title: Hard superconducting nitrides.
abstract: 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 delta-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.
authors.affiliation: Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA.
authors.forename: Xiao-Jia
authors.initials: XJ
authors.lastname: Chen
authors.forename: Viktor V
authors.initials: VV
authors.lastname: Struzhkin
authors.forename: Zhigang
authors.initials: Z
authors.lastname: Wu
authors.forename: Maddury
authors.initials: M
authors.lastname: Somayazulu
authors.forename: Jiang
authors.initials: J
authors.lastname: Qian
authors.forename: Simon
authors.initials: S
authors.lastname: Kung
authors.forename: Axel Nørlund
authors.initials: AN
authors.lastname: Christensen
authors.forename: Yusheng
authors.initials: Y
authors.lastname: Zhao
authors.forename: Ronald E
authors.initials: RE
authors.lastname: Cohen
authors.forename: Ho-kwang
authors.initials: HK
authors.lastname: Mao
authors.forename: Russell J
authors.initials: RJ
authors.lastname: Hemley
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