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The boron carbide chain is a hard and brittle solid that is used in industrial applications like slurry pumping nozzles, grit blasting nozzles, and abrasion resistant plates. Its combination of high shear strength and elastic modulus makes it an extremely effective material for ballistic armour. However, a number of factors can cause premature failure, including shock-induced localised amorphisation and the propagation of microcracks across the surface.
The B4C crystal structure consists of 12-atom icosahedra linked by three-atom chains (either B-B-C or C-C-C). In the carbon-rich limit, these can form two kinds of intercluster bonds. The first, involving pairs of polar and equatorial atoms, is defined by the chemical formula B12C3, and is known as a rhombohedral exo-cluster bond. Its low DE (delocalised energy) is due to steric effects hindering access of reacting atoms to the polar icosahedra.
A second, involving pairs of equatorial and polar atoms in neighboring clusters, is defined by the chemical formula B13C2. It is a covalent exo-cluster bond and its low DE is attributed to an electron deficiency. The electron density study of B13C2 reveals a strong covalent interaction between BP atoms in adjacent clusters. The value of the atomic interaction tensor, rBCP, and the Laplacian, 2rBCP, are higher than those for the intercluster bonds in a-boron21 and g-boron22.
The stoichiometric B13C2 structure is free of intrinsic disorder and is built of B12 icosahedral clusters and C-BC-C chains. The stoichiometric structure is an insulator or large band-gap semiconductor according to DFT calculations2,12. On the carbon-rich side of B13C2, the stoichiometric chemistry has low reactivity with oxidizing agents at elevated temperatures, which is due to the small volume of BC atoms. In contrast, non-stoichiometric B12C3 has a high reactivity because of the presence of a unique, electron-deficient 3e2c bond at the BP site.