α-CsCu5Se3：Discovery of A Low-cost Bulk Selenide with High Thermoelectric Performance.

Discovery of low-cost, less toxic and earth-abundant thermoelectric materials is of great challenge. Herein, with the aid of a unique and safe boron-chalcogen method, we discover a new tetragonal α-CsCu5Se3, featuring a previously un-recognized structure in the ternary family of Cs/Cu/Se. The structure is constructed by a Chinese-knot-like Cu8Se8 building unit that is further linked into a 3D network. The α-CsCu5Se3 exhibits thermal stability that is superior to the recently established thermoelectric materials of Cu2-xSe, CsAg5Te3 suffering unfavorable phase transitions. Distinct from the liquid-like migration in Cu2-xSe, the α-CsCu5Se3 obeys a typical crystalline solid thermal transport behavior dominated by the Umklapp scattering. Compared to the isostructural CsAg5Te3, α-CsCu5Se3 shows a 30% volume decrease that leads to stronger orbital overlapping that markedly decrease the band effective mass (m*). With smaller m* and softer Cu-Se bond, the α-CsCu5Se3 eventually realizes a 200% increase in the power factor (8.17 μW/cm/K2, the highest among the copper-rich alkali metal chalcogenides) and a figure of merit (ZT) of 1.03 at 980 K. Further, the doping in α-Cs(Cu0.96Sb0.04)5Se3 boosts the lattice anharmonicity by the lone pairs that via intensifying the Umklapp scattering and slowing the phonon velocity ensure a low lattice thermal conductivity (0.40 W/m/K), and finally lead to a ZTmax of 1.30 at 980 K. Our discovery represents a step toward the low-cost, earth-abundant and high-performance chalcogenide materials that will shed useful lights on future exploration in the related fields.