The thermoelectric performance of a semiconductor is intrinsically limited by the coupling of the transport factors, originating in the mechanism of band gap opening. Here we report the anomalous semiconductor AgSbSe2-alloyed PbSe, in which the spin-orbit coupling (SOC) induced band gap allows the decoupling of electrical conductivity and Seebeck coefficient. In such a SOC semiconductor, the decrease of lattice constant increases the band gap from 0.23 eV up to 0.5 eV, seemingly in contrast to the previously measured negative pressure coefficient of band gap. Very different thermoelectric behaviors from the pristine PbSe were observed in PbSe–AgSbSe2, and finally a high peak ZT of 1.65 at 858 K and a remarkable average ZT (300–858 K) of 0.86 were obtained. This study reveals the novel relativistic effects in PbSe–AgSbSe2 system, and its thermoelectric performance is believed to have a large potential to be further improved.
