Structures of the boron-rich boron carbides from neutron powder diffraction: Implications for the nature of the inter-icosahedral chains
Structures of the boron-rich boron carbides from neutron powder diffraction: Implications for the nature of the inter-icosahedral chains
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DOI：
10.1021/jp953235j
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发表时间：
1996-05-09
影响因子：
--
通讯作者：
Morosin, B
中科院分区：
文献类型：
--
作者：
Kwei, GH;Morosin, B
使用中子粉末衍射数据对 10、13、16 和 20 at 碳化硼样品进行结构细化的结果。审查碳百分比。陶瓷粉末样品获得的结果显示，线性三元链的中心原子位置处存在明显较大的空位浓度（高达 25%）。从富硼端元 B9C 的先前 X 射线结构推导出来的模型表明，足够的散射密度从中心链位置转移到相邻空隙，以解释观察到的空位。另一方面，来自研磨的、Cu熔体生长的单晶的中子粉末衍射给出了不显示此类空位的结构。这与对同一陶瓷样品进行连续运行的结果一起表明，陶瓷材料中的空位是其合成所固有的，而不是由辐射损伤造成的，正如所暗示的那样。我们的结果支持了对有关碳化硼中三元C-B-C链性质的其他实验结果的最新解释，即与中心原子的键合非常弱。此外，研磨单晶的低温结构研究表明，即使在标称的 B4C 组成下，链端“C”位点也存在位点无序。这种紊乱很可能是由硼（或 C-B-B）链的存在引起的。
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.