Preparation, thermoelectric properties, and crystal structure of boron-doped Mg2Si single crystals

Abstract

Mg2Si is a potential thermoelectric (TE) material that can directly convert waste energy into electricity. In expectation of improving its TE performance by increasing electron carrier concentration, the element boron (B) is doped in Mg2Si single crystals (SCs). Their detailed crystal structures are definitely determined by using white neutron holography and single-crystal x-ray diffraction (SC-XRD) measurements. The white neutron holography measurement proves that the doped B atom successfully substitutes for the Mg site. The SC-XRD measurement confirms the B-doping site and also reveals the presence of the defect of Si vacancy (VSi) in the B-doped Mg2Si SCs. The fraction of VSi increases with increasing B-doping concentration. In the case of B-doped Mg2Si polycrystals (PCs), VSi is absent; this difference between the SCs and PCs can be attributed to different preparation temperatures. Regarding TE properties, the electrical conductivity, σ, and the Seebeck coefficient, S, decreases and increases, respectively, due to the decrease in the electron carrier concentration, contrary to the expectation. The power factor of the B-doped Mg2Si SCs evaluated from σ and S does not increase but rather decreases by the B-doping. The tendencies of these TE properties can be explained by considering that the donor effect of the B atom is canceled by the acceptor effect of VSi for the B-doped Mg2Si SCs. This study demonstrates that the preparation condition of Mg2Si should be optimized to prevent the emergence of an unexpected point defect.

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

“…Comparison of the atomic image with the expected location of Sn and interstitial atoms did not give a reasonable result, similar to the case of the Bdoped Mg 2 Si SC. 26 In other words, the B dopant substituted for the Mg site. It should be noted that the third-nearest Mg atoms were clearly observed relative to the firstand secondnearest ones in Figure 1a.…”

“…Considering the fact that the B-doping to the Mg 2 Si SC induced V Si , 26 it is concluded that the B-doping has the effect of generating vacancy defects in the Mg 2 Sn and Mg 2 Si SCs owing to the increased chemical pressure. It should be noted that the V Mg fraction of the B-doped Mg 2 Sn SCs is much higher than the V Si fraction of the B-doped Mg 2 Si SCs (e.g., 2.1(14)% at x = 0.75% 26 ). The high V Mg fraction could be one reason why the disorder was only observed in the Mg atoms in the B-doped Mg 2 Sn SC presented in Figure 1a.…”

“…was fixed to the nominal composition due to the difficulty in the determination of the small fraction of the light element B. With regard to the TE properties of the B-doped Mg 2 Si SCs, it is reported that the σ decreases as x increases, mainly due to the decrease of n. 26 This is because the V Si of the B-doped Mg 2 Si SCs acts as an acceptor and cancels the donor effect of the B dopant. Similarly, the σ of the B-doped Mg 2 Sn SCs slightly decreased as x increased (Figure 3a), accompanied by the decrease in n (Figure 3b).…”

“…In the case of Mg 2 Si SCs, the B-doping decreases the lattice constant and induces V Si . 26 Thus, in this study, we investigate the effect of B-doping on the crystal structure and TE properties of Mg 2 Sn SCs, to determine whether the B dopant can be utilized for future enhancement of the zT of Sb-doped Mg 2 Sn SC. For the crystal structure analysis, we use white neutron holography, 26−35 a kind of atomic resolution holography that can directly reveal a three-dimensional local structure around a dopant without any structure model.…”

“…50−52 Thanks to the white neutron holography, we successfully revealed that the B-doping site in the Mg 2 Si SCs is the Mg site. 26 Similarly, we determine the Bdoping site in the Mg 2 Sn SCs using the white neutron holography. In addition, we perform SC X-ray diffraction (SC-XRD) to evaluate the V Mg fraction.…”