Retarding moisture-induced chemical degradation of Yttrium Tellurides by tailoring grain boundary chemistry

Grain boundary engineering has been extensively applied to improve thermoelectric performance, but its potential to enhance chemical stability remains underexplored. Here, we demonstrate that modifying grain boundary chemistry can effectively suppress the chemical degradation of Y₂Te₃ under ambient conditions. Scanning transmission electron microscopy and atom probe tomography reveal that H₂O preferentially infiltrates along grain boundaries, initiating oxidation of Y₂Te₃ into Y-O-H phases and causing chemo-mechanical breakdown of the matrix. This process, remarkably, can be retarded by just 1 at.% of Bi incorporation due to its segregation along grain boundaries. Density functional theory calculations reveal the thermodynamic and kinetic origins of Bi segregation, and show how segregated Bi modifies the local electronic and chemical environment of grain boundaries, thereby linking GB chemistry to both chemical stability and thermoelectric performance. These findings establish multifunctional grain boundary engineering as a generalizable strategy for the design of next-generation thermoelectric materials.