Exploration and Optimization of Tellurium‐Based Thermoelectrics

12.3.2. Electronic Structure Calculations
In all studied cases, the space group was set to 4/ corresponding to the closest space group
to 4/ where no further symmetry is lost by substituting Sn and Bi atoms on Tl sites. Due to the
lack of Te–Te bonds in the system, the Te atoms can be considered Te 2‐ , while the other atoms can be
treated as the typical Tl + , Sn 2+ and Bi 3+ . This is the same approach taken with literature calculations as
well as those found previously in this section. The calculations in Tl10‐xLaxTe6 [236] utilized a similar
strategy to these calculations and also resulted in p‐type and metallic results.
Figure 12.1 DOS calculations for Tl 9Sn 1‐xBi xTe 6: Tl 8.5SnBi 0.5Te 6 (left), Tl 9Sn 0.5Bi 0.5Te 6 (centre), Tl 8.5Sn 0.5BiTe 6 (right)
The Sn‐heavy model was calculated by replacing one more Tl site with a Sn atom wielding Tl8.5Sn1Bi0.5Te6,
an intrinsic semiconductor with a band gap on the order of 0.2 eV with a similar appearance to the
ternary itself, shown in Figure 10.2; the density of states (DOS ) calculated for x = 0.5, y = 0.5 predicts p‐
type extrinsic semiconductor behaviour with an electron deficiency of ~ 0.15 eV below the band gap,
which is then predicted to be on the order of 0.3 eV; the Bi‐rich model, Tl8.5Sn0.5Bi1Te6, is predicted to
show metallic behaviour as it has no gap between valence and conduction bands. The valence band is
predominantly occupied by Te‐p states.
12.3.3. Physical Property Measurements
12.3.3.1. Tl9Sn1‐xBixTe6
The Seebeck coefficient for Tl9Sn1‐xBixTe6 displays behaviour typical of p‐type semiconductors
with a reasonably high charge carrier concentration. Seebeck coefficient values increase steadily across
the full range of temperature measurement in a constant and linear fashion. The lowest Seebeck
coefficient values correspond to Tl9Sn0.8Bi0.2Te6 with 41 V∙K ‐1 at room temperature and increases to 77
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