Ag-Doping-Mediated Interlayer Coordination Engineering: Enabling Thermoelectric ZT = 1 in TMDs-Derived Narrow-Gap...

Thermoelectric materials enable direct thermal-to-electrical energy conversion for waste heat recovery, yet their figure of merit ZT is constrained by intrinsic transport trade-offs. Herein, we target CuCrTi₂Se₆, a new narrow-bandgap semiconductor and quaternary chalcogenide derived from two-dimensional transition-metal dichalcogenides (TMDs), and tune its coordination via Ag doping, driving its peak ZT to 1.0 at 773 K and single-leg efficiency to 6.3% at ΔT = 500 K. The larger atomic mass and size of Ag generate local stress fields that drive Cu migration from octahedral to tetrahedral interlayer sites, shortening Cu-Se bonds, enhancing bond covalency, and increasing carrier mobility from 28 to 35 cm²·V^-1·s^-1. Concurrently, Ag doping reduces Cu-vacancy formation energy, increasing hole concentration while elevating valence band degeneracy to enhance the Seebeck coefficient. On the phononic side, weak Ag-Se bonds induce lattice softening, and strong point-defect scattering from Ag-Cu mass/strain fluctuations synergistically reduces lattice thermal conductivity from 0.46 to 0.31 W·m^-1·K^-1. Benefiting from the simultaneous optimization of electronic and phononic transport, Cu_0.95Ag_0.05CrTi₂Se₆ achieves a peak ZT nearly 70% higher than pristine CuCrTi₂Se₆. This work establishes coordination environment regulation as an effective strategy for tuning chemical bonding and achieving coupled optimization of thermoelectric transport in layered materials.