first reported in Te‐doped Mg 3+ δ Sb 1.5 Bi 0.5 with a high zT value of about 1.6 at 725 K by Tamaki et al .…”