The unique crystal structure of BaBiTe3 containing Te···Te resonant bonds and its narrow band gap motivated the systematic study of the thermoelectric transport properties of BaBiTe3-xSex (x = 0, 0.05, and 0.1) presented here. This study gives insight in the chemical bonding and thermoelectric transport properties of BaBiTe3. The study shows that the presence of Te···Te resonant bonds in BaBiTe3 is best described as a linear combination of interdigitating (Te1-)2 side groups and infinite Ten chains. Rietveld X-ray structure refinements and extrinsic defect calculations reveal that the substitution of Te by Se occurs preferentially on the Te4 and Te5 sites, which are not involved in Te···Te bonding. This work strongly suggests that both multiband effects and native defects play an important role in the transport properties of BaBiTe3-xSex (x = 0, 0.05, and 0.1). The carrier concentration of BaBiTe3 can be tuned via Se substitution (BaBiTe3-xSex with x = 0, 0.05, and 0.1) to values near those needed to optimize the thermoelectric performance. The thermal conductivity of BaBiTe3-xSex (x = 0, 0.05, and 0.1) is found to be remarkably low (ca. 0.4 Wm-1K-1 at 600 K), reaching values close to the glass limit of BaBiSe3 (0.34 W m-1 K-1) and BaBiTe3 (0.28 W m-1 K-1). Calculations of the defect formation energies in BaBiTe3 suggest the presence of native BiBa+1 and TeBi+1 antisite defects, which are low in energy and likely responsible for the native n-type conduction and the high carrier concentration (ca. 1020 cm-3) found for all samples. The analyses of the electronic structure of BaBiTe3 and of the optical absorption spectra of BaBiTe3-xSex (x = 0, 0.05, 0.1, and 3) strongly suggest the presence of multiple electron pockets in the conduction band (CB) in all samples. These analyses also provide a possible explanation for the two optical transitions observed for BaBiTe3. High-temperature optical absorption measurements and thermoelectric transport analyses indicate that bands higher in the conduction band converge with the conduction band minimum (CBM) with increasing temperature and contribute to the thermoelectric transport properties of BaBiTe3 and BaBiTe2.95Se0.05. This multiband contribution can account for the ∼50% higher zTmax of BaBiTe3 and BaBiTe2.95Se0.05 (∼0.4 at 617 K) compared to BaBiTe2.9Se0.1 (∼0.2 at 617 K), for which no such contribution was found. The increase in the band offset between the CBM and bands higher in the conduction band with respect to the selenium content is one possible explanation for the absence of multiband effects in the thermoelectric transport properties of BaBiTe2.9Se0.1.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)
- Materials Chemistry