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

Well crystallized Fe-based oxyhydroxy-fluoride with the FeO(OH0.2F0.8)·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 FeO0.8OH1.2·0.2Cl akaganeite because of the random distribution of F−/OH−/O2− in the vicinity of this Fe cation.