Exploration and Optimization of Tellurium‐Based Thermoelectrics

Chapter 7. Layered Ternary Structures in (SnTe)x(Bi2Te3)y
7.1. Introduction to Layered Bismuth Tellurides
Since the discovery in the 1950s of bismuth telluride’s thermoelectric potential, researchers
have been attempting to understand why Bi2Te3 is such an effective TE material and expanding on ways
to further improve it. With all of the studies performed, Bi2Te3 and its composites warrant a book of
their own. Now with respect to thermoelectrics, one can consider such contributions as solid solution
work like (Sb,Bi)2Te3 or Bi2(Se,Te)3, [13] extensive studies on doping, [166‐168] crystal structures, [169]
calculations, [170, 171] phase diagrams and of course, physical properties such as Seebeck coefficient and
charge carrier concentration (Hall measurements). [81, 166, 167] Recent efforts towards nanoscopic studies
have produced thin films, nanowires, nanocrystals and numerous other property‐related studies. [172‐175]
In this chapter, new compounds with similar features are formed based on the parent Bi2Te3 structure
by combining it with other (in this case) binaries expecting the child compounds to show similar
characteristics and therefore similar, possibly enhanced physical properties.
Bi2Te3 is a layered compound of the rhombohedral space group 3. Forming perpendicular to
the c‐axis, are alternating layers of covalently bonded Bi and Te atoms in five segments (a block) starting
[169, 176]
and ending with Te: Te–Bi–Te–Bi–Te. The Bi–Te bond lengths in this layer are 3.07 and 3.26 Å.
The interactions between blocks are predominantly van der Waals forces. A Te–Te van der Waals
interaction of 3.65 Å connects the blocks in a stacked structure. This can be seen on the left side of
Figure 7.1 displayed below.
The search for related layered compounds led to investigations into the (TtQ)x(Pn2Q3)y (Tt = Ge,
Sn, Pb; Pn = Sb, Bi; Q = Se, [177] Te) materials, where x and y are integers. The resulting studies uncover a
series of increasingly longer unit cells with similar layering to that of bismuth telluride. Further
discussion regarding layering in the crystal structures is discussed below. It has since been postulated
that the lead‐based system’s phase diagram [178, 179] includes (PbTe)‐(Bi2Te3), (PbTe)‐(Bi2Te3)2, (PbTe)‐
(Bi2Te3)3, (PbTe)2‐(Bi2Te3) and arguably (PbTe)‐(Bi2Te3)4 which can be written as proper ternary
compounds: PbBi2Te4, PbBi4Te7, PbBi6Te10, Pb2Bi2Te5, and PbBi8Te13 respectively. Due to the extensive
layering along the c‐axis and the widespread occurrence of heavy p‐block elements, said structures may
have potential for thermoelectric applications. [16] Though crystal structures have been predicted for this
system, ordering beyond PbBi2Te4 cannot be verified due to the indistinguishable nature of Pb versus Bi
in XRD studies – a feature that could be more apparent in the Sn‐Bi‐Te system. Though many of the
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