Preparation, crystal structure, and superconductive characteristics of new oxynitrides (Nb1−xMx)(N1−yOy) where M=Mg, Si, and x≈y

New niobium oxynitrides containing either magnesium or silicon were prepared at 1000 °C by ammonia nitridation of oxide precursors obtained via the citrate route. The products had rock-salt type crystal structures. Crystallinity was improved by annealing in 0.5 MPa N₂ and the final compositions were (Nb_0.95Mg_0.05)(N_0.92O_0.08) at 1500 °C and (Nb_0.87Si_0.09□_0.04)(N_0.87O_0.13) at 1200 °C. The magnesium and oxide ions partially co-substitute the niobium and nitride ions in the octahedral sites of the δ-NbN lattice, respectively. Silicon ions were also successfully doped together with oxide ions into the rock-salt type NbN lattice. The Si doped product exhibited relatively large displacement at the octahedral sites and was accompanied by a small amount of cation vacancies. Superconductivity was improved by annealing to obtain critical temperatures/volume fractions of T_c=17.6 K/100% for Mg- and T_c=16.2 K/95% for the Si-doped niobium oxynitrides.