Preparation, crystal structure, and superconductive characteristics of new oxynitrides (Nb 1− x M x )(N 1− y O y ) 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 N 2 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. Doped silicon ions substituting Nb in (Nb 0.87Si 0.09□ 0.04)(N 0.87O 0.13) introduced a large displacement at the 4 a site mainly because of the accompanied vacancy. The silicon ions may also be slightly shifted from the regular octahedral 4 a site (green) towards the interstitial tetrahedral site (red) statistically in the rock-salt type lattice. [Display omitted] ► New superconducting niobium oxynitrides containing either magnesium or silicon were prepared. ► Their annealed products had chemical compositions of (Nb 0.95Mg 0.05)(N 0.92O 0.08) and (Nb 0.87Si 0.09□ 0.04)(N 0.87O 0.13) in the rock-salt type structure. ► Their superconductivities were T c/volume fraction=17.6 K/100% and 16.2 K/95%, respectively.