Theoretical Investigations on the Structural and Electronic Properties of WO3 Polymorphs

The structural characteristic and electronic properties of seven WO_3 bulk polymorphs have been studied by first-principles density functional theory(DFT) calculations. Our calculation results indicate that according to the energy of WO_3 per unit, the stability of different WO_3 systems reduces in turn in the following order: monoclinic WO_3, orthorhombic WO_3, tetragonal WO_3, triclinic WO_3, hexagonal WO_3 and cubic WO_3. And when the unit cell is distorted from the cubic framework, the alternatively long and short W–O bonds are formed, implying an improvement of bonding-antibonding splitting associated with the interactions between W and O atoms. The deformation of the WO6 octahedron enhances the stability of the system to some extent though the energy difference between various WO_3 phases is small(< 0.1 eV/per WO_3 unit). Besides, the band gap tends to enlarge accompanied with the symmetry decrease of WO_3 bulk. According to the band structures, the minimum band gaps for those WO_3 polymorphs are direct at the Γ point except for the simple cubic and hexagonal structures. In addition, for all the WO_3 phases, the compositions of the valance band maximum(VBM) and conduction band minimum(CBM) are the same, which are relative to the O 2 p and W 5 d orbitals, respectively.