Structural phase transition in the perovskite-type tantalum oxynitrides, Ca[1-x]Eu[x]Ta(O,N)[3]

Pure Ca[1-x]Eu[x]Ta(O,N)[3] were successfully synthesized in the whole range of Ca/Eu compositions by means of ammonia nitridation via a citrate precursor route. As-nitrided products with x < 0.4 were apparently orthorhombic, while those with x ≥ 0.4 crystallized in a cubic structure. The anionic composition was found to be essentially O2N and independent of Eu content(x). The as-nitrided EuTaO2N possessed a cubic perovskite-type structure, while high-temperature post-annealing led to a tetragonal EuTaO2N phase with better crystallinity. The re-nitridation transformed the post-annealed product into the original cubic lattice, which might be an average of the tetragonal micro-domains