Crystal structural change of iron hydroxide by nanoparticulation

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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.