Exploration and Optimization of Tellurium‐Based Thermoelectrics

The calculations on the ternary tellurides were first performed assuming no Sn‐Bi mixing. This
produces a similar model to the structure motifs given in the introduction section and is consistent for
all four calculated structures. Likewise, the calculated band gaps are comparable, with SnBi2Te4 showing
a gap of 0.48 eV, SnBi4Te7 showing a gap of 0.35 eV, SnBi6Te10 showing a gap of 0.61 eV, and Sn2Bi2Te5
having the largest calculated gap with 0.64 eV. The Te‐p orbitals in each case make up the majority of
the contributions below the Fermi level followed by Bi‐s and Sn‐s respectively – as above, the extra
contribution discussed by Imai and Watanabe [201] in PbBi4Te7. The conduction band is a broad mixture of
Sn‐p, Bi‐p, and Te‐p orbitals which due to covalent interactions, are also strongly present in the valence
band. As previously mentioned, sharp increases in the DOS at the Fermi level can be beneficial for
thermoelectricity which would suggest from these initial calculations that SnBi4Te7 is a very good choice
with its 0.35 eV band gap and very steep valence bands at the Fermi level. The ternaries themselves do
not appear to have any unique vacant states in or near the band gap such as orbitals pinned at the Fermi
level, [204] which may have led to further property improvements and/or interesting behaviours. As can
be seen from the DOS calculations performed (Figure 7.4), removal of a small quantity of electrons from
the stoichiometric ternaries should lead to the Fermi level being placed on a desirably sharp DOS
region:
Figure 7.4 DOS calculations for (a) SnBi 2Te 4, (b) SnBi 4Te 7, (c) SnBi 6Te 10, and (d) Sn 2Bi 2Te 5.
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