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.
Highlights
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
The high mechanical and thermal stability, low density and low costs of fabrication have made boron carbide the prime choice in a series of technological applications[1,2,3,4,5,6,7]
Boron carbide preserves the same structure for a range of compositions, and details of this crystal structure have been discussed in terms of chemical disorder of boron and carbon atoms as well as the presence of vacancies[1,8,9,10,11]
Summary
Electrons between C and BE again gives a formal charge of − 0.17 for BE, and it would result in a (B12)2− group[2] carbon is more electronegative than boron and should attract most of the bonding electrons. A detailed analysis of the electron density shows that the positive charge of BE is the result of a strong polar-covalent character of the C–BE bond, with the BCP much closer to BE than to C (Fig. 2; Table 1), but with a large value of ρ BCP as opposed to an expected small value for ionic bonding[20]. The electron deficient character of this bond is in complete agreement with the ionic charge of + 2.30 of BC The latter value is the result of the extremely small volume of the atomic basin of this atom, which demonstrates that the internal pressure has squeezed out most of the electrons of BC, reminiscent of the effect of pressure on the electrons in lithium metal[25]. See the Supplementary Information for details on procedures and the MP model
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