By structure prediction, we found a complex semiconducting Si metastable phase, named pentasilicon, which is stable at room temperature. The pentasilicon, which consists of sp2-sp3 hybrid bonds, has an ultralow thermal conductivity of about 1.7 W/mK at room temperature, which is similar to those of many high-performance thermoelectric materials. We found the low thermal conductivity of pentasilicon originated from the strong anharmonicity and crossover effect due to the strong interactions between LA mode and several low-frequency quasi-local vibrations, which originate from the sp2-sp3 hybrid bonding character. Additionally, such strong anharmonicity also leads to the negative thermal expansion in pentasilicon. Furthermore, we obtained the energy-dependent electron relaxation lifetimes by considering electron-phonon coupling, and predicted the thermoelectric properties of pentasilicon. For example, the maximum values of ZT are 0.35 and 0.27 at doping level of 10¹⁹/cm³ and 1.7 × 10²⁰/cm³ for p- and n-type pentasilicon, which are around 12 and 5 times of those in silicon, respectively, at 500 K. Our work provides an alternative strategy to improve the thermoelectric performance of Si-based materials.

通过结构预测，我们发现了一种复杂的半导体硅亚稳相，命名为五硅，它在室温下是稳定的。五硅由sp 2- sp 3 杂化键组成，在室温下具有约1.7 W/mK的超低热导率，与许多高性能热电材料相似。我们发现五硅的低热导率源于强非谐性和交叉效应，这是由于 LA 模式和几个低频准局域振动之间的强相互作用，这源于sp 2- sp3 混合键合特性。此外，如此强的非谐性也导致五硅的负热膨胀。此外，我们通过考虑电子 - 声子耦合获得了与能量相关的电子弛豫寿命，并预测了五硅的热电特性。例如，在掺杂水平为 10 ¹⁹ / cm ³和 1.7 × 10 ²⁰ / cm ³的 p 型和 n 型五硅中，ZT 的最大值分别为 0.35 和 0.27，约为硅中的 12 和 5 倍，分别为 500 K。我们的工作为提高硅基材料的热电性能提供了一种替代策略。