Density and elasticity of superconducting niobium nitride under high pressure

Hard superconducting materials are of considerable interest for specific electronic applications. Transition-metal (TM) nitrides have increasingly attracted attention because of their outstanding mechanical, optoelectronic, thermal, magnetic and/or superconducting properties and potential usage in a variety of technological areas, such as NbN exploited in superconducting and high hardness coatings. Previous hardness measurements on NbN by Vickers indentation method reported a Vickers hardness about 20 GPa and its bulk modulus was found close to that of cubic boron nitride. In addition, experimental studies and first-principles calculations have investigated the equation-of-state (EOS) for B1 structured NbN and provided important insights into the origin of its outstanding mechanical properties. In spite of its importance, to date, the high-pressure behavior and elastic properties of NbN are not well studied experimentally, in particular for the shear properties under pressure. In this study, we hot-pressed high quality (well-sintered, free of cracks, small grain size and homogeneous) polycrystalline NbN specimens and performed simultaneous measurements of compressional and shear wave travel times using ultrasonic interferometry techniques up to ~12 GPa at room temperature in a large-volume high-pressure apparatus. By fitting these experimental data to finite strain equations, the compressional and shear wave velocities, density, and the bulk and shear moduli as a function of pressure are all obtained. These new data not only allow us to compare with previous EOS data on NbN and those of other transition metal nitrides, but also enable us to further explore the constitutive relations between elastic moduli and hardness in these nitrides.