Comments concerning the crystal structure of B4C

The crystal structure of B4C at ∼20 atom% carbon is reported. The space group is R 3m, hexagonal cell lattice constants are a=5.6039(4)A, c=12.0786(14)A, v=1−9.50A3 Z=3, μ(MoKα)=0.05 mm−1. The excess carbon above the stoichiometric composition of B13C2 replaces up to 1/6 of the boron in the rhombohedral boron site (B2). The results of this refinement are reported and compared with earlier results reported on lower carbon samples. Evidence is that the icosahedron is essentially pure boron at and below carbon compositions of 13.3 a%.