The Polymorphic Crystallization of TiO2 under Hydrothermal Conditions. I. The Effect of Phosphate Ions on the Selective Crystallization of Anatase

Abstract

The hydrothermal growth of anatase has been investigated at 480–650 °C and 500–1000 atm. Only rutile was formed in KF solutions. On the other hand, anatase was found to crystallize in solutions containing 5 wt% KF+0.1–10 wt% K2HPO4 at temperatures up to 600 °C, accompanied by small amounts of K2Ti6O13 on the addition of 5–10 wt% of K2HPO4. The optimum growth conditions were: mineralizer solution, 5 wt% KF+1–2.5 wt% K2HPO4; temperature, 550–600 °C; pressure, 1000 atm. Anatase crystals were found as irregular aggregates of dipyramidal grains, blue and transparent. Individual crystals grown at a low supersaturation were characterized by well-developed {101} and {103} faces. The observed habit agrees with the expectations based on Hartman’s PBC theory. The presence of phosphate ions favored anatase formation in NH4F solutions as well, and the retrograde solubility of TiO2 was observed. The reaction scheme for anatase formation has been proposed. The polymorphic crystallization of TiO2 under hydrothermal conditions has been shown to be greatly affected by the complexing anions, such as fluoride or phosphate, in mineralizer solutions.