Mixed-valence hydrated iron fluoridotitanate: Crystal structure and thermal behavior

The digestion of the natural mineral ilmenite, FeTiO3, in hydrofluoric acid results in the formation of mixed-valency hydrated iron fluoridotitanate. High quality single crystals of the compound were obtained and their crystal structures at room temperature (RT, 296 K, trigonal, R (3) over bar, Z = 3) and at 233 K, which is below the ferroelastic phase transition (271.5-275.5 K (LT), monoclinic P2(1)/c or triclinic P (1) over bar, Z = 2) were determined by X-ray diffraction. Both iron and titanium are present in a mixed-valency state. The cationic sublattice is characterized by the presence of hydrated [Fe(H2O)(6)](2+), [Fe(H2O)(5)F](2+), and [Ti(H2O)(5)F](2+) octahedra, while only [TiF6](2-) octahedra form the anionic sublattice with partial isomorphous substitution of hydroxide ions for fluoride. The overall formula of the compound studied was determined to be {[Fe(H2O)(6)](1-x-y)[Fe(H2O)(5)F](x)[Ti(H2O)(5)F](y)}[Ti (OH)(z)F6-z] (x, y <= 0.1; z <= 0.5). Its thermal behavior is characterized by two-stage dehydration followed by pyrohydrolysis with the HF evolution in accordance with the existence of three crystallographically distinct O-H center dot center dot center dot F hydrogen bonds.