The BC8 silicon allotrope has a lattice thermal conductivity 1-2 orders of magnitude lower than that of diamond-cubic silicon. In the current work, the phonon density of states, phonon dispersion, and lattice thermal conductivity are investigated by inelastic neutron scattering measurements and first-principles calculations. Flat phonon bands are found to play a critical role in the reduction of lattice thermal conductivity in BC8-Si. Such bands in the low-energy range enhance the phonon scattering between acoustic and low-energy optical phonons, while bands in the intermediate-energy range act as a scattering bridge between the high- and low-energy optical phonons. They significantly enlarge the phonon-phonon scattering phase space and reduces the lattice thermal conductivity in this novel silicon allotrope. This work provides insights into the significant reduction of the lattice thermal conductivity in BC8-Si, thus expanding the understanding of novel silicon allotropes and their development for electronic devices.

BC8 硅同素异形体的晶格热导率比金刚石-立方硅低 1-2 个数量级。在目前的工作中，通过非弹性中子散射测量和第一性原理计算研究了声子态密度、声子色散和晶格热导率。发现平坦的声子带在降低 BC8-Si 的晶格热导率方面起着关键作用。低能范围内的这些波段增强了声学和低能光声子之间的声子散射，而中能范围内的波段则充当了高能和低能光声子之间的散射桥梁。它们显着扩大了声子 - 声子散射相空间并降低了这种新型硅同素异形体的晶格热导率。