Bixbyite- and anatase-type phases in the system Sc-Ta-O-N

The aim of our study was to modify the basis compound ß-TaON, which crystallizes in the monoclinic baddeleyite-type, by incorporation of appropriate dopant ions, in order to obtain anion-deficient cubic fluorite-type phases, which are of interest as solids with mobile nitrogen ions. For this purpose, scandium-doped tantalum oxide nitrides were prepared by ammonolysis of amorphous oxide precursors. An unexpected variety of phases with different structural features was observed: bixbyite-type phases of general composition Sc_xTa_1−x(O,N)_y with 0.33≤x≤1 and 1.7≤y≤1.9, yellow colored metastable anatase-type phases such as Sc_0.1Ta_0.9O_1.2N_0.8 or Sc_0.15Ta_0.85O_1.3N_0.7 and, additionally, anosovite-type phases Sc_xTa_3−xO_2xN_5−2x with 0≤x≤1.05. Selected phases were investigated by UV/vis spectroscopy. Anatase- and anosovite-type compounds show brilliant colors. In the anatase-type phase, a possible anion ordering was examined by theoretical methods. Additionally, energy calculations on phase stability were performed for Sc_xTa_1−xO_1+2xN_1−2x in the baddeleyite, rutile, and anatase structure types with varying amounts of dopants.