Evolution of thermoelectric properties of SnBi2Te4 via same lone-pair doping by Sb; Carrier mobility preservation and optimization

초록

Ternary layered chalcogenides belonging to the SnTe center dot Bi2Te3 homologous series, particularly the septuplelayered SnBi2Te4 are room-temperature thermoelectric materials due to their intrinsically low lattice thermal conductivity and tunable narrow-band-gap electronic structure. However, the thermoelectric performance of SnBi2Te4 is limited by an excessive hole concentration arising from native defects, while conventional substitutional doping typically degrades carrier mobility. In this study, a series of Sb-doped SnBi2Te4 samples of Sn1xSbxBi2Te4 (x = 0, 0.015, 0.030, 0.045, 0.060) was systematically investigated to examine a same-lone-pair (5 s2) doping strategy based on nominal Sb substitution at the Sn site in SnBi2Te4. The rhombohedral SnBi2Te4 structure was retained across the doping range. At 300 K, the Hall carrier concentration nH decreased from 1.96 & times; 1020 cm- 3 for the pristine sample to 5.22 & times; 1019 cm- 3 at x = 0.060, indicating effective carrier compensation upon Sb incorporation. At x = 0.015, the carrier-compensation behavior and enhanced carrier mobility are consistent with a possible reduction in pre-existing Sn/Bi antisite-related scattering. As a result, the nondegenerate mobility significantly increased from 46 to 63 cm2 V- 1 s-1 at x = 0.015 and was well preserved between 53 and 57 cm2 V- 1 s- 1 up to x = 0.045 - 0.060. The lattice thermal conductivity exhibited a gradual increase with Sb content, while Debye-Callaway analysis indicated that the thermal-transport evolution reflects changes in the overall phonon-scattering environment rather than a single scattering mechanism. Meanwhile, the total thermal conductivity was reduced primarily by the decrease in electronic thermal conductivity associated with reduced electrical conductivity. Combined with the favorable balance between carrier-concentration regulation and mobility preservation, this reduction in total thermal conductivity enabled the x = 0.015 sample to achieve a maximized power factor of 1.17 mW m- 1 K-2 at 300 K and a peak zT of 0.38, marking a 30% and 45% enhancement relative to the pristine sample, respectively. Consequently, further substitution beyond x = 0.015 did not produce additional zT enhancement because the benefits of carrier-concentration regulation and reduced thermal conductivity were progressively offset by the decrease in md* and, at x >= 0.045, the deterioration of carrier mobility.

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