Synthesis and characterization of inorganic pigments based on transition metal oxynitrides
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alum zirconium oxynitrides in the Ta–Zr–O–N system of Ta(3−x)Zrx(O/N)6 and Ta(3−x)ZrxN(5−x)Ox (0 艋 x 艋 0.6) compositions have been synthesized by the nitridation of the amorphous tantalum zirconium oxides, obtained by sol-gel technique, with flowing ammonia at temperatures in the range 1023–1123 K for several hours. Different coloration has been obtained, going from yellow to red, whose intensity is a function of the nitrogen content in each sample. The structure of these oxynitrides has been refined from powder x-ray diffraction by the Rietveld method. The present study shows that the baddeleyite phase is formed in the preliminary stages of the nitridation and then transformed to the Ta3N5-type phase. Below 1123 K, a baddeleyite-type structure with a monoclinic symmetry, space group P21/c, is observed in these phases. When the nitration temperature increases, a marked increase in the nitrogen content takes place, and these materials adopt the Ta3N5 structure, space group C2/m. Diffuse reflectance spectra and CIE-LAB color coordinates suggest that the solid solutions Ta(3−x)Zrx(O/N)6 and Ta(3−x)ZrxN(5−x)Ox (0 艋 x 艋 0.6) are expected to be promising candidates for new ecological pigments.
I. INTRODUCTION
Inorganic pigments are traditionally based on transition metal oxides1; however, some of them contain heavy or transition metals that can adversely effect the environment and human health if critical levels are exceeded. Because of their high toxicity, there is a growing need to develop new pigments that replace or surpass the properties of the traditional materials. In general, the color of a semiconductor appears brilliant and pure when a selective absorption of visible light is related to an electronic interband transition between the occupied valence band and the vacant conduction band, leading to a steep absorption edge in the visible spectrum.2 The width of the band gap is determined by the extent of overlap of the valence orbitals and by the difference between the electronegativities of the cations and anions involved. Most metal oxides have band gaps too wide to allow for absorption in the visible region of the electromagnetic spectrum. If nitrogen is introduced as a nonmetal with lower electronegativity than the oxygen, the gap energy can be reduced by as much as ∼1 eV,
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0272 J. Mater. Res., Vol. 21, No. 9, Sep 2006
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which makes oxynitrides and nitrides promising candidates for inorganic pigments. In fact, some oxynitrides have already been found to be colored.3–8 Unlike other transition elements, tantalum is able to form with nitrogen two ionic-covalent nitride-type compounds, the oxynitride TaON and the binary nitride Ta3N5, both containing tantalum in its highest oxidation state, Ta+5. While Ta2O5 is white, Ta3N5 powders present a bright red color; it is worth noting that this material will be commercially produced as pigment in the near future9–11 and an
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