Ionization Degree-Driven Interface Decoupling in the Layered Ca ₂ Mg ₂ Cd ₂ Sb ₄ Superlattice with High Thermoelectric Performance

Thermoelectric (TE) materials enable direct heat-to-electricity conversion, vital for alleviating energy crises. However, the inherently high lattice thermal conductivity ( κ l ) of CaMg 2 Sb 2 severely limits further TE performance optimization. In current work, a nanolayered Ca 2 Mg 2 Cd 2 Sb 4 superlattice is constructed via precise alternating stacking of CaMg 2 Sb 2 and CaCd 2 Sb 2 sublayers, driven by a pronounced contrast in ionization degree between the highly ionized [Mg 2 Sb 2 ] 2– layer (76.6%) and the weakly ionized [Cd 2 Sb 2 ] 2– layer (32.8%). Using first-principles calculations, machine-learning interatomic potentials, and Boltzmann transport theory, the thermal and electronic transport, and TE performance were systematically evaluated. In the Ca 2 Mg 2 Cd 2 Sb 4 superlattice, alternating insulating [Mg 2 Sb 2 ] 2– and conductive [Cd 2 Sb 2 ] 2– layers induces interfacial decoupling, substantially enhancing phonon scattering while preserving efficient carrier transport. The pronounced four-phonon scattering is enhanced at elevated temperatures, further reducing κ l to 0.43 W m –1 K –1 at 700 K. The Ca 2 Mg 2 Cd 2 Sb 4 superlattice exhibits an anisotropic electronic structure that effectively balances carrier mobility and Seebeck coefficient ( S ), thereby optimizing the power factor ( PF ). Consequently, the optimal dimensionless figure-of-merits ( ZTs ) for n -type and p -type Ca 2 Mg 2 Cd 2 Sb 4 superlattices reach 1.68 and 1.38 at 700 K, respectively. The findings establish Ca 2 Mg 2 Cd 2 Sb 4 as a high-performance TE material and demonstrate ionization degree-driven layered structural engineering as an effective strategy for decoupling phonon and carrier transport in advanced thermoelectrics.