Tantalum and niobium perovskite oxynitrides: Synthesis and analysis of the thermal behaviour

Ta5+ and Nb5+-based oxynitride perovskites of the ABO2N type (A = Ca, Sr, Ba) were synthesised by ammonolysis of complex oxide precursors. These precursors were either crystalline perovskites or amorphous xerogels prepared by solid-solid reaction and by soft chemistry methods, respectively. Phase purity of the oxynitrides was verified by X-ray diffraction (XRD) and their crystal structures were determined by Rietveld refinements. The morphology of the obtained powders was characterised by scanning electron microscopy (SEM). Thermal stability was investigated by thermogravimetric analysis (TGA) coupled with mass spectroscopy. Oxidation studies reveal an intermediate product that gives rise to a characteristic weight gain in the TG curve. This intermediate was found for all the examined oxynitrides in oxidising atmosphere. Investigations by Raman scattering revealed the presence of dinitrogen (NN) loosely bound to B and NB bonds (B = transition metal) in the intermediate compounds. Mass spectral analysis confirmed molecular nitrogen evolution indicating that N2 is retained during the oxidation reaction. At higher temperatures (T = 800-1000 °C) the dinitrogen is released leading to the formation of the corresponding oxides.