Structure of graphite-like BC2 layer in Sc2B1.1C3.2: An intermediate between BC and BC3

Abstract

The ternary borocarbide Sc2B1.1C3.2 is known to have a layered structure in which graphite-like BC2 layers and NaCl-like Sc2C layers are alternately stacked. Due to the complexity of the structure, X-ray analysis is unable to determine the precise structure of the BC2 layer, namely, how the boron atoms are arranged in the layer. Furthermore, the stacking manner of the two layers remains unclear. In this study, we have investigated the entire structure of Sc2B1.1C3.2 through first-principles calculations. The calculations reveal that the material is the most stable when B atoms are arranged to form 3×3 –R30° superlattices in the graphite-like layer. The interlayer bonding is mostly van der Waals-like, and the total energy is almost independent of the stacking method. The BC2 layer with the 3×3–R30° structure can be regarded as an intermediate between similar graphite-like materials, BC and BC3. The electronic density of states of Sc2B1.1C3.2 exhibits metallic features, and electrons are transferred from Sc to B and C atoms, which compensates the lack of electrons in the BC2 layer. The origin of the undulation in the BC2 layer is not the inner electronic structure but the interaction between the BC2 and Sc2C layers. The weak localization found in the in-plane resistivity in experiments is explained by the domain boundaries of the 3×3–R30° structure rather than the randomness of the arrangement of B atoms.