An investigation on the surface properties of B4C for advancing its nuclear applications

B₄C is an important material in diverse nuclear applications. However, a systematic examination of its surface properties is still missing. In this work, we employ first-principles simulations to investigate the energetic stability of 16 distinct slab models representing (001), (100), (101), (110), and (111) surfaces, which are constructed by minimizing dangling bonds. Our results show that C-terminated (001) surface exhibits significantly greater stability than other surfaces under both the carbon and boron-rich conditions. Besides, we also study the defect formation energies on the C-terminated (001) surface and compare them with the cases in bulk. The high formation energies of the defects suggest a low likelihood of their occurrence on this surface, despite their formation energies being lower compared to bulk cases. Furthermore, mid-gap surface states are revealed for the top atomic layers of the C-terminated (001) surface, which are deduced at the deeper layers, and the band structures of the middle layers of this slab recover to the bulk band gap. These surface mid-gap states allow electron excitation from the valence band to these states, resulting in a reduced optical band gap compared to the bulk band gap of B₄C. This provides a plausible explanation for the significantly smaller band gap observed in experiments compared to the larger gap predicted by theoretical models. Our study not only sheds light on the surface properties of B₄C but also lays the groundwork for advancing this material for more advanced nuclear applications.