Niobium oxynitrides with defective rock salt-type structures

The effect of crystal structural defects on superconductivity was investigated in two types of rock salt-type niobium oxynitride structures, (Nb0·87Si0.09□0.04) (N0·87O0.13) and Nb4(N,O)5. The first oxynitride had been reported to have an enhanced critical superconducting current Jc by the doping of Si in the crystal lattice together with some amount of cationic vacancies in our previous publications. In the present study, it was thermally annealed at various high temperature and under nitrogen pressure to study the effect of changing the defect structure on the superconductivity. The doped Si was gradually excluded as SiO2 impurities forming nano crystalline rock salt-type δ-NbN together with NbO2 impurities. The superconducting volume fraction and critical current density decreased following the annealing. The second niobium oxynitride examined, Nb4(N,O)5, has a cationic vacancy-ordered rock salt-type crystal structure. Wet ammonolysis led to an improvement of crystallinity in Nb4·19(N4·50O0.50) in I4/m space group with a = 0.6861 (2) nm and c = 0.4284 (1) nm. Magnetic measurements showed that the well-ordered cationic vacancies was especially important to have its superconductivity below 10 K with an increase in superconducting volume fraction to 47.1%. •Well extended crystal structures including structural defects were important for superconductivity.•(Nb0·89Si0.08□0.03) (N0·70O0.30) with high Jc gradually decomposed in high temperature annealing.•A wet ammonia flow was effective for superconducting Nb4(N,O)5 with well-ordered cationic vacancy.