Investigation of Fe-based oxyhydroxy-fluoride with hollandite-type structure

Well crystallized Fe-based oxyhydroxy-fluoride with the FeO(OH_0.2F_0.8)·0.2H₂O chemical composition has been prepared from hydrolysis of Fe trifluoride under supercritical CO₂ conditions. Investigation by Mössbauer spectroscopy and neutron diffraction show that this compound crystallize in the monoclinic symmetry (SG: I2/m, a = 10.447(7) Å, b = 3.028(2) Å, c = 10.445(4) Å, β = 90.00(3)°). Taking into account the Fe-O(F) bond distances, F⁻ anions are mainly located on the common vertices of Fe octahedra whereas OH⁻ groups occupy mainly the shared edges of the Fe octahedra. Two various highly distorted octahedral sites have been identified with Fe-O/F bond distances varying from 1.90 Å to 2.31 Å. One Fe site is more distorted than in FeO_0.8OH_1.2·0.2Cl akaganeite because of the random distribution of F⁻/OH⁻/O²⁻ in the vicinity of this Fe cation.