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 (TiO 2 ) are known: anatase, brookite, and the α-lead dioxide (α-PbO 2 )-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 TiO 6 octahedra. Inorganic structural chemistry considers that, in the rutile and α-PbO 2 types, Ti ions occupy half of the octahedral voids in the hexagonal close packing of oxide ions, and the TiO 6 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-PbO 2 -type zigzag strands. Additionally, I characterize the crystal structures of four TiO 2 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 d 0 ions result in an unusual variety of polymorphs in TiO 2 .