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

•High quality mixed-valence single crystals of hydrated iron hexafluoridotitanate (HITF) were obtained.•Crystal structures of RT and LT modifications of HITF with a transition from a disordered state to an ordered one were determined.•Hydrated iron and trivalent titanium octahedra occupy the cationic sublattice.•Three types of hydrogen bonds of the O–H•••F type are responsible for the thermal behavior of HITF. 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. [Display omitted]