Crystal structural change of iron hydroxide by nanoparticulation

利用同步加速器x射线全散射实验中的原子对分布函数（PDF）分析研究了水合铁向赤铁矿的聚集诱导相变。由PDF衰减估计的六线水合铁的粒径从约4 nm增加到10 nm以上。然后水合铁开始转变为赤铁矿。铁水合体结构的晶格参数随粒径的增大而减小。随着晶格参数的不断减小，六元环发生收缩。当由三个FeO6八面体和三个FeO4四面体组成的六元环收缩时，FeO4四面体配位变为八面体。因此，六元环转变为由六个FeO6八面体组成的环，这与赤铁矿的结构相同。这种由六元环收缩引起的配位变化导致了水合铁向赤铁矿的相变。

Aggregation-induced phase transition of ferrihydrite to hematite was investigated using atomic pair distribution function (PDF) analysis of synchrotron x-ray total scattering experiments. The particle size of six-line ferrihydrite estimated from attenuation of the PDF increases from about 4 nm to above 10 nm. Then the ferrihydrite starts to transform to hematite. The ferrihydrite structure exhibits a strong tendency for the a lattice parameter to decrease with particle size. As the a lattice parameter decreases continuously, contractions of the six-membered rings occur. When a six-membered ring comprising three FeO6 octahedra and three FeO4 tetrahedra is contracted, the FeO4 tetrahedral coordination changes to octahedral. Consequently, the six-membered ring transforms to that comprising six FeO6 octahedra, which is the same configuration as that of hematite. This coordination change caused by the six-membered ring contraction induces the phase transition of ferrihydrite to hematite.