Exploration and Optimization of Tellurium‐Based Thermoelectrics

were inconclusive, this implies the sample may have decomposed into SnBi6Te10/SnBi4Te7 and Bi2Te3 –
hence the much lower 665 K peak – after melting. EDX data was also collected, verifying the
stoichiometric ratios for samples within an acceptable ±5 % of predicted elemental ratios. Each
compound studied in the (SnTe)x(Bi2Te3)y ternary system can be found recorded in Table 7.1 along with
XRD data after reheating samples and results following initial cooling schemes. As shown in the table
above, the compounds in this system were expanded to encompass (SnTe)1(Bi2Te3)4 and (SnTe)3(Bi2Te3)1.
The number of phases encountered experimentally increased with the layering complexity of the target
compound, so for example SnBi2Te4 could be obtained with relative ease while SnBi6Te10 would rarely be
encountered without two or more additional phases.
It has been hypothesized that x > y would not yield ternary compounds unless Tt = Ge; [179] GeTe
undergoes a phase transition at 700 K [196, 197] between the high temperature β‐phase (3) and the
low temperature α‐phase (31), while the other TtTe phases remain cubic above room temperature.
Studies in this work however have produced single‐phase p‐XRD patterns for Sn2Bi2Te5, fit with a
Ge2Bi2Te5 (31) pattern [198] during phase studies. Sn2Bi2Te5 was produced stoichiometrically, but can
also be produced with slight Bi‐deficiencies as shown in Table 8.2 during a phase range study on
SnxBi3‐xTe4 where, for example, Sn2Bi1.75Te5 (aka. Sn1.6Bi1.4Te4) was found pure phase as well. Differential
scanning calorimetry on the same sample displays a sharp single peak for melting at 863 K with no gains
or losses in sample mass. LeBail refinements on a pure phase sample produced lattice parameters of a =
4.42 Å, c = 17.6 Å, V = 298 Å 3 , based on data published by Matsunaga et al. on Ge2Bi2Te5; [198] attempts at
a following Rietveld refinement were largely unsuccessful as nearly all atoms’ temperature factors (Uiso)
converged negatively despite attempts involving fully ordered metal atoms and mixing metal sites
separately. Better results are still gained with mixing Sn/Bi as was suggested with the authors of the Ge
model, having ~60 % Ge on one site and ~40 % Ge on the alternate. This, along with LeBail refinements
for other pure phase compounds in this series, is summarized in Table 7.2. In most cases, attempts at a
full Rietveld refinement were problematic. Though successful, SnBi2Te4 and SnBi4Te7 will be discussed in
the following chapters respectively. SnBi6Te10, showed distinct software problems due to its most
accurate model having c 120 Å, – the LeBail refinement was eventually successful with high damping
and fewer profile parameters freed in GSAS. Likewise, this model is only hypothetical and detailed
crystallographic data is currently unavailable.
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