Structure and electronic properties of amorphous WO₃

The structure and electronic structure of amorphous WO₃ were studied with first-principles density-functional calculations. Upon amorphization, a large increase of the band gap is observed. The empty states exhibit a tendency towards localization. We studied the filling of these states as induced by addition of an alkali metal (Na) or removal (“deficiency”) of O. Na addition does not affect the structure much, and results in a filling of the lower part of the conduction-band continuum. Oxygen removal can induce large structural relaxations, which can give rise to the formation of W-W dimers (bond length <3.0 Å) and corresponding in-gap states. We suggest a reinterpretation of the nature of the active species in the intervalence charge-transfer mechanism that is believed to explain the electrochromic behavior of thin-film amorphous tungsten bronzes.