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Prediction of Giant Thermoelectric Power Factor in Type-VIII Clathrate Si 46

View Article: PubMed Central - PubMed

ABSTRACT

Clathrate materials have been the subject of intense interest and research for thermoelectric application. Nevertheless, from the very large number of conceivable clathrate structures, only a small fraction of them have been examined. Since the thermal conductivity of clathrates is inherently small due to their large unit cell size and open-framework structure, the current research on clathrates is focused on finding the ones with large thermoelectric power factor. Here we predict an extraordinarily large power factor for type-VIII clathrate Si46. We show the existence of a large density of closely packed elongated ellipsoidal carrier pockets near the band edges of this so far hypothetical material structure, which is higher than that of the best thermoelectric materials known today. The high crystallographic symmetry near the energy band edges for Si46-VIII clathrates is responsible for the formation of such a large number of carrier pockets.

No MeSH data available.


(a) Crystal structure of the type-VIII clathrate Si46 in real space. (b) Brillouin zone of the Si46 type- VIII clathrate showing the hole pockets at Γ = (0, 0, 0) point (brown), on the ΓH line (violet), on the NH line (green), at P = (1/4, 1/4, 1/4) point (blue), and at N = (1/2, 0, 0) points (red). The valley degeneracies for Γ, N, P, ΓH and NH are 1, 6, 2, 6, and 12, respectively. (c) The predicted conduction and valance band structures and the densities of states. The numbers with arrows indicate the multiplicity of each extrema. The ellipsoidal curvatures do not present the actual effective masses. The mass values are presented in the supplementary file.
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f1: (a) Crystal structure of the type-VIII clathrate Si46 in real space. (b) Brillouin zone of the Si46 type- VIII clathrate showing the hole pockets at Γ = (0, 0, 0) point (brown), on the ΓH line (violet), on the NH line (green), at P = (1/4, 1/4, 1/4) point (blue), and at N = (1/2, 0, 0) points (red). The valley degeneracies for Γ, N, P, ΓH and NH are 1, 6, 2, 6, and 12, respectively. (c) The predicted conduction and valance band structures and the densities of states. The numbers with arrows indicate the multiplicity of each extrema. The ellipsoidal curvatures do not present the actual effective masses. The mass values are presented in the supplementary file.

Mentions: Figure 1(a–c) presents the crystal lattice structure, the Brillouin zone with an interestingly large density of carrier pockets near the valance band edge, the electronic band structure and the density of states of this clathrate. The band structure is complex with multiple extrema in both the valence and conduction bands. Hence, the DOS of Si46-VIII varies with a larger slope near the band edges, for example, compared with that of the diamond structured Si. Figure 1-(b) shows the Brillouin zone of this material with degenerate hole pockets at the Γ, N, and P points and along the ΓH and NH lines. The figure presents 6 pockets along the ΓH line which are completely inside the Brillouin zone and 24 half-pockets along the NH line (green). Moreover, it predicts one pocket at the Γ = (0, 0, 0) point, 8 quarter-pockets at P = (1/4, 1/4, 1/4) points and 12 half-pockets at N = (1/2, 0, 0) points. Therefore, the degeneracies of the Γ, N, and P points and along the ΓH and NH lines are 1, 6, 2, 6, and 12, respectively, which add up to 27. This is significantly higher than that of the best thermoelectric materials known so far. For comparison, this number for (Bi, Sb)2Te3 p-type TE material is 18. It is notable that according to the group theory considerations, the maximum achievable Nv for extrema points in a band structure is 48 which can happen in cubic crystals43.


Prediction of Giant Thermoelectric Power Factor in Type-VIII Clathrate Si 46
(a) Crystal structure of the type-VIII clathrate Si46 in real space. (b) Brillouin zone of the Si46 type- VIII clathrate showing the hole pockets at Γ = (0, 0, 0) point (brown), on the ΓH line (violet), on the NH line (green), at P = (1/4, 1/4, 1/4) point (blue), and at N = (1/2, 0, 0) points (red). The valley degeneracies for Γ, N, P, ΓH and NH are 1, 6, 2, 6, and 12, respectively. (c) The predicted conduction and valance band structures and the densities of states. The numbers with arrows indicate the multiplicity of each extrema. The ellipsoidal curvatures do not present the actual effective masses. The mass values are presented in the supplementary file.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5382702&req=5

f1: (a) Crystal structure of the type-VIII clathrate Si46 in real space. (b) Brillouin zone of the Si46 type- VIII clathrate showing the hole pockets at Γ = (0, 0, 0) point (brown), on the ΓH line (violet), on the NH line (green), at P = (1/4, 1/4, 1/4) point (blue), and at N = (1/2, 0, 0) points (red). The valley degeneracies for Γ, N, P, ΓH and NH are 1, 6, 2, 6, and 12, respectively. (c) The predicted conduction and valance band structures and the densities of states. The numbers with arrows indicate the multiplicity of each extrema. The ellipsoidal curvatures do not present the actual effective masses. The mass values are presented in the supplementary file.
Mentions: Figure 1(a–c) presents the crystal lattice structure, the Brillouin zone with an interestingly large density of carrier pockets near the valance band edge, the electronic band structure and the density of states of this clathrate. The band structure is complex with multiple extrema in both the valence and conduction bands. Hence, the DOS of Si46-VIII varies with a larger slope near the band edges, for example, compared with that of the diamond structured Si. Figure 1-(b) shows the Brillouin zone of this material with degenerate hole pockets at the Γ, N, and P points and along the ΓH and NH lines. The figure presents 6 pockets along the ΓH line which are completely inside the Brillouin zone and 24 half-pockets along the NH line (green). Moreover, it predicts one pocket at the Γ = (0, 0, 0) point, 8 quarter-pockets at P = (1/4, 1/4, 1/4) points and 12 half-pockets at N = (1/2, 0, 0) points. Therefore, the degeneracies of the Γ, N, and P points and along the ΓH and NH lines are 1, 6, 2, 6, and 12, respectively, which add up to 27. This is significantly higher than that of the best thermoelectric materials known so far. For comparison, this number for (Bi, Sb)2Te3 p-type TE material is 18. It is notable that according to the group theory considerations, the maximum achievable Nv for extrema points in a band structure is 48 which can happen in cubic crystals43.

View Article: PubMed Central - PubMed

ABSTRACT

Clathrate materials have been the subject of intense interest and research for thermoelectric application. Nevertheless, from the very large number of conceivable clathrate structures, only a small fraction of them have been examined. Since the thermal conductivity of clathrates is inherently small due to their large unit cell size and open-framework structure, the current research on clathrates is focused on finding the ones with large thermoelectric power factor. Here we predict an extraordinarily large power factor for type-VIII clathrate Si46. We show the existence of a large density of closely packed elongated ellipsoidal carrier pockets near the band edges of this so far hypothetical material structure, which is higher than that of the best thermoelectric materials known today. The high crystallographic symmetry near the energy band edges for Si46-VIII clathrates is responsible for the formation of such a large number of carrier pockets.

No MeSH data available.