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 N₂ dumbbells in which the two N atoms are singly bonded to a [N₂]^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.