Exploration and Optimization of Tellurium‐Based Thermoelectrics

Chapter 13. Modifications of Tl9SbTe6 via the addition of Tt 2+
13.1. Introduction
Like the preceding chapters, interest in Tl9SbTe6 led to the investigation of its thermoelectric
properties [232, 250] as well as interest in the subsequent optimization of the properties. Measurements on
the Seebeck coefficient at room temperature for Tl9SbTe6 was initially reported at 90 V∙K ‐1 . [250] The ZT
was first measured as 0.27 at room temperature. [251] The forthcoming investigations can then be split
into two series: Those with Sn‐inclusions and those with Pb‐inclusions, leading to the Tl9SnxSb1‐xTe6 and
the Tl9PbxSb1‐xTe6 formulae respectively. As with the original Bi‐study, one would expect the production
of a p‐type semiconductor with values of x approaching values of 0.1 and beyond. As such,
improvements to the system should be feasible. With the similarities between
Tl9SbTe6/Tl9SnTe6/Tl4SnTe3, the possibility of alloying with mid‐range values of x could produce different
behaviours altogether so studies will involve both large and small values of x.
13.2. Experimental Section
13.2.1.1. Synthesis
All reactions were prepared from the elements (Tl: 99.99 %, granules 1‐5mm, ALDRICH; Sn:
99.999 %, powder ‐100 mesh, ALFA AESAR; Pb: 99.9 %, granules 3 mm, ALFA AESAR; Sb: 99.5 %, powder
‐100 mesh, ALFA AESAR; Te: 99.999 %, ingot, ALDRICH), with total sample masses between 500 mg and
1000 mg for in‐lab samples. The systems were again explored by heating the elements their respective
stoichiometric molar ratios inside glassy carbon crucibles and evacuated (approx. 10 ‐3 mbar) inside silica
tubes. The silica tubes were sealed under a hydrogen‐oxygen flame, and samples were heated in
resistance furnaces to 923 K, followed by slow cooling from 873 to 723 K within five days. Mixed‐phases
were crushed into powder, resealed and annealed at 723 K for between five and ten days; the melting
point of 753 K for Tl9SbTe6 [252] should ensure a suitable annealing temperature. A wide range of x was
initially chosen for the series in order to gain an overall understanding of the phase range as well as the
properties. Therefore 0.0 ≤ x ≤ 0.7 in increments of 0.05 was initially studied followed by increments of
0.025 between 0.0 ≤ x ≤ 0.2 for each series.
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