High pressure synthesis of superconducting nitrides in the MoN-NbN system

We have synthesized a range of Mo-Nb nitrides at 2.5 GPa and T = 1600 degrees C or 2200 degrees C using high pressure-high temperature techniques. Following syntheses at 1600 degrees C Mo-rich compositions are hexagonal and Nb-rich phases are cubic, with a narrow two-phase region indicating the presence of a solvus. The maximum Mo content in the cubic phase derived from delta-NbN is 40-44 mol%, and the maximum Nb content in the hexagonal delta-MoN phase is 46-47 mol%. There was little variation in the superconducting transition temperature T(c) for hexagonal delta-(Mo, Nb) N samples produced in the study. Previous studies showed maximum T(c) values of 12-15 K for pure delta-MoN as a function of N-site ordering in high-P, T experiments. Here we recorded T(c) 14 K for the limiting Mo-rich composition prepared in the study. We observed T(c) = 11.5-16 K for cubic delta-NbN depending on high-P, T synthesis and annealing conditions. This value falls to 5 K for the cubic Mo(0.37)Nb(0.63)N(0.98) solid solution phase. Some high-P, T synthesis or annealing experiments were carried out at T 2200 degrees C. At this temperature delta-MoN decomposes to produce gamma-MoN(0.54). A minor phase in this sample achieves T(c) = 9.5 K. A new superconducting hexagonal oxynitride MoN(0.74)O(0.38) with T(c) = 16 K was also produced during this study.