Structures of the Boron-Rich Boron Carbides from Neutron Powder Diffraction:  Implications for the Nature of the Inter-Icosahedral Chains

Abstract

Results from structure refinement using neutron powder diffraction data for boron carbide samples with 10, 13, 16, and 20 at. % carbon are reviewed. Those obtained for ceramic powder samples show an apparent large vacancy concentration (as high as 25%) at the central atom position of the linear three-membered chains. A model deduced from the previous X-ray structure of the boron-rich end member, B 9 C, suggests the shift of sufficient scattering density from the central chain position into adjacent voids to account for the observed vacancies. On the other hand, neutron powder diffraction from ground, Cu-melt-grown single crystals gives a structure which shows no such vacancies. This, together with results from sequential runs with the same ceramic sample, suggests that the vacancies in the ceramic materials are intrinsic to their synthesis and do not result from radiation damage, as has been suggested. Our results support the more recent interpretation of other experimental results concerning the nature of the three-membered C-B-C chains in the boron carbides, i.e., that the bonding to the central atom is very weak. Further, low-temperature structural studies of the ground single crystals suggest that even at the nominal B 4 C composition, there is site disorder at the chainend "C" site. It is most likely that this disorder arises from the presence of boron (or C-B-B) chains.

Heavily Boron Doped Diamond Powder: Synthesis and Rietveld Refinement

“…The crystal data and structure refinement are shown in Tables 1 and 2. The unit cell parameters and the weight fraction in the mixture were refined for B4C, while the atomic parameters were taken from [18].…”

Atomic Structure and Vibrational Properties of IcosahedralB4CBoron Carbide

“…Neutron diffraction [6,7] cannot distinguish 11 B from 12 C because their scattering lengths are too close [6]. X-ray diffraction has allowed to identify a C-B-C chain [8,9], but the location of the remaining C atom in the B 11 C icosahedron remains unsettled because the X-ray form factors of boron and carbon atoms are also too close.…”

Electronic structures and mechanical properties of boron and boron-rich crystals (Part I)

“…In terms of the bond length and phonon properties, it is indisputable that the chain bond is the strongest among the bonds of this crystal [77]. Despite, there are many observations indicating the occurrence of defects in the chain [40,164], which led some researchers to interpret it as soft bond [127,165]. This contradiction has not been completely resolved.…”

“…The icosahedron-based structure was first identified for boron carbide, which is formally referred to as B 12 C 3 or more shortly as B 4 C [33,34]. The true structures became more complicated as the study progressed [35][36][37][38][39][40][41]. The composition varies in some range, typically from B 12 C 3 to B 13 C 2 , but more C-poor compounds exist [42].…”