ON THE POSITION OF CARBON-ATOM IN THE TAU-PHASE OF CARBON-DOPED MN-AL PERMANENT-MAGNETS

In order to investigate the site occupancy of the carbon atom in the ordered b.c.t. structure (L1(0), P4/mmm) of the tau-phase of carbon-doped Mn-Al permanent magnets a structure 'analysis by X-ray diffraction is attempted. The experiments are performed using a series of (Mn0.53Al0.47)1-xCx (x = 0 to 0.0315) pseudo-binary alloys, and indicate that the intensity ratio I200/I100 of fundamental to superlattice reflections increases with increasing carbon content. To interpret this observation the X-ray intensity ratios from four probable model structures, each assuming different location of carbon atoms either at a substitutional sublattice (Al) site, (1/2, 1/2, 1/2) or octahedral interstitial sublattice site(s) (1/2, 1/2, 0) or/and (0, 0, 1/2), are calculated and compared with the experimental results. The analysis shows that a reasonable agreement can be realized with the experimental observation only when the carbon atoms are assumed to be located on specific octahedral interstitial sublattice sites at (1/2, 1/2, 0) positions, lying in the Mn atom layers. This interpretation of selective site occupancy of carbon atoms in the tau-phase is in accordance with the previous reasoning based on lattice parameter measurements. The assumptions introduced in the structure analysis are also discussed.