Helmut Werheit | Prof. Dr. rer. nat	website:	http://www.werheit.mynetcologne.de/
		e-mail:	helmut.werheit@koeln.de   helmut.werheit@uni.duisburg-essen.de

 

Structures of some boron-rich structures

 

Boron carbide

Symmetry type of the structure is (space group 166). Initially, the rhombohedral unit cell was assumed to be composed of B₁₂ icosahedra at its vertices and a linear CCC chain on the main diagonal parallel to the crystallographic c-axis, leading to the chemical composition B₁₂C₃. After the central chain atom was proved to be solely boron, the structure formula (B₁₂) CCC was modified to (B₁₁C) CBC. This structure model is depicted in Fig. 1 using the program VESTA[i]. The apparently plausible formula B₄C is long since proved to be incorrect ⁷, but nevertheless it has often been used as a synonym for boron carbide.

According to the present state of art, the chemical compound B₄C or B₁₂C₃ does not exist, at least not, when it is prepared by the common high-temperature preparation methods melting or hot-pressing, typically exceeding 2300 K. Actual chemical compounds of boron carbide exist in the large homogeneity range extending from the carbon-rich limit B_4.3C [ii] to the boron-rich limit B_~10.8C [iii]. Various structure elements have been identified; their concentrations depend on the actual chemical composition, and the distribution is assumed to be statistical, as no superlattice has been identified so far:

 ·                B₁₂ icosahedra

·                B₁₁C icosahedra with the C atom accommodated in one of the 6 polar sites

·                CBC chains

·                CBB chains

·                B□B arrangements (□, vacancy)

 Thus, boron carbide has no real crystal structure characterized by a well defined unit cell, but exhibits at the most an elementary cell with a twelve-atom icosahedron and a mostly three-atom chain, both in varying compositions, which are randomly distributed.

Therefore, the most prominent methods of structure analysis, x-ray as well as neutron scattering and NMR, fail in determining the actual microstructure of boron carbide, indeed for different reasons. X-ray and neutron diffraction, averaging a large volume with differently composed elementary cells, yield the atomic sites correctly, but are unable to determine their individual occupancies. Moreover, these methods are decisively impeded by the difficulty of distinguishing B and C atoms because of their very similar scattering cross section.

NMR spectra of boron carbide contain the overlapping resonances of ¹¹B and ¹³C isotopes respectively, whose local environments vary in the volume analyzed. This aggravates decisively the general problem of NMR, whose interpretation is unambiguous at most in neat structures. Otherwise, it depends on the more or less arbitrarily chosen structure models used for fitting the spectra. In the case of boron carbide, the random mixture of differently composed cells and structure elements makes a reliable analysis of the spectra nearly impossible.