[en] Two techniques are usually employed to prepare the so-called δ-NbN phase having good superconducting properties: reactive sputtering in a nitrogen-argon atmosphere, and thermal diffusion of nitrogen into niobium followed by a rapid quench cooling. Sputtered NbN films have granular type microstructure, yielding high B/sub C2/ - and J/sub C/ - fluxoid properties (4), and are well-adapted for the protection of surfaces with simple forms. However for complicated geometries it is rather difficult to obtain homogeneous NbN films by this technique. Thermal diffusion technique has been used for the first time about forty years ago (5), but has been rarely employed since then, due to the difficulty for obtaining the δ - NbN phase at temperature below 1300⁰C. For their application to RF cavities, the NbN obtained by thermal diffusion is intended not to be of the granular type structure, having thus lower RF residual losses R/sub res/ and higher RF breakdown fields B/sub crit/. The results reported here are related to NbN obtained by the thermal diffusion technique on bulk niobium at temperatures well below 1300⁰C. 14 references, 3 figures