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 (Nb0.95Mg0.05)(N0.92O0.08) at 1500 °C and (Nb0.87Si0.09□0.04)(N0.87O0.13) 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 Tc=17.6 K/100% for Mg- and Tc=16.2 K/95% for the Si-doped niobium oxynitrides.

Graphical AbstractDoped silicon ions substituting Nb in (Nb0.87Si0.09□0.04)(N0.87O0.13) introduced a large displacement at the 4a site mainly because of the accompanied vacancy. The silicon ions may also be slightly shifted from the regular octahedral 4a site (green) towards the interstitial tetrahedral site (red) statistically in the rock-salt type lattice.Figure optionsDownload as PowerPoint slideHighlights
► New superconducting niobium oxynitrides containing either magnesium or silicon were prepared.
► Their annealed products had chemical compositions of (Nb0.95Mg0.05)(N0.92O0.08) and (Nb0.87Si0.09□0.04)(N0.87O0.13) in the rock-salt type structure.
► Their superconductivities were Tc/volume fraction=17.6 K/100% and 16.2 K/95%, respectively.