Unexpected ground-state crystal structures and mechanical properties of transition metal pernitrides MN2 (M= Ti, Zr, and Hf)

Titanium pernitride (TiN2) with CuAl2-type structure (I4/mcm, Z = 4), the first reported member to the family of early transition metal pernitride was recently synthesized via direct chemical reactions between titanium mononitride and nitrogen molecules at 73 GPa and 2400 K. Using an unbiased structure searching method combined with first principles calculations, we here have fully investigated the energy landscape of MN2 (M = Ti, Zr, and Hf) at high pressure up to 100 GPa and suggested that the synthesized I4/mcm structure is a metastable form for TiN2 at ambient conditions. A novel tetragonal I4/mmm (Z = 4) structure composed of the MN6 octahedrons connected by peculiar double N=N bonded dimers, was identified as the universal thermodynamic ground-state structure for three MN2. Under high pressure, the I4/mmm structure undergoes a first-order phase transition to the discovered I4/mcm structure with a large volume drop, accompanying the increase of coordination and oxidation number of metal M atoms in MN2. The configuration of M+2 [N-2](-2) for MN2 in the I4/mmm structure was demonstrated by electronic structure and chemical bonding analyses. The mechanical properties including elastic parameters and elastic anisotropy behaviors of MN2 within I4/mmm structure were systematically studied. Results on the elastic anisotropy and ideal tensile strengths indicate that the (001) planes may be viewed as their cleavage planes. (C) 2018 Elsevier B.V. All rights reserved.