Anharmonic thermal motion of atoms in thermoelectric Mg2Si studied via convergent-beam electron diffraction

Abstract

We study the thermal motion of the atoms in the thermoelectric material Mg 2 Si by using electron diffraction. The nanodiffraction or convergent-beam electron diffraction technique we use allows us to observe simultaneously many Bragg reflections with large reciprocal lattice vectors. Previous observations of anharmonicity by singlecrystal x-ray diffraction are confirmed, and we determine the anharmonicity parameter of the Mg atom in the tetrahedral environment with high accuracy, β = (−4.27 ± 0.14) × 10 −12 ergÅ −3 around room temperature. In an alternative picture the Mg atom tends to vibrate around positions displaced 4.50 ± 0.14 pm from the center of the tetrahedron.

Nano- and Microstructure Engineering: An Effective Method for Creating High Efficiency Magnesium Silicide Based Thermoelectrics

“…The larger molar weight results in the reduction of the specific heat from 0.90 J g –1 K –1 for Mg 2 Si to 0.44 J g –1 K –1 for Mg 2 Sn, , which is partially offset by the increase in density. Finally, the anharmonic thermal motion of atoms, , the lattice strain , and the increased number of grain boundaries through ball milling could potentially scatter phonons to further reduce the lattice thermal conductivity.…”