Real-Space Visualization of the Intrinsic Aging in n -type Mg 3 ( Sb , Bi ) 2 Thermoelectrics

Long-term stability in materials is commonly associated with resistance to external perturbations. Yet metastable defect populations may relax spontaneously even in the absence of environmental stimuli, reshaping macroscopic properties over extended timescales. Here, we show the real-space visualization of intrinsic aging of n -type Mg 3 ( Sb , Bi ) 2 under inert and room-temperature conditions after two-year storage by atomic-scale characterizations. Density-functional-theory calculations and machine-learning molecular dynamics simulations prove that the aging originates from relaxation of a metastable Mg-rich state, in which low vacancy-formation energies and migration barriers enable thermodynamically favorable and kinetically accessible Mg redistribution. Multiscale characterizations further reveal that grain boundaries (GBs) act as fast-diffusion pathways and effective sinks, establishing a hierarchical Mg redistribution process from grain interiors to GBs and surfaces. These results identify intrinsic defect physics as one of the fundamental stability constraints in functional materials and underscore the importance of controlling defect thermodynamics and transport pathways for achieving practically durable materials.