Boron carbide, B13-x C2-y (x = 0.12, y = 0.01)

Abstract

Boron carbide phases exist over a widely varying compos-itional range B12+x C3-x (0.06 < x < 1.7). One idealized structure corresponds to the B13C2 composition (space group R-3m) and contains one icosa-hedral B12 unit and one linear C-B-C chain. The B12 units are composed of crystallographically distinct B atoms BP (polar, B1) and BEq (equatorial, B2). Boron icosa-hedra are inter-connected by C atoms via their BEq atoms, forming layers parallel to (001), while the B12 units of the adjacent layers are linked through inter-icosa-hedral BP-BP bonds. The unique B atom (BC) connects the two C atoms of adjacent layers, forming a C-B-C chain along [001]. Depending on the carbon concentration, the carbon and BP sites exhibit mixed B/C occupancies to varying degrees; besides, the BC 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: [B12] and [B11C] icosa-hedra (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 icosa-hedron formed by BEq (B2) thus rendering the formula B0.87(B0.98C0.02)12(B0.13C0.87)2 for the current boron carbide crystal.