Off-centering effect and crystal symmetry modification lead to high thermoelectric performance in diamondoid Cu

Recently, dimensionless figure of merit (ZT) 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 0.3 W m−1 K−1. Consequently, a maximum ZT of 1.3 was obtained at 800 K for the Cu1.85Ag0.15Sn0.9Cd0.1Se3.