Crystal structures of transition metal pernitrides predicted from first principles

We have extensively explored the stable crystal structures of early-transition metal pernitrides (TMN2, TM = Ti, V, Cr, Mn, Zr, Nb, Mo, Hf, and Ta) at ambient and high pressures using effective CALYPSO global structure search algorithm in combination with first-principles calculations. We identified for the first time the ground-state structures of MnN2, TaN2, NbN2, VN2, ZrN2, and HfN2 pernitrides, and proposed their synthesis pressures. All predicted crystal structures contain encapsulated N-2 dumbbells in which the two N atoms are singly bonded to a [N-2](4-) pernitride unit utilizing the electrons transferred from the transition metals. The strong nature of the single dinitrogen bond and transition metal-nitrogen charge transfer induce extraordinary mechanic properties in the predicted transition metal pernitrides including large bulk modulus and high Vickers hardness. Among the predictions the hardness of MnN2 is 36.6 GPa, suggesting that it is potentially a hard material. The results obtained in the present study are important to the understanding of structure-property relationships in transition metal pernitrides and will hopefully encourage future synthesis of these technologically important materials.