Crystal Structure of Tungsten Oxides

Tungsten trioxide (WO₃) is the most widely used tungsten oxide today. Perfect tungsten oxides are ReO₃-type cubic crystal structure materials in which octahedral WO₆ are interconnected by parting the angles. In the WO₆ octahedron, the W atom is located in the center and the remaining six O atoms form the octahedral framework. With changes in temperature and pressure, the WO₆ octahedra tilt and rotate at certain angles, leading to the formation of several different phases: tetragonal, orthorhombic, monoclinic, triclinic, and cubic phases.

Inside WO₃, there are some sites and tunnels between octahedra, so small diameter atoms such as H⁺, Li⁺ and K⁺ can be transferred into WO₃ and stored. There is another phase of WO₃, the hexagonal phase, which can be obtained from the hydration process of hydrated tungsten oxides. After the WO₆ octahedral stacking, trigonal and hexagonal tunnels are formed along the c-axis. These tunnels facilitate the rapid transfer of ions and electrons, so the electrochemical activity of hexagonal tungsten oxides is better than that of other phases of WO₃. The most common tungsten trioxide phases are the monoclinic phase (m-WO₃), and the hexagonal phase (h-WO₃).

Oxygen deficiency is very common in naturally occurring WO3, leading to the presence of sub-stoichiometric tungsten oxides, WO₃-x (0 < x < 1), where the valence of W may be +3, +4, or +5. Of these, W₁₈O₄₉, W₂₀O₅₈ and W₂₄O₆₈ are the most common, and their internal oxygen deficiency promotes their electrical conductivity. This property makes WO₃ an n-type semiconductor whose conductivity can be tuned by controlling the amount of O vacancies in it.

Tungsten oxides made by liquid-related methods are usually hydrated tungsten oxides before heat treatment, i.e., WO₃-xH₂O, which mainly include WO₃-0.33H₂O, WO₃-0.5H₂O, WO₃-H₂O, and WO₃-2H₂O. The crystal structure of WO₃-xH₂O is mainly determined by the value of x. For example, the structure of WO₃-H₂O is that the H₂O is located in the interlayer gap of the WO₆ octahedron, while the structure of WO₃-2H₂O is that in addition to the similar H₂O molecules in WO₃-H₂O, another class of H₂O molecules is directly connected to the tungsten atoms at the bottom or top of the octahedron.

This structure facilitates the easy transport of ions and electrons, especially protons, through their internal hydrogen bonding network. Typically, hydrated tungsten oxide has better electrical conductivity than pure tungsten oxide, translating into enhanced electrochemical properties.

Reference: Han W, Shi Q, Hu R. Advances in electrochemical energy devices constructed with tungsten oxide-based nanomaterials[J]. Nanomaterials, 2021, 11(3): 692.

• < Prev
• Next >