Homogenizing a liquid-like Cu sublattice for high-performance Cu₂Te_0.5Se_0.5 thermoelectric materials

Liquid-like thermoelectric materials are continuously drawing significant attention because of their extraordinary performance. However, liquid-like Cu atoms usually aggregate into clusters, resulting in charge carrier concentrations that are far above the optimized value. Herein, we demonstrate that the dynamic Cu atoms can be homogenized throughout the matrix by a phase-dependent diffusion process. We directly visualized that the typical equilibrium process periodically aggregates Cu atoms into clusters along the grain boundaries, leaving abundant vacancies within the dynamic cation sublattice and thereby causing carrier concentrations that are much higher than the optimal value. We developed a solid-state non-equilibrium quenching process to freeze the homogeneous Cu sublattice in a symmetry cubic phase at ambient environment and removed the unfavorable Cu segregation. This substantially decreased the hole concentration from ∼1.2 × 10²² cm⁻³ for the equilibrium sample to ∼0.8 × 10²² cm⁻³ for the quenched sample, approaching the calculated optimal value. Notably, the highly uniform Cu sublattice reduced the lattice thermal conductivity to the predicted minima of 0.3 W m⁻¹ K⁻¹ from 300 to 1000 K. The reduced hole concentration and lattice thermal conductivity collectively gave rise to a high ZT value of 1.5 at 1000 K, lying in the ZT range of all previously reported stoichiometric copper chalcogenides.