Resonant bonding, multiband thermoelectric transport and native defects in n-type BaBiTe_(3-x)Se_x (x = 0, 0.05 and 0.1)

Abstract

The unique crystal structure of BaBiTe_3 containing Te···Te resonant bonds and its narrow band gap motivated the systematic study of the thermoelectric transport properties of BaBiTe_(3–x)Se_x (x = 0, 0.05, and 0.1) presented here. This study gives insight in the chemical bonding and thermoelectric transport properties of BaBiTe_3. The study shows that the presence of Te···Te resonant bonds in BaBiTe_3 is best described as a linear combination of interdigitating (Te^(1–))_2 side groups and infinite Te_n 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 BaBiTe_(3–x)Se_x (x = 0, 0.05, and 0.1). The carrier concentration of BaBiTe_3 can be tuned via Se substitution (BaBiTe_(3–x)Se_x with x = 0, 0.05, and 0.1) to values near those needed to optimize the thermoelectric performance. The thermal conductivity of BaBiTe_(3–x)Se_x (x = 0, 0.05, and 0.1) is found to be remarkably low (ca. 0.4 Wm^(–1)K^(–1) at 600 K), reaching values close to the glass limit of BaBiSe_3 (0.34 W m^(–1) K^(–1)) and BaBiTe_3 (0.28 W m^(–1) K^(–1)). Calculations of the defect formation energies in BaBiTe_3 suggest the presence of native Bi_(Ba)^(+1) and Te_(Bi)^(+1) antisite defects, which are low in energy and likely responsible for the native n-type conduction and the high carrier concentration (ca. 10^(20) cm^(–3)) found for all samples. The analyses of the electronic structure of BaBiTe_3 and of the optical absorption spectra of BaBiTe_(3–x)Se_x (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 BaBiTe_3. 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 BaBiTe_3 and BaBiTe_(2.95)Se_(0.05). This multiband contribution can account for the ∼50% higher zT_(max) of BaBiTe_3 and BaBiTe_(2.95)Se_(0.05) (∼0.4 at 617 K) compared to BaBiTe_(2.9)Se_(0.1) (∼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 BaBiTe_(2.9)Se_(0.1).

Files

Additional details