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Abstract:
A unique combination of useful properties in boron-carbide, such as extreme hardness, excellent
fracture toughness, a low density, a high melting point, thermoelectricity, semi-conducting behavior,
catalytic activity and a remarkably good chemical stability, makes it an ideal material for a wide range
of technological applications. Explaining these properties in terms of chemical bonding has remained
a major challenge in boron chemistry. Here we report the synthesis of fully ordered, stoichiometric
boron-carbide B13C2 by high-pressure–high-temperature techniques. Our experimental electron-density
study using high-resolution single-crystal synchrotron X-ray diffraction data conclusively demonstrates
that disorder and defects are not intrinsic to boron carbide, contrary to what was hitherto supposed. A
detailed analysis of the electron density distribution reveals charge transfer between structural units in
B13C2 and a new type of electron-deficient bond with formally unpaired electrons on the C–B–C group in
B13C2. Unprecedented bonding features contribute to the fundamental chemistry and materials science
of boron compounds that is of great interest for understanding structure-property relationships and
development of novel functional materials.
