Rare-earth and tungsten oxynitrides with a defect fluorite-type structure as new pigments

The thermal nitridation in flowing ammonia of different rare-earth tungstat es leads to new oxynitride compositions in which the +VI oxidation state of tungsten can be kept. From the tungstates Re,WO,, fluorite-type oxynitride s are prepared as brown-colored powders of the general composition A(4)X(6. 6) (A = cations, X = anions) intermediate between the CaF2 (A(4)X(8)) fluor ite and Mn2O3 (A(4)X(6)) bixbyite stoichiometries. Starting from the tungst ates Re14W4O33 and Re6WO12 which have a defect fluorite-type structure, a p rogressive substitution of nitrogen for oxygen within the anionic network i s shown to be possible with the formation of two oxynitride solid solution domains ranging from A(4)X(7.33)square (0.67) and A(4)X(6.85)square (1.15), respectively, to A(4)X(6)square (2) in both cases. The color of the nitrid ed powders changes continuously from white to yellow with the nitrogen enri chment as a function of the nitridation temperature and time, and the diffu se reflectance spectra confirm that the absorption edge is progressively sh ifted towards higher wavelength values. These observations can be explained by a decrease in the energy band gap, as nitrogen gives with cationic elem ents more covalent bonds than oxygen. So, this progressive N3-/O2- anionic substitution gives access to a new class of pigments with, in addition, the possibility to tune the absorption edge position to a precise value. (C) 2 001 Elsevier Science B.V. All rights reserved.