Exploration and Optimization of Tellurium‐Based Thermoelectrics

Chapter 9. Complex Layered Materials SnBi4Te7 and SnBi6Te10
Much like their SnBi2Te4 counterpart, SnBi4Te7 and SnBi6Te10 have aroused interest in the
thermoelectric field for their likeness to Bi2Te3 and complex layering motifs. Whist the point was made
in Chapter 7, their mysterious formation scheme makes the syntheses challenging – hence the
available literature only pertains to phase ranges and diagrams. Nevertheless it was shown that both of
these compounds could be produced phase‐pure and each had measureable properties comparable to
each other. With respect to the individual compounds, do their more complex layering arrangements
benefit more suitably from doping/substitution experiments than SnBi2Te4?
9.1. Synthesis and Structural Analysis
Compounds were synthesized according to 7.2 and were weighed to a minimum precision of
±0.0005 g. While it was necessary to optimize the heating conditions for these systems, the smaller
phase ranges make stability and pure phase samples difficult to achieve. All layered compounds were
heated to 923 K to ensure a homogeneous melt followed by their respective optimizations: SnBi4Te7
was cooled to 823 K quickly followed by annealing for 100 hours, then it was cooled to room
temperature, crushed, and returned to 823 K for another 100 hours – this strategy produced
approximately 75 % pure phase samples with an additional anneal often required; SnBi6Te10 was cooled
from 843 K to 823 K over 100 hours for elemental syntheses, while mixed‐phase samples were crushed
and heated at 798 K for again ~100 hours – these attempts produced pure‐phase samples for ~33 % of
the cases. Doping reactions on the Sn site were tried using Ga and In with 0.01 ≤ x ≤ 0.15. Substitution
of some Bi atoms was also tried with Ga, In, Tl, Nb, and Ta with 0.00 ≤ x ≤ 0.25 in increments of 0.05,
including additional reactions 0.01, 0.02, …, 0.04.
If designated pure phase via XRD, selected samples were examined by DSC/TG and EDX. For
example the following data over several crystalline spots were collected: Ga0.05Sn0.95Bi4Te7 (0.5, 8.9, 32.9,
57.7 at‐%). It was found in several cases that Ga was over‐estimated by 3 or 4 at‐% likely due to
integration errors (i.e. small Ga‐Ka with Bi‐Ll and augmented background). In the case of the attempted
substitution with [V] elements Nb and Ta, the corresponding binaries NbTe2 and TaTe2 were found in
most reactions. As such no further discussion regarding [V]x…Bin‐x… will ensue.
Attempts at growing suitable single crystal samples for examination via CCD were unsuccessful
for both the SnBi4Te7 and SnBi6Te10 compounds due to the excessive twinning of the plate‐like crystals
and the poor quality of the plates emerging from the reaction vessels. Though every strategy explained
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