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

Based on density functional theory and evolutionary algorithms the thermodynamically stable structures of MnN and their stability - fields were revealed in the pressure range of 0-50 GPa. The MnN-4̄3 polymorph (zinc blende-type) was shown to be the ambient pressure phase, while experimentally known MnN-4/ (distorted NaCl-type) is a high-temperature phase quenchable to the ambient condition. At 5.0 GPa and below 500 K the MnN-4̄3 transforms into a NiAs-type structure, MnN-6/. With temperature increase, MnN-6/ undergoes the phase transition into MnN-4/ Above 41.2 GPa the polymorph with a distorted NiAs-type structure, MnN-, 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 ( > 20 GPa) and therefore they could not be classified as hard materials.