Hollandites’ crystal chemistry, properties, and processing: a review

ABSTRACT This review provides a broad overview of the structural characteristics, compositional flexibility, and structure–property relationships of hollandite materials. Hollandites have a general formula AxB 8O16, x ≤ 2, with ‘A’ cations located in one-dimensional tunnels formed by a framework of ‘B’–O octahedra. With numerous possibilities for chemical and structural modifications, hollandite family provides many opportunities to manipulate its properties for specific applications. First, we review the chemistry, structure–property relationship, and processing techniques for various applications. The primary focus is on the cumulative effects of A- and B-cation interaction, and the resultant parameters including unit cell symmetry, cation order–disorder, electronic and/or magnetic coupling, that dictate the material's applicability. Then, selected applications, such as crystalline hosts for radioactive caesium disposal, electrode material for Li-ion batteries, and ferromagnetic materials, are outlined from a structure–property relationship perspective. Finally, processing strategies in correlation with structural evolution and applications are briefly addressed.