Structural arrangements of the ternary metal boride carbide compounds MB2C4 (M=Mg, Ca, La and Ce) from first-principles theory
Résumé
The structural arrangements of the ternary metal borocarbides MB2C4 (M=Mg, Ca; La and Ce) are investigated using density-functional theory (DFT) calculations within the generalized gradient approximation (GGA). Results indicate that these compounds adopt a layered structure consisting of graphite-like B2C4 layers alternating with metal sheets. Within the hexagonal layers, the coloring with the -C-C-C-B-C-B- sequence is energetically more stable than that with the -C-C-C-C-B-B- one. The electronic structures of these compounds, mainly determined by the B2C4 sheets, can be rationalized with the simple valence electron distribution M2+[B2C4]2−xe−, with the metals essentially acting as two-electron donors with respect to the boron-carbon network, the other x electrons remaining in the relatively narrow d and/or f bands of the metals. Accordingly, MB2C4 are narrow band-gap semiconductors (ΔE≈0.2-0.4 eV) with M=Mg and Ca. On the other hand, with M=La and Ce, the compounds are conducting with a relatively high density of states at the Fermi level predominantly metal in character with substantial B/Cπ* antibonding state admixture.