STRUCTURAL MODEL FOR FERRIHYDRITE
STRUCTURAL MODEL FOR FERRIHYDRITE
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DOI：
10.1180/claymin.1993.028.2.02
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发表时间：
1993-06-01
期刊：
影响因子：
1.5
通讯作者：
MANCEAU, A
中科院分区：
文献类型：
--
作者：
DRITS, VA;SAKHAROV, BA;MANCEAU, A
重新考虑了六线和两线水合铁的结构。首次对所提出的不同结构模型进行了X射线衍射(XRD)曲线的模拟，结果表明，这两种模型都不能反映水合铁氧体的实际结构。根据实验和模拟的X射线衍射曲线的一致性，表明FH是由三个组分组成的混合物：(1)由阴离子组成的无缺陷FH。六方晶胞参数为a=2.96埃，c=9.40埃，空间群为P31cbar。(Ii)缺陷Fh，其中Ac1Bc2A型和Ab1Cb2A型结构碎片以相同的概率出现并完全随机交替；这些碎片中的Fe原子在a=5.126埃的六方超晶胞中具有相同的有序分布。(3)超分散赤铁矿，相干散射域(CSD)平均尺寸为10-20埃。6线和2线FH的主要结构差异在于其CSD的大小，而后者的CSD非常小。用这种新的结构模型计算的最近Fe-Fe距离与EXAFS光谱在相同样品上测定的结果非常接近。
The structure of 6-line and 2-line ferrihydrite (Fh) has been reconsidered. X-ray diffraction (XRD) curves were first simulated for the different structural models so far proposed, and it is shown that neither of these corresponds to the actual structure of ferrihydrite. On the basis of agreement between experimental and simulated XRD curves it is shown that Fh is a mixture of three components: (i) Defect-free Fh consisting of anionic ABACA ... close packing in which Fe atoms occupy only octahedral sites with 50% probability; the hexagonal unit-cell parameters are a = 2.96 angstrom and c = 9.40 angstrom, and the space group is P31cBAR. (ii) Defective Fh in which Ac1Bc2A and Ab1Cb2A structural fragments occur with equal probability and alternate completely at random; Fe atoms within each of these fragments have identical ordered distribution within the hexagonal super-cell with a = 5.126 angstrom. (iii) Ultradispersed hematite with mean dimension of coherent scattering domains (CSD) of 10-20 angstrom. The main structural difference between 6-line and 2-line Fh is the size of their CSD which is extremely small for the latter structure. Nearest Fe-Fe distances calculated for this new structural model are very close to those determined by EXAFS spectroscopy on the same samples.