Polar and non-polar structures of NH4TiOF3

Ammonium oxofluorotitanate, NH₄TiOF₃, is probably the best known precursor for the synthesis of anatase mesocrystals. Transformation of NH₄TiOF₃ into TiO₂ through thermal decomposition, accompanied by hydrolysis, preserves some structural features of the precursor. Currently, any discussion of the mechanism of this transformation is difficult, as the exact crystal structure of the starting compound is not available and no intermediate structures are known. This article describes the outcome of single-crystal and powder X-ray diffraction studies, revealing the existence of two polymorphs of the parent NH₄TiOF₃ at different temperatures. A second-order phase transition from the polar Pca2₁ α phase (1), stable at room temperature, to the Pma2 β phase (2) above ∼433 K has been demonstrated. The direction of the pseudo-fourfold axis in NH₄TiOF₃ coincides with the orientation of the fourfold axis of anatase mesocrystals, consistent with a topotactical transformation.