Hidden role of intrinsic Sb-rich nano-precipitates for high-performance Bi_2-xSb_xTe₃ thermoelectric alloys

Nanostructuring is a universal strategy to improve the figure of merit (zT) of thermoelectric (TE) materials by the substantial decrease of lattice thermal conductivity due to the intensive scattering of phonons at interfaces. However, nanostructured bismuth antimony telluride alloys with foreign nanosized phases usually suffer from the degradation of carrier mobility at incoherent interfaces. The results reported here describe an approach to generate intrinsic Sb-rich precipitates that have a synchronal effect on micro-grained Bi_0.5Sb_1.5Te₃ alloy. The designed formation of nanosized Sb-rich precipitates by dissolving excess Pb into Bi_0.5Sb_1.5Te₃ matrix intensifies the phonon scattering and modulates the carrier transport simultaneously. By employing density functional theory calculation, transmission electron microscope and atom probe tomography, the role of Pb substituting for Sb site on the evolution of Sb-rich nanophase is clarified and the compositions and crystal structures of the precipitated Sb-rich phases are analyzed, revealing various interface structures. The optimized Bi_0.5Sb_1.5Te₃ + 0.22 wt.% Pb sample shows a maximum zT value of 1.32 at 400 K with an outstanding average zT value of 1.2 over 300 K to 500 K. This work identifies the hidden role of intrinsic Sb-rich precipitates in Bi_0.5Sb_1.5Te₃ alloys and provides an effective way beyond nanostructuring to develop high-performance bismuth antimony telluride thermoelectric alloys.