Effects of manganese and nickel addition on the preparation of niobium oxynitrides

Niobium oxynitrides were prepared by ammonolysis of the amorphous oxide precursors, obtained via citrate gelation, containing either a Mn or Ni additive. The product at 1000 °C, even with only 3 at% Ni, was a mixture of ɛ- and TiS-type Nb(N,O), containing a small amount of Ni metal impurity. Mn-containing products were δ-type (Nb,Mn)(N,O) with a NaCl structure. Upon post-annealing under a 0.2 MPa N2 atmosphere at 1500 °C, the crystallinity of the δ-phase improved. The local structure around the substituted Mn2+ was greatly distorted with respect to the regular octahedral site in a δ-type crystal lattice. The microstructure of the post-annealed products containing Mn, as revealed by SEM observation, pointed to the appearance of a liquid phase. The superconductivity (Tc = 15 K) of δ-Nb(N,O) gradually vanished in δ-(Nb,Mn)(N,O), with increasing Mn amount above 5 at% Mn.