Marcasite vs.arsenopyrite structural choice in MN2 (M = Ir, Os and Rh) transition metal nitrides

Abstract

A first-principles density functional theory study of the recently reported IrN₂ and OsN₂ as well as the as yet not synthesized RhN₂ transition metal nitrides is presented. It is found that although IrN₂ has a slight preference for the arsenopyrite structure, OsN₂ prefers the marcasite structure and RhN₂ has no clear preference between these two structures. The electronic structure of these nitrides is discussed on the basis of a simple model for the octahedral MN₄ chains present in both the marcasite and arsenopyrite structures. It is shown that the distortion from the marcasite to arsenopyrite structure in these nitrides is not a Peierls distortion, contrary to what is often claimed. Both metal–metal interactions along the MN₄ chains and interchain interactions through the dinitrogen units compete in these compounds. IrN₂ is found to be a semiconductor whereas OsN₂ and RuN₂ should be three-dimensional metals. It is predicted that RhN₂ can exist in two metallic forms with the marcasite and arsenopyrite structures, which should exhibit different anisotropies.