NEUTRON POWDER DIFFRACTION REFINEMENT OF BORON CARBIDES - NATURE OF INTERICOSAHEDRAL CHAINS

Our Rietveld structure refinements of neutron powder diffraction data of boron carbides with 10, 13, 16, and 20 at.% carbon indicate 15-25% vacancies at the central position of the intericosahedral chains. In a ddition, we find a comparable number of interstitials residing off the intericosahedral chains. However, X-ray diffraction studies on unirra diated boron carbide single crystals do not generally find these defec ts. This discrepancy may indicate that the observed defects are the re mnants of defect cascades created by the energetic fission products of B-10 during the neutron diffraction measurements. The tendency to for m vacancies in the center of the intericosahedral chain is consistent with the weak and soft binding of chain-center atoms that we previousl y inferred from other measurements.