Off-centering effect and crystal symmetry modification lead to high thermoelectric performance in diamondoid Cu<sub>2</sub>SnSe<sub>3</sub>

<p indent="0mm">Recently, dimensionless figure of merit (<italic>ZT</italic>) beyond 1.0 has been demonstrated in diamondoid Cu₂SnSe₃, positioning it as a low-cost and ecofriendly thermoelectric alternative, and sparking intense interest in further optimizing its thermoelectric properties. However, its intrinsic monoclinic structure and the compact packing of tetrahedral coordination lead to a light valence band and high thermal conductivity, which impede further performance improvement in Cu₂SnSe₃. Thus, developing effective approaches to altering the local structure of Cu₂SnSe₃ is important for further optimizing its thermoelectric performance. Herein, we identified and thoroughly investigated the atomic off-centering behavior in Cu₂SnSe₃, and demonstrated the electronic and phonon transport properties of Cu₂SnSe₃ can be significantly enhanced by carefully modifying its monoclinic lattice into a distorted zinc-blende cubic structure via introducing Cd and off-centering Ag elements. This structure transition leads to band convergence and a reduction in the deformation potential of the material, resulting in a 7-fold enhancement in the density-of-state effective mass and an 85% increase in carrier mobility. Additionally, the off-centering effect results in strong acoustic-optical phonon scattering and an ultra-low lattice thermal conductivity of <sc>0.3 W m⁻¹ K⁻¹.</sc> Consequently, a maximum <italic>ZT</italic> of 1.3 was obtained at <sc>800 K</sc> for the Cu_1.85Ag_0.15Sn_0.9Cd_0.1Se₃.