The Polymorphic Crystallization of Titanium(IV) Oxide under Hydro-thermal Conditions. II. The Roles of Inorganic Anions in the Nucleation of Rutile and Anatase from Acid Solutions

Titanic acid was treated with hydrochloric, nitric, sulfuric, and hydrofluoric acids at 350 °C and 49 MPa to give titanium(IV) oxide. With increasing acid concentration, its polymorphic forms changed from anatase to rutile in hydrochloric and nitric acids, but from anatase to rutile, then back to anatase in sulfuric and hydrofluoric acids. Rutile and anatase crystals were grown from 1.7–9.5 mol dm−3 hydrofluoric acid at 360–560 °C by the temperature-drop method. Lower temperatures and higher concentrations of the acid favored anatase formation. The mechanisms for crystallization of the two forms are discussed by considering the chemical behavior of titanium(IV) complexes involved in their primary nucleation.