Exploration and Optimization of Tellurium‐Based Thermoelectrics

produced mixtures of both rhombohedral and cubic structures. Having x < 0.5, especially with large
values of y, additionally produced the binary Cu2Te, which was also noticed as a side product in the
reaction with x = 1 and y = 2. During this study, the Ba3Cu17‐xS11.5‐yTey phase was treated as
Ba3Cu17‐xS11‐yTey, as the slightly different Ba:Q ratio (3 : 11.5 = 0.26 vs. 3 : 11 = 0.27) was not a
determining factor. Cu deficiencies are very common among Cu chalcogenides, likely due to the
tendency to mixed valent Cu, so that x has to be > 0 is not surprising.
A differential scanning calorimetry, DSC, measurement (5.3) performed under argon flow,
revealed Ba3Cu16.5S4Te7 to be stable until approximately 900 K. There were no additional peaks
encountered and the TG revealed that its mass was stable throughout the measurement range.
16.2.1.2. Analysis
To prove the absence of heteroelements such as silicon stemming from the reaction container,
two samples of nominal composition Ba3Cu17S8Te3 (rhombohedral variant) and Ba3Cu17S5Te6 (cubic
variant) were analyzed via energy dispersive analysis of X‐rays (6.2). The scans were performed with an
acceleration voltage of 20 kV under high dynamic vacuum. No heteroelements were detected during
the examination. The obtained Ba : Cu : S : Te ratios were reasonable with respect to the accuracy of
the technique, yielding 9 : 57 : 29 : 6 at‐%, as averaged over six crystals, compared to the nominal 9.7 :
54.8 : 25.8 : 9.7 in case of Ba3Cu17S8Te3, and 10 : 54 : 18 : 18 averaged over four crystals, compared to
the nominal 9.7 : 54.8 : 16.1 : 19.4 in case of Ba3Cu17S5Te6.
16.2.1.3. Structure Determination
X‐ray single crystal structure studies were performed on black block‐like single crystals. The
Bruker CCD diffractometer (4.2) was used for the data collections, performed by scans of 0.3° in in
two groups of 600 frames (each with an exposure time of 60 seconds) at = 0° and 90°. The data were
corrected for Lorentz and polarization effects. Absorption corrections were based on fitting a function
to the empirical transmission surface as sampled by multiple equivalent measurements using SADABS
incorporated into the APEX II package. [127]
The structure solution and refinements were carried out with the SHELXTL program package. [131]
The lattice parameters indicated either rhombohedral or cubic symmetry, depending on the S:Te ratio.
Using a higher S than Te amount yields rhombohedral symmetry, to which no additional systematic
absences were identified, leaving 3 as the space group of highest symmetry, as in
Ba3Cu17‐x(Se,Te)11. [276] Thus, that model was used as a starting point for the refinements. As in case of
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