Ag-Doping-Mediated Interlayer Coordination Engineering: Enabling Thermoelectric <i>ZT</i> = 1 in TMDs-Derived Narrow-Gap Semiconductor CuCrTi ₂ Se ₆

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, Cu0.95Ag0.05CrTi2Se6 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.