Exploration and Optimization of Tellurium‐Based Thermoelectrics

however was successfully analyzed from an exploratory synthesis attempt. The data collections were
carried out on a BRUKER SMART APEX CCD at room temperature utilizing Mo‐Kα radiation. The crystal
was picked from a reaction of attempted TlSnBi3Te6 stoichiometry. Data was collected by scans of 0.3°
in in at least two blocks of 900 frames at = 0° and = 120°, with exposure times of 40 seconds per
frame yielding 1893 used reflections between 5.9 and 59.8 2. The data was corrected for Lorentz and
polarization effects.
Structure refinements were performed with the SHELXTL package [130] and completed using the
atomic positions published for SnBi2Te4 [193] assuming mixed Tl/Sn/Bi occupancies on the 3a site and 6c
site, with Tl:Bi based on EDX data due to their indistinguishable scattering. Attempts to fix Sn on the 3a
site of the crystal resulted in a negative occupancy factor for Sn, such that the site (Table 8.4) was
assumed to include heavier atoms. Refinements converged smoothly without showing any
abnormalities.
Using powder data and Rietveld refinements modeled from the early reports of SnBi2Te4 [193]
where identical quantities of Sn and Bi were mixed on both the 3a (i.e. “Sn site”) and 6c (i.e. “Bi site”)
evenly such that [Sn] = 33.3 % and [Bi] = 66.7 % on both sites. Three different refinement scenarios (in
Table 8.1) converged successfully with RP\RB values between 0.066\0.064 and 0.0492\0.0461 with both
Te sites fixed at 100% occupancy, assuming no deficiencies on any site. In some cases, the current
model caused the Ueq values to converge to negative values – namely M(2) and Te(1) – but to fix only
these values would result in a negative Ueq for M(1) and in some cases also for Te(2). Thus, for the sake
of this comparison, all Ueq were fixed at 0.025 Å; this action still yielded similar and acceptable R values.
Fixing M(1) as Sn and M(2) as Bi yielded reasonable ~6.5 % R values, but better R values were obtained
by allowing the originally published quantity of mixing (i.e. 33.3 % Sn on both M sites) and onward to
higher quantities of Sn on the M(1) site. The best R values were not surprisingly found after allowing
the occupancies of both M sites to refine, but serves as a thought exercise since the stoichiometry is far
beyond 1 Sn : 2 Bi.
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