Formation of barium-tantalum oxynitrides

Abstract

Oxynitride powders suitable for sintering to form dielectric ABO₃-type perovskites have been prepared by nitriding barium carbonate and tantalum oxide. Both heat treatment in air and reaction in flowing ammonia yield isostructural oxynitride phases, but the amount and electronic state of the oxynitrides depend on the nitriding gas conditions. A red-brown oxynitride of high nitrogen content is described as Ba₂Ta₂O₃N₂ containing Ta (IV) ions, but after heat treatment in air the colour changes to white indicative of Ta(V) and a stoichiometry of BaTaO₂N results. A third possibility is identified in an oxynitride containing both Ta (IV) and Ta (V) with an intermediate nitrogen content. Through studies involving thermal analysis and X-ray diffraction, the materials are characterized in powder form and possible sintering conditions are suggested which involve a rapid heating rate and a densification temperature of 1500 °C.