Boron carbide, B(13-x)C(2-y) (x = 0.12, y = 0.01)

Boron carbide phases exist over a widely varying compos­itional range B(12+x)C(3-x) (0.06 < x < 1.7). One idealized structure corresponds to the B(13)C(2) composition (space group R-3m) and contains one icosa­hedral B(12) unit and one linear C—B—C chain. The B(12) units are composed of crystallographically distinct B atoms B(P) (polar, B1) and B(Eq) (equatorial, B2). Boron icosa­hedra are inter­connected by C atoms via their B(Eq) atoms, forming layers parallel to (001), while the B(12) units of the adjacent layers are linked through inter­icosa­hedral B(P)—B(P) bonds. The unique B atom (B(C)) connects the two C atoms of adjacent layers, forming a C—B—C chain along [001]. Depending on the carbon concentration, the carbon and B(P) sites exhibit mixed B/C occupancies to varying degrees; besides, the B(C) site shows partial occupancy. The decrease in carbon content was reported to be realized via an increasing number of chainless unit cells. On the basis of X-ray single-crystal refinement, we have concluded that the unit cell of the given boron-rich crystal contains following structural units: [B(12)] and [B(11)C] icosa­hedra (about 96 and 4%, respectively) and C—B—C chains (87%). Besides, there is a fraction of unit cells (13%) with the B atom located against the triangular face of a neighboring icosa­hedron formed by B(Eq) (B2) thus rendering the formula B(0.87)(B(0.98)C(0.02))(12)(B(0.13)C(0.87))(2) for the current boron carbide crystal.