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High thermoelectric figure of merit, zT, of ∼1.85 at 725 K along with significant cyclable temperature stability was achieved in Pb-free p-type Ge_1–xSb_xTe samples through simultaneous enhancement in Seebeck coefficient and reduction of thermal conductivity. Sb doping in GeTe decreases the carrier concentration due to the donor dopant nature of Sb and enhances the valence band degeneracy by increasing the cubic nature of the sample, which collectively boost Seebeck coefficient in the temperature range of 300–773 K. Significant thermal conductivity reduction was achieved due to collective phonon scattering from various meso-structured domain variants, twin and inversion boundaries, nanostructured defect layers, and solid solution point defects. The high performance Ge_0.9Sb_0.1Te sample shows mechanical stability (Vickers microhardness) of ∼206 H_v, which is significantly higher compared to other …