The manganese nitrides η-Mn3N2 and θ-Mn6N5+x:: nuclear and magnetic structures

Neutron powder diffraction measurements on eta -Mn3N2 and theta -Mn6N5 + x (x = 0.26) revealed the nuclear and magnetic structures of these metallic interstitial compounds. At ambient temperatures the crystal structures of both materials derive from an fcc arrangement of Mn with N occupying octahedral sites. The unit cells are face-centred tetragonal, eta -Mn3N2: a = 4.2046(1) Angstrom, c = 12.131(1) Angstrom approximate to 3 x 4.041 Angstrom; theta -Mn6N5.26: a = 4.2193(1) Angstrom, c = 4.1287(1) Angstrom. Both phases show an antiferromagnetic ordering of the first kind with magnetic moments between 3.3 and 3.8 mu (B). The data for eta -Mn3N2 confirm a previously reported ordered arrangement of N on 2/3 of the octahedral sites and the collinear antiferromagnetic superstructure with the magnetic moments oriented perpendicular to the tetragonal c axis. However, a close inspection of the data reveals some systematic ambiguities of the magnetic structure which are analysed in detail. In theta -Mn6N5 + x (x = 0.26) the N atoms show no long-range occupational order on the octahedral sites. At ambient temperatures the collinear magnetic moments are tilted by 23 degrees relative to the tetragonal c axis. High temperature neutron powder diffraction measurements reveal a simultaneous decay of the sublattice magnetisations and the tetragonal distortion of the cell dimensions. At about 550 K the magnetic moments align parallel to the c axis, and up to the critical temperature of 660 K they decrease continuously to zero. Parallel to this, the tetragonal distortion of the unit cell reduces and finally approaches a face-centred cubic structure. The consequences of the results with respect to the Mn/N phase diagram are discussed.