Orientation domains in the intermediate product Na₃TiOF₅ during the synthesis of anatase TiO₂ nanosheets with exposed reactive {001} facets

A powder of oxyfluorotitanate Na₃TiOF₅ crystals is acquired as an intermediate product during the synthesis of anatase titanium dioxide (TiO₂) nanosheets with a large percentage of exposed reactive {001} facets. By application of transmission electron microscopy techniques, mainly energy dispersive spectroscopy and selected-area electron diffraction, coherent domain variants of Na₃TiOF₅ are shown to possess monoclinic structure with space group P2₁/n. The occurrence of these orientation domain variants is attributed to the reduction of crystal symmetry as a result of the phase transition from the high-temperature cubic phase to the low-temperature monoclinic phase. Through a detailed group theory analysis, the orientation domains are shown to exhibit 12 variants and 11 domain boundaries, which can be categorized into three types of perpendicular twins and two types of antiparallel twins. This work may provide meaningful insight for understanding the growth mechanism of anatase TiO₂ with a high percentage of reactive facets.