Thermoelectric performance in a Si allotrope with ultralow thermal conductivity: a first-principles study combining phonon-limited electronic transport calculations

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 10(19)/cm(3) and 1.7 x 10(20)/cm(3) 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.