Prediction and characterization of the marcasite phase of iron pernitride under high pressure

Using the unbiased structure searching CALYPSO techniques and the first-principles calculations, we predicted a phase transition in iron pernitride (FeN2) at 22 GPa from the hexagonal R-3m structure to an orthorhombic Pnnm structure. Although the Pnnm structure is a well-known marcasite phase of transition-metal pernitrides, to our knowledge, this is the first report about the pressure-induced phase transition to the Pnnm structure in the FeN2 compound. The dynamically and mechanically stable for the Pnnm phase are evaluated, further confirming this phase transition. By assessing the elastic property of the Pnnm structure, the result indicates that it displays excellent brittle and hardness characters. Viewing it's structure, a dinitrogen unit and a coordinating iron atom characterized by N-sharing six-fold FeN6 octahedrons are found. Based on the quantum-chemical analysis, the dinitrogen unit was formulated as a quasi-molecular double-bonded N22− and a mixed covalent NN and FeN bonding nature was identified in the structure. These structural and covalent bonding characters play a crucial role to its excellent mechanical properties. The present results extend the well-known marcasite phase in other transition-metal pernitrides to the FeN2 compound and provide a perspective toward the understanding of the synthesis and chemical bonding of N-rich FeN2 under high pressure.