4145 3.4. Thermal Conductivity and Heat Capacity 4146 4. Theoretical Developments 4147 4.1. Semiempirical Models 4147 4.1.1. Extensions of van der Waals−Platteeuw Theory 4147 4.1.2. Model Potential and Molecular Dynamics Calculations 4148 4.2. First-Principle Methods 4148 4.2.1. Ab Initio Calculations 4148 4.2.2. Theoretical Challenges 4149 5. Conclusions 4149 6. Acknowledgments 4150 7. Note Added after ASAP Publication 4150 8. References 4150

Detailed study of the equation of state, elasticity, and hardness of selected superconducting transition-metal nitrides reveals interesting correlations among their physical properties. Both the bulk modulus and Vickers hardness are found to decrease with increasing zero-pressure volume in NbN, HfN, and ZrN. The computed elastic constants from first principles satisfy c11 > c12 > c44 for NbN, but c11 > c44 > c12 for HfN and ZrN, which are in good agreement with the neutron scattering data. The cubic ␦-NbN superconducting phase possesses a bulk modulus of 348 GPa, comparable to that of cubic boron nitride, and a Vickers hardness of 20 GPa, which is close to sapphire. Theoretical calculations for NbN show that all elastic moduli increase monotonically with increasing pressure. These results suggest technological applications of such materials in extreme environments.elasticity ͉ elastic constants ͉ equations of state ͉ hardness ͉ binary compounds H ard superconducting materials are of considerable interest for specific electronic applications. Superconductivity has been discovered in diamond, generally believed to be the hardest material having very high shear and bulk moduli (1, 2), with a superconducting transition temperature (T c ) near 4 K when doped with boron (3). However, the transition-metal compounds having the sodium chloride (B1) structure (e.g., NbN, NbC, ZrN, or HfN) are also hard superconductors but with relatively higher T c s. The transition temperatures of solid solutions of NbN and NbC can reach a maximum value of 17.8 K, which is close to those found for the cubic A15-type compounds such as Nb 3 Sn and V 3 Si (4). The refractory characteristics of these transitionmetal nitrides and carbides have been applied as coatings to increase the wear resistance, for instance, in cutting tools as well as for magnetic storage devices. The unusual hardness enhancement in these materials has been theoretically shown to originate from a particular -band of bonding states between the nonmetal p orbitals and the metal d orbitals that strongly resists shearing strains (5). At the moment, there is a need to investigate elastic and mechanical properties of these superconductors under simulated extreme working conditions.Here, we report both experimental and theoretical studies of the equation of state, elasticity, and hardness of selected superconducting transition-metal nitrides. We find that the cubic ␦-NbN superconducting phase possesses a bulk modulus of 348 GPa, comparable to that of cubic boron nitride, and a Vickers hardness of 20 GPa, which is close to sapphire (Al 2 O 3 ) (6). The results indicate that these nitrides are good candidates for engineering hard superconducting materials. Experimental and Theoretical DetailsEquations of state studies were based on angle-dispersive synchrotron powder x-ray diffractometry with a diamond anvil cell. The diffraction experiments were carried out at the synchrotron beam line 16ID-B of the Advanced Photon Source High Pressure Collaborative Access Team. A 500 ϫ 500-m ...

Among transition metal nitrides, tungsten nitrides possess unique and/or superior chemical, mechanical, and thermal properties. Preparation of these nitrides, however, is challenging because the incorporation of nitrogen into tungsten lattice is thermodynamically unfavorable at atmospheric pressure. To date, most materials in the W−N system are in the form of thin films produced by nonequilibrium processes and are often poorly crystallized, which severely limits their use in diverse technological applications. Here we report synthesis of tungsten nitrides through new approaches involving solid-state ion exchange and nitrogen degassing under pressure. We unveil a number of novel nitrides including hexagonal and rhombohedral W 2 N 3 . The final products are phase-pure and well-crystallized in bulk forms. For hexagonal W 2 N 3 , hexagonal WN, and cubic W 3 N 4 , they exhibit elastic properties rivaling or even exceeding cubic-BN. All four nitrides are prepared at a moderate pressure of 5 GPa, the lowest among high-pressure synthesis of transition metal nitrides, making it practically feasible for massive and industrial-scale production.