Structure searching and phase relationships in MnN up to 50 GPa: a DFT study

Abstract

Based on density functional theory and evolutionary algorithms the thermodynamically stable structures of MnN and their stability P–T fields were revealed in the pressure range of 0–50 GPa. The MnN-F3m polymorph (zinc blende-type) was shown to be the ambient pressure phase, while experimentally known MnN-I4/mmm (distorted NaCl-type) is a high-temperature phase quenchable to the ambient condition. At 5.0 GPa and below 500 K the MnN-F3m transforms into a NiAs-type structure, MnN-P6₃/mmc. With temperature increase, MnN-P6₃/mmc undergoes the phase transition into MnN-I4/mmm. Above 41.2 GPa the polymorph with a distorted NiAs-type structure, MnN-Pnma, was shown to be the thermodynamically stable phase. Thus, we have shown that the recently synthesized above ∼55 GPa new high-pressure phase of MnN should have the distorted NiAs-type structure. In addition, for all considered MnN polymorphs the mechanical properties were estimated. It was shown that all MnN polymorphs fall below the minimum threshold for hard materials (H_V > 20 GPa) and therefore they could not be classified as hard materials.