(GeTe)_1-(AgSnSe₂): Strong Atomic Disorder-Induced High Thermoelectric Performance near the Ioffe-Regel Limit

In thermoelectrics, the material's performance stems from a delicate tradeoff between atomic order and disorder. Generally, dopants and thus atomic disorder are indispensable for optimizing the carrier concentration and scatter short-wavelength heat-carrying phonons. However, the strong disorder has been perceived as detrimental to the semiconductor's electrical conductivity owing to the deteriorated carrier mobility. Here, we report the sustainable role of strong atomic disorder in suppressing the detrimental phase transition and enhancing the thermoelectric performance in GeTe. We found that AgSnSe₂ and Sb co-alloying eliminates the unfavorable phase transition due to the high configurational entropy and achieve the cubic Ge_1-SbTe_1-(AgSnSe₂) solid solutions with cationic and anionic site disorder. Though AgSnSe₂ substitution drives the carrier mean free path toward the Ioffe-Regel limit and minimizes the carrier mobility, the increased carrier concentration could render a decent electrical conductivity, affording enough phase room for further performance optimization. Given the lowermost carrier mean free path, further Sb alloying on Ge sites was implemented to progressively optimize the carrier concentration and enhance the density-of-state effective mass, thereby substantially enhancing the Seebeck coefficient. In addition, the high density of nanoscale strain clusters induced by strong atomic disorders significantly restrains the lattice thermal conductivity. As a result, a state-of-the-art zT ≈ 1.54 at 773 K was attained in cubic Ge_0.58Sb_0.22Te_0.8(AgSnSe₂)_0.2. These results demonstrate that the strong atomic disorder at the high entropy scale is a previously underheeded but promising approach in thermoelectric material research, especially for the numerous low carrier mobility materials.