Effect of pressure on the composition and superconducting T-c value of NbN prepared by combustion synthesis

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 delta-NbN above 100 bar and of gamma-Nb4N3 + delta-NbN at lower pressures. The lattice parameter of delta-NbN vs. the applied pressure presents a maximum corresponding to the transition from a predominantly defective nitrogen sublattice (N/Nb<1) to a predominantly defective niobium 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 about 50 micron with a critical superconducting temperature up to 17.2 K were prepared. The films consist of a delta-NbN external layer, of a gamma-Nb4N3 intermediate layer and of a beta-Nb N inner layer.