The nitridation of niobium by thermal processing at 1300 degrees C in different nitrogen (N-2) gas pressures ranging from 2.6 x 10(-4) to 3.3 x 10(0) Pa was investigated. The NbNx films were grown on the niobium substrate by reactive thermal heating. The effect of nitrogen background pressure on the structure and morphology of the formed NbNx phase was studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). The electronic structure of the NbNx films was investigated by X-ray absorption near edge structure (XANES). The phase formation followed a sequence of alpha-Nb(N)-> beta-Nb2N when the nitrogen pressure was increased. As the pressure was increased to > 1.3 x 10(-3) Pa, the nitride film develop into the alpha-Nb(N) phase mixed with the beta-Nb2N phase. Increasing nitrogen pressure results in more beta-phase concentration accompanied by an increase of other phases. Higher pressure promotes the diffusion of adsorbed nitrogen, and consequently the formation of different phases of NbNx. The diffusion of nitrogen in the Nb surface and its reactivity governs the phase formation of NbNx. XANES measurements at the Nb M-edge showed strong hybridizations between the Nb 4d and N 2p states due to larger covalent contribution to the Nb-N bonding. On the basis of these observations our density of state calculation revealed that presence of the beta-Nb2N phase in NbNx results in a large covalent contribution than that by the alpha-NbN phase. (c) 2018 Elsevier B.V. All rights reserved.