Exploration and Optimization of Tellurium‐Based Thermoelectrics

scheme involving a homogeneous melt at 923 K, slow cooling between 873 and 773 K, followed by
crushing and annealing at 798 K for ten days was only successful for the original ternary compound.
Further optimization experiments on substitution samples, namely x = 0.1, 0.15, 0.20 in GaxSn1‐xBi6Te10,
led to nearly pure phase results in a shaker furnace with a homogeneous melt at 923 K followed by four
days at 843 K, then four days at 798 K. Those samples with fewer than 5% additional phases were also
examined via EDX. All spots were found to contain all expected elements within acceptable elemental
percentages, supporting the hypothesis of a uniform and homogeneous composition. In0.15SnBi5.85Te10,
for example averaged 3, 6, 35, and 56 % respectively over eight crystals.
9.3.2. Electronic Structure Calculations
Standard calculations with the LMTO software were performed with SnBi6Te10 on select
variations. Unlike the other compounds however, supercells were restricted by the fact that calculating
with c = 102 Å was on the limit of the program’s capabilities. Attempts to double the unit cell along the
a or b axis were nevertheless successful yielding calculations in 2/ or symmetries for even or odd
number of triel atoms added respectively. Calculations were performed on a basis of 1000 evenly
dispersed k‐points with Tr = Ga, In, and Tl. Selected calculation results are shown below:
Figure 9.9 DOS calculations on SnBi 6Te 10 variants: (a) Ga 0.5Sn 0.5Bi 6Te 10, (b) Ga 0.5SnBi 5.5Te 10, (c) Tl 0.5SnBi 5.5Te 10, (d) TlSnBi 5Te 10.
While similar in nature to SnBi6Te10, all quaternary compounds calculated show extra states at
the top of the valence band from the added triel elements while little changes occur in the conduction
band. Ga doping and substitution respectively show extra orbital states in the valence band shrinking
the band gap and predicting a more p‐type material. Tl – the large extreme of the triel elements –
shows the extra states occurring inside the band gap, displaying increasing breadth as the Tl content
increases. This again implies the added impurity states for the heavier triels appear broad compared to
110