Prediction of potential hard sodium carbaboride compounds assuming sp-bonded covalent clathrates

Boron-carbon clathrates have attracted great attention due to their unique sp 3 -bonded structure and excellent electronic properties. Here, by performing first-principles calculations, we predicted six stoichiometric Na-B-C clathrates (NaBC 11 , Na 2 B 2 C 10 , NaB 2 C 10 , Na 2 B 4 C 8 , NaB 4 C 8 , and Na 2 B 6 C 6 ) based on Na-doped boron-carbon clathrates. As a result, NaBC 11 , Na 2 B 2 C 10 , and NaB 2 C 10 were found to become energetically favorable. Under ambient conditions, the electronic structure calculations show that NaBC 11 and Na 2 B 2 C 10 are indirect band gap semiconductors, and NaB 2 C 10 , Na 2 B 4 C 8 , and NaB 4 C 8 exhibit metallic features. Na 2 B 2 C 10 and Na 2 B 4 C 8 are found to be synthesized at 22.7 and 14.2 GPa, respectively. Interestingly, the formation enthalpies of Na x B 2 C 10 and Na x B 4 C 8 ( x = 0, 1, and 2) clathrates decrease in turn with the increased number of Na atoms in the same synthetic paths. Moreover, the ideal indentation strengths of NaBC 11 , Na 2 B 2 C 10 , and NaB 2 C 10 approach 40 GPa, indicating that they are hard materials with superior hardness. These findings offer valuable insights for advancing the synthesis of boron-carbon clathrates. We predicted six stoichiometric sodium carbaboride compounds (NaBC 11 , Na 2 B 2 C 10 , NaB 2 C 10 , Na 2 B 4 C 8 , NaB 4 C 8 , and Na 2 B 6 C 6 ) based on Na-doped boron-carbon clathrates via first-principles calculations.