One example of a material with a boron–carbon network structure is B₅₀C₂. The model originally proposed by Ploog et al. (Fig. 1a) had traditionally been considered to be the correct structure, but a new model was recently proposed by Baker et al. (Fig. 1b). Our theoretical calculations revealed that B₅₀C₂ has a higher free energy than the neighboring phases and is therefore unstable throughout the entire pressure–temperature range below 3000 K and 30 GPa. Furthermore, by exploring structures with different carbon contents, we found that B₄₉C₃ is the most stable composition and is more stable than the neighboring phases B₁₀₆C and B₁₃C₂ under pressure and temperature conditions exceeding 17 GPa and 1500 K, respectively (Fig. 2b). At the same time, B₄₉C₃ is consistently more stable than the B₅₀C₂ + B₄₈C₄ phases over the entire pressure–temperature range examined (Fig. 2d). Therefore, B₅₀C₂ reported in previous experiments may in fact be B₄₉C₃.
Experimentally, B₅₀C₂ has been reported when the starting materials contain hydrogen. We therefore investigated the possibility of hydrogen incorporation and found that, for example, the composition B₅₀C₂H₂ can be stabilized by the incorporation of hydrogen (Fig. 3). The calculated electronic density of states of this compound exhibits semiconducting behavior, consistent with the experimental results (Fig. 4, left). Furthermore, its phonon dispersion indicates that the structure is dynamically stable (Fig. 4, right).