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

摘要

Magensium-doepd tantalum oxynitrides, which were prepared by ammonolysis of amorphous mixed oxides, have been investigated using qunatum-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 14(1)md, 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 magenesium, 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. (c) 2008 Ellsevier Inc. All rights reserved