Strain dependent bonding in solid C3N4:: High elastic moduli but low strength

Several solid C3N4 phases have been predicted to have strength approaching or even exceeding that of diamond based on calculated elastic moduli. In this paper, we show by first-principles calculation that the strength of the previously predicted hardest C3N4 phase is lower compared to not only diamond but also cubic boron nitride by a surprisingly large amount. We examine the electronic origin of this phenomenon and show that the excessive electrons on the N atoms play a key role. The results illustrate that a close examination of atomistic deformation modes and strain dependent bonding is critical for the design of superhard materials.