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All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on Mg3Bi2 by Materials Project

Mg3Bi2 crystallizes in the trigonal P-3m1 space group. The structure is three-dimensional. there are two inequivalent Mg sites. In the first Mg site, Mg is bonded to four equivalent Bi atoms to form distorted MgBi4 tetrahedra that share corners with six equivalent MgBi6 octahedra, corners with six equivalent MgBi4 tetrahedra, edges with three equivalent MgBi6 octahedra, and edges with three equivalent MgBi4 tetrahedra. The corner-sharing octahedra tilt angles range from 9–59°. There are three shorter (2.95 Å) and one longer (3.04 Å) Mg–Bi bond lengths. In the second Mg site, Mg is bonded to six equivalent Bi atoms to form MgBi6 octahedra that share corners with twelve equivalent MgBi4 tetrahedra, edges with six equivalent MgBi6 octahedra, and edges with six equivalent MgBi4 tetrahedra. All Mg–Bi bond lengths are 3.18 Å. Bi is bonded to seven Mg atoms to form a mixture of distorted corner and edge-sharing BiMg7 pentagonal bipyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Compromise between band structure and phonon scattering in efficient n-Mg 3 Sb 2-x Bi x thermoelectrics

n-type Mg 3 Sb 2 -based materials have become a top candidate for efficient thermoelectric applications within 300–700 K, due to its high band degeneracy, inherently high carrier mobility and low lattice thermal conductivity, as well as its advantages of less toxicity and abundance. Existing works showed that Mg 3 Bi 2 -alloying largely help ensure the exceptional performance, leaving a key issue to be uncovered on the primary mechanisms favoring or limiting the thermoelectric performance of Mg 3 Sb 2-x Bi x alloys. Furthermore we focus on the alloy composition dependent transport properties at various temperatures, with a large volume of experimental data. It is revealed that, with increasing x, the reduction in both inertial mass and lattice thermal conductivity is significantly beneficial, but the closure in band gap leads to a strong compensation due to the bipolar effect. Such a compromise between band structure and phonon scattering results in optimal Mg 3 Bi 2 -alloying concentrations to be about 50%–75% at 300 K, 50%–60% at 450 K and 50% at 600 K, which successfully guiding this work to realize extraordinary thermoelectric figure of merit at these temperatures.

36 MATERIALS SCIENCE↗