Structures of the boron-rich boron carbides from neutron powder diffraction: Implications for the nature of the inter-icosahedral chains

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, B9C, 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 B4C composition, there is site disorder at the chain-end ''C'' site. It is most likely that this disorder arises from the presence of boron (or C-B-B) chains.