Crystal structure and superconductive characteristics of Nb0.89Al0.11 oxynitrides

The crystal structure and superconductive characteristics of the niobium–aluminum oxynitrides were investigated. Single-phase products were successfully obtained starting from a cation ratio of Nb0.89Al0.11. The as-nitrided product crystallized in a cation-deficient rock-salt type structure with a chemical formula of (Nb0.60Al0.08□0.32)(O0.21N0.79), while annealing at 1773 K under a nitrogen pressure of 0.5 MPa led to a highly crystallized product with a simple rock-salt type structure represented as (Nb0.89Al0.11)(O0.17N0.85). Upon post-annealing, both the critical temperature (Tc) and the superconductive volume fraction (VSC) of the oxynitride were significantly enhanced from Tc≈7 K and VSC=23% for the as-nitrided product to Tc=17.3 K and VSC=91% for the post-annealed product.