Oxidation State of Chromium in Red-Shade Ceramic Pigments Based on Solid Solutions with Pyrochlore Structure

摘要

At present, a wide variety of high-quality ceramic pigments is available on the market. An exception make the red-shade pigments, as most of them present environmental risks in their production, insufficient thermal stability or low color intensity. The chromium-doped oxides form an important group of red-shade pigments and some of them are commonly used in practice. Several examples of industrial red-shade pigments containing Cr(lll) are: CaSnSiO_5:Cr~(3+) (sphene), Al_2O_3:Cr~(3+) (corundum), MnAl_2O_4:Cr~(3+) (spinel). The sphene-based pigment gives probably the best approximation to a red hue, after the pigments based on solid solutions of CdS:CdSe, yet insufficient to substitute them completely. The chromium pigments are relatively stable and environmentally safe, thus, the investigations aiming to improve the already known ones or develop new ones are continuing. Here, we report some results of an investigation on recently discovered by us ceramic pigments based on the pyrochlore structure. This structure has been used previously as the base for yellow-shaded pigments but never for red-shaded ones, at least up to our knowledge. Solid solutions with pyrochlore structure having the general formula A_2B_(2-x)Cr_xO_(7-2/x) (where A= Y~(3+), Nd~(3+), B= Sn~(4+), Zr~(4+), Ti~(4+)) were studied in view of their possible application as ceramic pigments. This stoichiometry was suggested assuming that the ion B(IV) is substituted by Cr(lll) and the charge compensation is realized through formation of oxygen vacancies, so that an oxygen vacancy is produced for every two substituted Sn(IV) atoms. Oxygen defect formation is a common phenomenon in the pyrochlore structure.