Metallic Transport in Pseudocubic Quaternary Diamondoid Thermoelectric Semiconductor

Recently diamondoid semiconductors have attracted significant attention for thermoelectric applications due to their intrinsic low thermal conductivity and unique electrical transport properties. However, because of their compact tetrahedral coordination and strong chemical bonding, diamondoid materials are typically difficult to dope, resulting in low electrical conductivity, generally below 200 S cm^-1. In this work, we report an unusual metallic transport behavior in CdSe-doped pseudocubic diamondoid semiconductor Cu₂ZnSnSe₄. The intrinsic Cu₂ZnSnSe₄ possesses a zinc blende pseudocubic lattice. Owing to the structural compatibility of CdSe with Cu₂ZnSnSe₄, Cd is easily incorporated into the lattice. We find that substituting Sn⁴⁺ with Cd²⁺ increases the carrier concentration to ∼10²¹ cm^-3 without distorting the pseudocubic lattice. Due to the simultaneous improvement in carrier concentration and mobility, an unusually high room-temperature electrical conductivity of 1200 S cm^-1 is achieved in Cu₂ZnSn_0.9Cd_0.1Se₄, which exhibits metallic transport behavior. Furthermore, the thermal conductivity of the material is reduced through Ag alloying, which softens the chemical bonding and induces an off-centering effect. As a result, a peak ZT of 0.8 at 800 K is achieved in Cu_1.95Ag_0.05ZnSn_0.9Cd_0.1Se₄, highlighting the strong thermoelectric potential of this quaternary diamondoid system.