Density-functional studies of tungsten trioxide, tungsten bronzes, and related systems

Tungsten trioxide adopts a variety of structures which can be intercalated with charged species to alter the electronic properties, thus forming ``tungsten bronzes.'' Similar effects are observed upon removing oxygen from WO₃. We present a computational study of cubic and hexagonal alkali bronzes and examine the effects on cell size and band structure as the size of the intercalated ion is increased. With the exception of hydrogen (which is predicted to be unstable as an intercalate), the behavior of the bronzes are relatively consistent. NaWO₃ is the most stable of the cubic systems, although in the hexagonal system the larger ions are more stable. The band structures are identical, with the intercalated atom donating its single electron to the tungsten $5d$ valence band. A study of fractional doping in the NaₓWO₃ system (0x1) showed a linear variation in cell parameter and a systematic shift in the Fermi level into the conduction band. In the oxygen-deficient WO_3-x system the Fermi level undergoes a sudden jump into the conduction band at around $x=0.2$. Lastly, three compounds of a layered WO₄∙α,ω-diaminoalkane hybrid series were studied and found to be insulating, with features in the band structure similar to those of the parent WO₃ compound that relate well to experimental UV-visible spectroscopy results.