Thermal oxidation of oxynitride perovskites in different atmospheres

Oxynitride perovskites with bright and variable colour have the potential to be employed as non-toxic pigments, and to substitute colourants that contain harmful heavy metals. For this application it is extremely important to have a precise knowledge about the materials stability. The thermal stability of oxynitride perovskites in different atmospheres was measured by thermogravimetry in combination with mass spectroscopy (MS). The studied compounds, namely LaTiO₂N, SrNbO₂N and SrTaO₂N, were heated up to 1200 °C in argon–oxygen mixtures with varying oxygen contents. It was found that the thermal behaviour of the studied oxynitrides changes drastically with the oxygen concentration. When heated up in pure argon the oxynitrides transform to oxides containing transition metals of lower oxidation state and/or binary nitrides. For oxygen contents between 0.5% and 20% the samples were completely oxidised. The oxidation rate increases with oxygen content. MS analysis of the gaseous products (including N₂, NO and NO₂) reveals a complex reaction mechanism, which is strongly modified by the composition of the atmosphere.