Exploration of AB3Si3 (A = Na/K/Rb/Cs) compounds under moderate pressure

We discovered the composition of ternary AB3Si3 (A = Na/K/Rb/Cs) compounds in the moderate pressure range of 0-100 GPa using first-principles structural prediction and systematically analyzed their structures, stability, electronic and optical properties within the framework of density functional theory. The AB3Si3 compounds exhibit a diverse phase diagram, including nine structures that are selected based on formation energies, along with a known clathrate RbB3Si3 structure with Pm3̄n symmetry. All predicted phases are thermodynamically and dynamically stable within the studied pressure range. In particular, the KB3Si3 compound with a direct band gap of 1.0 eV is identified as a promising candidate for photovoltaic materials beyond silicon-based materials, among which boron atoms form a unique regular octahedral structure; in contrast, NaB3Si3 and RbB3Si3 compounds are shown to have metallicity. Our findings enrich crystal structures of alkali-metal borosilicides and provide valuable insights into their potential applications.

Prediction of potential hard sodium carbaboride compounds assuming sp3-bonded covalent clathrates

Boron-carbon clathrates have attracted great attention due to their unique sp3-bonded structure and excellent electronic properties. Here, by performing first-principles calculations, we predicted six stoichiometric Na-B-C clathrates (NaBC11, Na2B2C10, NaB2C10, Na2B4C8, NaB4C8, and Na2B6C6) based on Na-doped boron-carbon clathrates. As a result, NaBC11, Na2B2C10, and NaB2C10 were found to become energetically favorable. Under ambient conditions, the electronic structure calculations show that NaBC11 and Na2B2C10 are indirect band gap semiconductors, and NaB2C10, Na2B4C8, and NaB4C8 exhibit metallic features. Na2B2C10 and Na2B4C8 are found to be synthesized at 22.7 and 14.2 GPa, respectively. Interestingly, the formation enthalpies of NaxB2C10 and NaxB4C8 (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 NaBC11, Na2B2C10, and NaB2C10 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.