Multi-phase structure of thermally diffused niobium nitride

In order to study superconducting niobium nitrides for RF applications, Nb-N samples obtained by diffusion of nitrogen in bulk niobium were investigated. X-ray diffraction and AC susceptibility measurements were performed. From these measurements, the composition and the critical parameters (H/sub c1/ and J/sub c/) of the material were deduced. The results for several samples are compared. It is shown that the proposed method allows detection of the presence of different superconducting phases and granularity of materials. It was used to understand the RF behaviour of Nb-N (high residual surface resistance). Although the measured samples did not show granularity, the presence of several materials with different superconducting properties was observed. Low-T/sub c/ phases were always detected in the samples, sometimes also at the surface.