Exploration and Optimization of Tellurium‐Based Thermoelectrics

Chapter 8. Tuning of the Physical Properties for SnBi2Te4
Zhukova et al. [193] discovered the crystal structure of SnBi2Te4 in 1971, solved in 3 having a
unit cell of 4.41×4.41×41.51 Å 3 . This structure is a layered variant of the Bi2Te3‐type crystal structure,
shown previously. Placing the Sn atoms on the 0 0 0 Wyckoff position (3a) yields the diagram below
(Figure 8.1). Bi sits on the 0 0 0.4278 (6c) site and Te occupies two sites: 0 0 0.136 and 0 0 0.29, both 6c.
As can be seen, the unit cell is comprised of covalently bonded sections separated by van der Waals gaps
(dTe–Te = 3.6 Å) with Sn atoms in the centre of each block. Due to each block occurring as alternating cis
then trans, each unit cell is comprised of three blocks. While several models propose a mixed
occupancy between Sn and Bi, data has been solved in both for example PbBi2Te4 where
indistinguishable Pb, is usually fixed on the 3a position. Shown in Figure 8.1 below is a representation of a
triple unit cell along a:
Figure 8.1 Triple unit cell of SnBi2Te4 along a.
Both crystal structure and physical properties of SnBi2Te4 have been studied to certain extents
by a variety of research groups. Most recently, detailed melting studies predict SnBi2Te4 melts
eutectically at 840 K at composition of approximately 45 % SnTe : 55 % Bi2Te3. [181] Additionally, studies
[178, 180, 193]
have explored the phase diagram of the system with attempts at indexing and unit cell size;
thermoelectric properties of the stoichiometric ternary were discussed in Chapter 7; [179, 183, 188, 194] and
electron microscope studies on alloying, solubility, and precipitates within the SnTe/PbTe‐Bi2Te3 system
[192, 206]
have been undertaken.
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