A density-functional study on the stability of anatase-type phases in the system Mg-Ta-O-N

Magnesium-doped tantalum oxynitrides, which were prepared by ammonolysis of amorphous mixed oxides, have been investigated using quantum-theoretical methods. For small magnesium amounts (5 cat%), density-functional total-energy calculations indicate an anatase-type structure consisting of stretched, corner-sharing TaO₃N₃ octahedra with a tetrahedrally distorted equatorial plane. The calculated structural parameters are in excellent agreement with those obtained using X-ray powder diffraction and synchrotron radiation. Additionally, the quantum-chemical results show a clear preference for an ordered anionic distribution (space group I4₁md, no. 109) of the host lattice, which is locally disturbed around Mg²⁺. For thermodynamical reasons, the excess oxygen anions, which replace nitrogen on account of the lower charge of the dopant cation, segregate next to magnesium, thus forming local MgO “domains”. For higher magnesium contents (?10%), minor phases of rutile-type structure have to be expected, which is in good agreement with experimental data.