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 Nb_0.89Al_0.11. The as-nitrided product crystallized in a cation-deficient rock-salt type structure with a chemical formula of (Nb_0.60Al_0.08□_0.32)(O_0.21N_0.79), 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 (Nb_0.89Al_0.11)(O_0.17N_0.85). Upon post-annealing, both the critical temperature (T_c) and the superconductive volume fraction (V_SC) of the oxynitride were significantly enhanced from T_c≈7 K and V_SC=23% for the as-nitrided product to T_c=17.3 K and V_SC=91% for the post-annealed product.