High-pressure behaviors of a niobium nitride single crystal*

We report the measurements of the high-pressure lattice structures, Raman-scattering spectra, and superconducting transition temperatures up to 50 GPa on a non-stoichiometric niobium nitride single crystal. The material remains the simple $B1$ NaCl structure over the whole pressure range with a bulk modulus of 348 GPa, comparable to that of cubic boron nitride. The pressure-induced phonon frequency shifts are obtained based on the Raman-scattering data. These results together with the calculated electronic density of states are used to explain our observed constant superconducting transtion at 12.6 K under pressures in this material.