Superhard lithium carbaboride clathrates

Abstract

Lightweight strong boron-carbon clathrates adopting sodalite structure have been inspirated by the successful synthesis of the SrB3C3 compounds, which assuming the cubic bipartite sodalite structure. In this work, we designed series of stoichiometric LiBxC12-x and Li2BxC12-x (x = 1–6) compounds based on boron substitution of carbon in carbon sodalite structure using first-principles calculations. We found that LiB2C10, LiB3C9, LiB4C8, Li2B3C9 and Li2B4C8 are dynamically stable at high pressure and can be recovered under normal pressure. Strikingly, compared with inserting one Li atom into the B-C framework, inserting two Li atoms exhibit lower enthalpy value and better symmetry. Li2B4C8 is identified as the most stable structure among these compounds, and Li2B4C8 could potentially be synthesized at moderate pressure. The electron localization function calculations reveal that there is a strong covalent bond between B and C atoms in the clathrates, which forming the rigid three - dimensional crystalline networks. Moreover, the estimated Vickers hardness values of these five stable clathrates are 42 ∼ 51 GPa, which can be classified as superhard materials. At the same time, the pure shear strength of LiB2C10 is found to be 48.1 GPa, which exceeds the threshold value of 40 GPa. The pure shear strength of LiB3C9 (38.0 GPa) and Li2B3C9 (38.8 GPa) are found to be close to 40 GPa. Our work presents a new strategy for exploring superhard sodalite-like clathrate candidates.