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

Abstract

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, R3¯, Z = 3) and at 233 K, which is below the ferroelastic phase transition (271.5–275.5 K (LT), monoclinic P21/c or triclinic P1¯, 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)5F]2+, and [Ti(H2O)5F]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)5F]x[Ti(H2O)5F]y}[Ti(OH)zF6–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•••F hydrogen bonds.