Promoted thermoelectric performance in cubic-phase GeTe via grain-boundary phase elimination under phase diagram guidance

Eliminating the ferroelectric phase transition to obtain a cubic phase under ambient conditions is deemed an ultimate goal for p-type GeTe-based materials to be feasible for thermoelectric generator applications at mid-temperature range. Alloying stoichiometric AgSbTe2 or non-stoichiometric Ag1-delta Sb1+delta Te2+delta into GeTe can regulate its cubic-rhombohedral phase transition temperature well below 300 K; nevertheless, the existence of the high-resistivity Ag8GeTe6 grain boundary phase impeded the further improvement of thermoelectric performance in both cases. In this work, as guided by the phase equilibrium diagram, we successfully eliminated the Ag8GeTe6 grain boundary phase by meticulously tailoring the alloying ratio x and regulating Ag/Sb ratio delta in (GeTe)(x)(Ag1-delta Sb1+delta Te2+delta)(100-x) while retaining its cubic crystalline structure. Furthermore, the formation of Ge2Sb2Te5 imbedded in GeTe grains involved discrete van der Waals planar gaps, which can further help reduce the lattice thermal conductivity. As a result, a peak figure of merit ZT(max.) similar to 2.0 at 673 K and average ZT(ave.) of similar to 1.5 at 323-773 K were obtained in our cubic-phase (GeTe)(78)(Ag0.77Sb1.23Te2.23)(22), and the fabricated eight-pair thermoelectric power generator exhibited an outstanding conversion efficiency of similar to 6.3% with output power of similar to 1.32 W at a temperature difference of 480 K. This work demonstrated that eliminating the high-resistivity grain boundary phase is a facile way to realize enhanced thermoelectric performance, and could shed light on further research on other thermoelectric materials.