Effect of pressure on the composition and superconducting Tc value of NbN prepared by combustion synthesis

Abstract The synthesis of niobium nitride by combustion of packed niobium powder under nitrogen atmosphere was studied in the pressure range 3.5–700 bar without solid-phase dilution. Surface melting of niobium was only detected at 30 bar. The reaction products consist of δ-NbN above 100 bar and of γ-Nb 4 N 3 +δ-NbN at lower pressures. The lattice parameter of δ-NbN vs. the applied pressure presents a maximum corresponding to the transition from a predominantly defective nitrogen sublattice (N/Nb 1). Bulk metal–nitride components were obtained using a `chemical oven' configuration and combining the high pressure with the high combustion temperatures of the niobium powder. Thick nitride films of ≈50 μm with a critical superconducting temperature up to 17.2 K were prepared. The films consist of a δ-NbN external layer, of a γ-Nb 4 N 3 intermediate layer and of a β-Nb 2 N inner layer.