Crystal structure and physical properties of OsN2 and PtN2 in the marcasite phase

We have theoretically evaluated a recently synthesized OsN2 that was shown to have a large bulk modulus, and we propose a structure that is isostructural with marcasite with the space group Pnnm. The calculated lattice constants, bulk modulus, and x-ray diffraction spectrum of OsN2 are in good agreement with the experimental results. Calculations of bulk modulus and hardness indicate that the marcasite-type OsN2 is an ultraincompressible material instead of a superhard one. The possibility of the marcasite PtN2 has also been investigated, showing that the marcasite PtN2 could exist as a metastable phase at pressures up to 50 GPa, being elastically stable with a slightly higher energy (32 meV/atom at 0.1 MPa) than that in the pyrite structure.