Crystal Structure and Electron Density of Tantalum Oxynitride, a Visible Light Responsive Photocatalyst

Tantalum oxynitride (TaON) is investigated by neutron and synchrotron diffraction analysis to clarify the structural and electronic features responsible for the visible-light activity of this material. In combination with density functional theory (DFT) calculations, the present measurements confirm the TaON photocatalyst to be monoclinic with space group P21/c (a = 4.94941(4) Å, b = 5.01662(4) Å, c = 5.16430(2) Å, α = 90°, β = 99.6107(4)°, γ = 90°, V = 126.426(2) Å3 at 298 K). Complete occupational ordering of the anions occurs in alternate layers normal to the [100] direction. Oxygen and nitrogen atoms in TaON are coordinated with three and four tantalum atoms, respectively. Bond valence sums for the oxygen and nitrogen atomic sites are calculated to be 2.1 and 3.2, respectively. Electron density analyses based on synchrotron data in combination with DFT calculations reveal covalent bonds between Ta and O atoms and between Ta and N atoms. The hybridization of anion 2p and Ta 5d orbitals attributed to the covalent bonds between Ta and O or N atoms results in increased dispersion of the valence band, raising the top of the valence band and leading to a visible-light response. The minimum electron density at the Ta−O bond decreases with increasing bond length, whereas no such trend was observed for the Ta−N bond.