The Complex Crystal Chemistry of Niobium Tungsten Oxides

Niobium tungsten oxides are currently intensively studied because of their potential use as high-performance anode materials in lithium ion batteries, showing fast ion exchange and high cycling stability. Such properties originate from a varied structural chemistry in the pseudobinary system Nb2O5/WO3, which is based upon multifaceted octahedral frameworks derived from the ReO3 type. Different structure types like the block phases and the tetragonal tungsten bronze derivatives are realized to accommodate oxygen:metal ratios in the range 2.5 ≤ O/ΣM ≤ 3.0 (M = Nb,W). This review starts with an overview about the synthesis of the existing phases and their structures. The building principles of structures occurring in this system are described. In a given phase, the substitution of Nb5+ by equimolar amounts of various tetravalent metals and W6+ gives rise to isostructural solid solution series. The option to oxidize reduced phases at different temperatures introduces further complexity and generates unprecedented structural variants. Possibilities and limitations of X-ray diffraction and various electron microscopy methods for a comprehensive structural characterization of pure phases and less-ordered arrangements are discussed in a historical context. Finally, the outstanding electrochemical performance and thermoelectric properties of niobium tungsten oxides are addressed and perspectives for future applications discussed.