Unraveling Non-polymorphic Phase Evolution in Mg-Ag-Sb for Designing Thermally Recoverable Thermoelectrics

P-type MgAgSb with excellent room-temperature thermoelectric performance holds great promise for cooling and power generation. However, its practical application has been significantly limited by phase transitions, which are long considered a polymorphic transformation where the compositions remain identical but the crystal structures differ. Here we report the non-polymorphic nature of the complex phase transitions in Mg-Ag-Sb during thermal cycling, yielding four compositionally distinct ternary compounds that crystallize in three different structures. During heating, the tetragonal α -phase MgAgSb decomposes into the tetragonal β -phase Mg 5 Ag 4.32 Sb 4.5 , and subsequently into the cubic γ H -phase Mg 5 Ag 2.11 Sb 4 , with the progressive precipitation of Ag 3 Sb and Sb phases. Upon cooling, the γ H -phase Mg 5 Ag 2.11 Sb 4 converts to the cubic γ L -phase Mg 5 Ag 1.18 Sb 3.63 , accompanied by the α phase with substantial Ag 3 Sb and Sb phases. The electron localization function and bonding analysis indicate that the Mg-Ag-Sb compound is characterized by ionic interactions, wherein the Ag-Sb bond is notably weaker compared to the Mg-Sb bond. Molecular dynamics simulations reveal that significant Mg and Ag migration within the Sb sublattices underlies the phase evolution. Surprisingly, the rapid atomic diffusion also enables the restoration of the α -phase MgAgSb upon annealing at low temperatures. Successful recovery of the α phase can be realized even after 500 thermal cycles between 423–673 K, demonstrating unprecedented thermal recoverability.