Superhard transition metal tetranitrides: XN 4 (X = Re, Os, W)

The structural, mechanical, and electronic properties of rhenium, osmium, and tungsten tetranitrides, XN 4 (X = Re, Os, W) with the orthorhombic ReP 4 -type structure have been investigated by first-principles calculations using density functional plane-wave pseudopotential method. The calculated formation enthalpies and elastic constants show that these tetranitrides are energetically and mechanically stable. It is appeared from the calculated band structures and density of states that ReN 4 and new proposed WN 4 are metallic, while OsN 4 is semiconductor with a band gap of 0.64 eV. The hardness values of all compounds obtained from different hardness methods indicate that these tetranitrides are superhard materials.