Exploration and Optimization of Tellurium‐Based Thermoelectrics

(AgSbTe2)1‐x(GeTe)x that has the same rock salt structure as PbTe for x > 0.8. With x < 0.8, the TAGS
structure transforms into rhombohedral‐type where x = 0.7 is preferential for structure‐formation. [45]
TAGS performs optimally between 0.8 < x < 0.85 [44, 46] and is a popular thermoelectric chalcogenide that
has been used in several of the applications mentioned in 1.3.1.
1.3.3.2. The Low‐Temperature Thermoelectric, CsBi4Te6
In particular, CsBi4Te6 is a well‐known chalcogenide for low‐temperature thermoelectric
applications, as its best performance is at temperatures below 298 K. CsBi4Te6 can be considered a
variant of Bi2Te3 with [Bi4Te6] – anion layers in the structure. It is analogous to localizing one extra
electron around the Bi atoms which in turn will form Bi–Bi bonds (3.24 Å). Alternating between these
slabs are layers of Cs + cations which have atomic displacement parameters iv 1.6 times larger than that of
Bi or Te; evidence for a rattling effect likely further reducing heat transport through the material.
Likewise, the structure – namely the [Bi4Te6] – sheets – is quite anisotropic, having the majority of its
bonds along the b axis and likely functioning with reduced dimensionality for electron flow. [16, 47, 48] The
unit cell of CsBi4Te6 is illustrated in Figure 1.6.
Figure 1.6 Unit cell of CsBi 4Te 6 (Bi–Te bonds – black, Bi–Bi bonds – red).
1.3.3.3. Cage Structures – Clathrates and Skutterudites
Clathrates are well‐known materials that can accept and trap foreign atoms within their large
cage‐like structures. Clathrates, or cage‐structures, comprise a series of 20‐atom dodecahedra and 24‐
atom tetrakaidecahedra, which form around large “rattling” (loosely bound in an oversized
crystallographic site) guest atoms, typically with a cubic space group v 3 for inorganic type I. [51] A
iv That is, the extent and uniformity of atoms’ vibrations in a crystal structure.
v Further explained in 2.1 of this thesis, with details included in most crystallographic textbooks. [49, 50]
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