Chemical-Pressure-Induced Point Defects Enable Low Thermal Conductivity for Mg₂Sn and Mg₂Si Single Crystals

Abstract

The development of thermoelectric (TE) materials, which can directly convert waste heat into electricity, is vital to reduce the use of fossil fuels. Mg 2 Sn and Mg 2 Si are promising TE materials because of their superior TE performance. In this study, for future improvement of the TE performance, point defect engineering was applied to the Mg 2 Sn and Mg 2 Si single crystals (SCs) via boron (B) doping. Their crystal structures were analyzed via white neutron holography and SC X-ray diffraction. Moreover, nanostructures and TE properties of the B-doped Mg 2 Sn and Mg 2 Si SCs were investigated. The B-doping increased the chemical pressure on the Mg 2 Sn and Mg 2 Si SCs, leading to inducing vacancy defects as a point defect. No apparent change was observed in electronic transport, but thermal transport was significantly prevented. This study demonstrates that the vacancy defects can be controlled by the chemical pressure and can aid in achieving high TE performance for the Mg 2 Sn and Mg 2 Si SCs. KEYWORDS: Mg 2 Sn and Mg 2 Si single crystals, boron-doping, chemical pressure, point defects, thermoelectric properties

Realizing p-type Mg2Sn Thermoelectrics via Ga-Doping and Point Defect Engineering

“…In other words, V Mg and Ga atoms are considered to be point defects. The dislocation core with a density lower than ∼3 × 10 16 cm –2 does not effectively decrease κ lat ; ,, thus, phonon scattering by dislocation cores in the Mg 2 Sn 1– x Ga x SCs is neglected in this study (the calculation details for κ el , κ bip , and κ lat can be found in the Supporting Information and the parameters are presented in Tables S3 and S4). The calculated κ lat curve and the measured κ lat values are plotted against Γ as shown in Figure c.…”

“…Figure 3a shows a high-magnification TEM image of the x = 0.005 ingot, which contains stripe patterns, that is, Moirépatterns, corresponding to the V Mg region, where V Mg aggregated as reported for undoped, Sb-doped, and B-doped Mg 2 Sn SCs. 20,22,24 The other regions have few or no V Mg , resulting in a perfect arrangement of the Ga-doped Mg 2 Sn(111) surface, which is called the single-crystal region. V Mg regions were also found in the high-magnification TEM image of the x = 0.01 ingot (Figure S3).…”

“…Furthermore, Ga doping is expected to increase the V Mg fraction, leading to a low κ lat . This expectation is based on our recent research in which we showed that the V Mg fraction increased upon increasing the chemical pressure applied to Mg 2 Sn SCs . When Ga atoms substitute for Sn atoms, the chemical pressure should increase because the smaller atomic radius of Ga (122.1 pm) than that of Sn (140.5 pm) causes negative lattice strain, which decreases the formation energy of V Mg .…”

“…Mg 2 Sn and its derivatives Mg 2 (Si, Ge, Sn), which are composed of cheap, environmentally friendly, and abundant elements, are potential TE materials for operation at moderate temperatures between 400 K and 800 K. ,− Recently, we prepared Mg 2 Sn single crystals (SCs) by melting under an Ar atmosphere, which contained Mg vacancies (V Mg ) as point defects that scattered phonons and gave the prepared Mg 2 Sn SCs a lower κ lat than that of a previously reported Mg 2 Sn polycrystals (PCs) . Such point defect engineering has recently been used to improve the zT value of bulk materials. − We enhanced the n-type characteristics of the Mg 2 Sn SC containing V Mg by increasing its electron carrier concentrations via Sb doping .…”

High power factor in epitaxial Mg2Sn thin films via Ga doping

“…9,10 An effective approach to improve the thermoelectric properties of Mg-based II-IV semiconductors is the combination of single crystals and doping. [11][12][13][14][15][16] In polycrystals, there exist many grain boundaries of large crystallographic angle difference, which can act not only as scatterers of carriers but also as trapping sites. Additionally, they can make interface energy levels that can cause band bending through the pinning effect.…”