Chlorine Occupancy Dependence of Crystal Structure of Pure β-Phase of Iron-Oxyhydroxide

Abstract

Crystal structure of β-phase of iron-oxyhydroxide (Akaganeite) with different chlorine occupancy was studied by X-ray diffraction with Rietveld analysis. The structure depends on chlorine occupancy, u in tunnel sites of the phase. The crystal system was assigned to monoclinic (I2/m) throughout u range whereas the lattice and structure parameters changed with u. The chemical formula was estimated to be Fe₈³⁺O₁₆²⁻H_8+2u⁺Cl_2u⁻(H₂O)_2w (0.25≤u≤0.56 and w\\simeq0.2) where w denotes fraction of water molecules in tunnel sites. A sample with u=0.44 showed the highest crystal symmetry throughout the u range. The u-dependence of some structural parameters was discussed from the view point of oxygen displacements.