Ultra-incompressibility and anomalous shear deformation resistance of transition metal polynitrides TMN8 (TM = Ti, Zr, Hf)

Abstract

The recently proposed transition metal polynitride, tetragonal TiN8 featured infinitely armchair-like N chains, has attracted much attention for its larger high-energy density value than the well-known TNT explosive. However, as a typical transition metal nitrogen-rich compounds with outstanding mechanical properties, the electronic properties and structure-mechanical relations remain to be further explored. Here, we performed first principles calculations that investigate the thermodynamic stabilities of tetragonal TMN8 (TM = Ti, Zr, Hf), and fully studied the mechanical and elastic anisotropic behavior of these materials. In particular, the stress-strain relations and the related evolutions of chemical bonding were systematically studied. The uniaxial ultra-incompressible nature of these TMN8 has been demonstrated by the calculated elastic moduli, originated from the strong N–N covalent bonds along the c-axis. Under (110) [11‾0] shear direction, the TMN8 exhibits an abnormally large breaking shear strain that produces a strong shear strength exceeding 40 GPa. This unusual behavior stems from the strongly and stably 3D structure framework consisted of strong N–N covalent bonds in infinite N chains connected by Ti atom through Ti–N covalent bonds in Ti–N hexahedrons. These obtained findings explicate the crystal structural configurations and chemical bonding characters that are responsible for the mechanical properties of TMN8 and provide insights for understanding other transition metal polynitrides.