Nitrogen ordering in ζ-manganese nitrides with hcp arrangement of Mn – MnN y with 0.39 < y < 0.48 – determined by neutron diffraction

Abstract

The crystal structures occurring in the ζ-region of the Mn–N phase diagram were investigated by X-ray and high temperature neutron diffraction within a range of compositions MnN y with 0.39< y<0.48. The ζ-region consists of several phases all based on an hcp arrangement of Mn. They differ by their arrangements of N atoms on the octahedral sites. A high temperature ‘γ-type’ phase of defect anti-NiAs type, a hcp× a hcp× c hcp, space group P6 3/mmc, is disordered with respect to nitrogen. Three long-range nitrogen ordered superstructure phases exist: (1) an ‘ε-type’ superstructure, observed below 570 K for MnN 0.388 and MnN 0.421 with 3 1/2 a hcp×3 1/2 a hcp× c hcp with respect to an hcp unit cell. Powder diffraction data can be equally well described in P6 322, P312 or P 3 ̄ 1 m ; (2) a ‘ζ-type’ superstructure, observed below about 870 K for MnN 0.457 and MnN 0.472 with anti-α-PbO 2 type structure with 2 a hcp×3 1/2 a hcp× c hcp, space group Pbna; (3) a ‘β-type’ superstructure observed above 870 K for MnN 0.484 with anti-CaCl 2 type superstructure, a hcp×3 1/2 a hcp× c hcp, space group Pmnn. These findings require a revision of the Mn–N phase diagram that is, up to now, presented with only one single ζ-phase in the literature.