Synergetic Enhancement of Power Factors and Suppression of Lattice Thermal Conductivities via Biaxial Strain in ScAgSe₂ and TmAgTe₂

The challenge of achieving high thermoelectric (TE) performance is mainly from the entanglement among Seebeck coefficient (S), electrical conductivity ($σ$), and lattice thermal conductivity (κL). In this work, we propose a synergetic strategy of enhancing power factor (PF, S²σ) and suppressing κL by applying a biaxial tensile strain in two silver chalcogenides ScAgSe₂ and TmAgTe₂ with TlCdS₂-type structure. The forbidden p-d orbital coupling at the $Γ$ point and allowed p-d orbital coupling at the A point and the middle of $Λ$ line leads to high electronic band dispersion along the $Γ$-A direction and a high-degeneracy valence band valley (Λ₂). The elongation of the Ag-Se bond under tensile strain weakens the orbital coupling between Ag-d and Se/Te-p orbitals and reduces the band energy at the A point, which aligns the valence band and achieving a high band degeneracy. Concurrently, the weaker Ag-Se/Ag-Te bond under a small tensile strain leads to lower phonon group velocity and strong three- and four phonon scatterings, leading to lower κL. Our first-principles calculations combined with electron-phonon coupling analysis as well as phonon and electron Boltzmann transport equations show that applying a 3% (2%) tensile strain can enhance the PF along the c-axis of ScAgSe₂ (TmAgTe₂) by 243% (246%) at a carrier concentration of 3×10²⁰ cm⁻³ and reduce the κL by 37% (26%) at 300 K. Consequently, 2 ~ 4 times of $ZT$ enhancement is obtained by 3% or 1% tensile strain in ScAgSe₂ (TmAgTe₂) at 300 K, achieving a maximum $ZT$ of 3.10 (3.62) at 800 K. Our material design strategy based on molecular orbital analysis reveals an effective route to boosting TE performance, and can be extended to other systems as well.