Inorganic Structural Chemistry of Titanium Dioxide Polymorphs

Apart from rutile, which crystallizes in the rutile-type structure characteristic of many metal dioxides, three major polymorphs of titanium dioxide (TiO2) are known: anatase, brookite, and the α-lead dioxide (α-PbO2)-type high-pressure form. Ti ions are commonly found in octahedra composed of six oxide ions, and their crystal structures are distinguished according to the linkage pattern of the TiO6 octahedra. Inorganic structural chemistry considers that, in the rutile and α-PbO2 types, Ti ions occupy half of the octahedral voids in the hexagonal close packing of oxide ions, and the TiO6 octahedra in each layer are joined via edge sharing to form linear and zigzag strands, respectively. Anatase and brookite, on the other hand, exhibit more complex three-dimensional edge-sharing octahedral connections, although their origins are not fully explained. I show that these configurations can be interpreted as distinct stacking structures of layers with α-PbO2-type zigzag strands. Additionally, I characterize the crystal structures of four TiO2 polymorphs in detail using stacking sequence descriptions based on anion close packings and explore their relationships in terms of inorganic structural chemistry. I note that the moderate covalent nature of the Ti-O bond and the local structural instability of d0 ions result in an unusual variety of polymorphs in TiO2.