Exploration and Optimization of Tellurium‐Based Thermoelectrics

common stoichiometry studied with clathrates is A8Tr16Tt30 (A = Sr, Ba; Tr = Al, Ga; Tt = Si, Ge). These
compounds are usually narrow band‐gap semiconductors, which can generally be improved by
optimizing the electrons near the Fermi level: minor variations in the valence electrons associated with
different guest atoms and cage‐vacancies therein. Clathrate uniqueness arises from a thermal
conductivity (1.2.3) < 1 W∙m ‐1 K ‐1 , known to be diminished by the rattling of guest atoms in the oversized
cage. [52] This is an excellent example of a phonon glass, or a material that relentlessly scatters phonons.
The crystal structure of Ba8Ga16Si30 is shown in Figure 1.7 (a).
Figure 1.7 Unit cells for (a) Clathrate Ba 8Ga 16Si 30 (E = Ga, Si) (b) Skutterudite LaFe 4Sb 12 & LaSb 12 icosahedron [52]
Mo3Sb7, [53] a Zintl compound vi,[50] (i.e. the ZT > 1 compound Yb14MnSb11 [54] ) was discovered as a
promising thermoelectric by the Kleinke group. [55] A related large family of compounds is termed
skutterudites (Figure 1.7 (b)) – named after Skutterud, Norway where the aristotype CoAs3 was discovered.
The aristotype crystallized in cubic symmetry as 3 and similar to the clathrates, skutterudites are
known for forming complex cage structures with numerous Sb–Sb interactions including structures
LaFe4Sb12 [56] and ‐Zn4Sb3. [57] Unlike clathrates however, strong guest‐host coupling in skutterudites
likely rules out any rattling effects, better demonstrating electron crystal – phonon crystal
characteristics instead of PGEC concepts (1.1). [52, 58] Skutterudites are promising enough to be
engineered into TEGs between 5 and 100 kW power generation in automobile tests. [42] While quite
similar to the skutterudite family, Mo3Sb7 does not have the filled CoAs3 structure necessary for
classification under the same name. The Mo3Sb7 structure comprises MoSb8 square antiprisms which
include Mo–Mo interactions and empty Sb8 cubes that are connected via Sb–Sb bonds to the next
square antiprisms – a representative of the Ir3Ge7 (3) structure‐type. [53] The tuning of this Ir3Ge7
compound, with respect to its thermoelectric properties, commenced in 1997 with experiments
involving substitution of Te for Sb in the structure. [55] For further attempted improvement of the
vi Compounds with polyanionic or polycationic bonds formed to complete the octet rule with counter ions.
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