Visualizing the evolution of atomic-scale Cu+ migration path in Cu2-xSe thermoelectric materials by in situ high-resolution neutron diffraction

• • Intra-tetrahedral Cu 8c ↔ 32f <111> hopping initiates at 398 K in β-Cu2Se, with a nuclear density isosurface of 0.505 fm Å-3, marking the onset of superionic behavior that enhances phonon scattering and reduces lattice thermal conductivity. • • In β-Cu1.95Se, the same hopping path appears at 393 K with a lower isosurface of 0.484 fm Å-3, indicating that Cu vacancies lower the activation barrier for Cu+ migration, which can increase ionic conductivity but risks Cu precipitation above 723 K. • • Inter-tetrahedral migration pathways (Cu 32f ↔ 32f <100> and 32f ↔ 4b ↔ 32f <111>) form above 448 K, as confirmed by line scans along [1̅11̅] up to 723 K, providing a direct link between long-range Cu+ motion and device degradation. • • The observed structural evolution explains the anomalous zT enhancement (exceeding 1.5 at 900 K) and identifies 723 K as a critical threshold where Cu+ mobility may compromise Seebeck coefficient and device stability, necessitating interface engineering to suppress precipitation.