α-CsCu_5Se_3: Discovery of a Low-Cost Bulk Selenide with High Thermoelectric Performance

摘要

The discovery of low-cost, less toxic, and earth-abundant thermoelectric materials is a great challenge. Herein, with the aid of a unique and safe boron— chalcogen method, we discover the new tetragonal α-CsCu_5Se_3, featuring a previously unrecognized structure in the ternary family of Cs/Cu/Se. The structure is constructed by a Chinese-knot-like Cu_8Se_8 building unit that is further linked into a 3D network, α-CsCu_5Se_3 exhibits thermal stability that is superior to that of the recently established thermoelectric materials Cu_(2-x)Se and CsAg_5Te_3 suffering unfavorable phase transitions. Distinct from the liquidlike migration in Cu_(2-x)Se, α-CsCu_5Se_3 obeys a typical crystalline solid thermal transport behavior dominated by Umklapp scattering. In compariosn to the isostructural CsAg)5Te)3, α-CsCu)5Se)3 shows a 30% volume decrease that leads to stronger orbital overlapping that markedly decreases the band effective mass (m*). With a smaller m* and a softer Cu—Se bond, α-CsCu_5Se_3 eventually realizes a 200% increase in the power factor (8.17 μW/ (cm K~2), 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(Cu_(0.96)Sb_(0.04))_5Se_3 boosts the lattice anharmonicity by the lone pairs that, via intensifying the Umklapp scattering and slowing the phonon velocity, ensures a low lattice thermal conductivity (0.40 W/ (m K)), and finally leads to a ZT_(max) value of 1.30 at 980 K. Our discovery represents a step toward low-cost, earth-abundant, and high-performance chalcogenide materials that will shed useful light on future exploration in the related fields.