Visualizing the evolution of atomic-scale Cu+ migration path in Cu2-xSe thermoelectric materials by in situ high-resolution neutron diffraction
Cu2-xSe is a leading p-type thermoelectric material owing to its phonon-liquid electron-crystal (PLEC) behavior, yet the atomic-scale mechanisms governing Cu+ migration remain unresolved. This study employs in situ high-resolution neutron diffraction coupled with maximum entropy method (MEM) analysis to map the temperature-dependent evolution of Cu+ nuclear density in β-Cu2Se and β-Cu1.95Se. At 398–423 K, intra-tetrahedral Cu 8c ↔ 32f <111> hopping emerges, with isosurface values of 0.505 fm Å-3 for β-Cu2Se and 0.484 fm Å-3 for β-Cu1.95Se. Above 448 K, inter-tetrahedral pathways form via Cu 32f ↔ 32f <100> or 32f ↔ 4b ↔ 32f <111> migration, as revealed by line scans along [1̅11̅] up to 723 K. The presence of Cu vacancies (x = 0.05) alters the onset and connectivity of these pathways, directly impacting phonon scattering and electron transport. These findings establish a structural basis for controlling Cu+ mobility, offering a rational route to mitigate Cu precipitation and enhance zT stability beyond 1.5 at 900 K. The work bridges microstructural dynamics and thermoelectric performance, providing critical guidance for defect engineering in superionic thermoelectrics.