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

The thermal nitridation in flowing ammonia of different rare-earth tungstates leads to new oxynitride compositions in which the +VI oxidation state of tungsten can be kept. From the tungstates Re,WO,, fluorite-type oxynitrides 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)) fluorite and Mn2O3 (A(4)X(6)) bixbyite stoichiometries. Starting from the tungstates Re14W4O33 and Re6WO12 which have a defect fluorite-type structure, a progressive substitution of nitrogen for oxygen within the anionic network is 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 nitrided powders changes continuously from white to yellow with the nitrogen enrichment as a function of the nitridation temperature and time, and the diffuse reflectance spectra confirm that the absorption edge is progressively shifted towards higher wavelength values. These observations can be explained by a decrease in the energy band gap, as nitrogen gives with cationic elements 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) 2001 Elsevier Science B.V. All rights reserved.