Synthesis, Crystal Structure, Magnetic Properties, and Stability of the Manganese-Rich “Mn₃AlC” κ Phase

κ-carbides of varying composition, seemingly responsible for age hardening in high-Al steel alloys, have been detected to precipitate both at grain boundaries and in the bulk grain of steels. Herein we report the bulk-phase synthesis of “Mn₃AlC” by arc plasma sintering and rapid solidification. Single crystals have been found suitable for X-ray diffraction using Mo radiation and yield a lattice parameter of a = 3.875(2) Å. We find a mixed occupation of the 1a position by Al and Mn, which, together with the C position being fully occupied, leads to the actual composition Mn_3.1Al_0.9C. Additional energy-dispersive X-ray–scanning electron microscopy measurements support the composition and corroborate the homogeneity. SQUID data collected on the polycrystalline ferromagnetic sample exhibit a Curie temperature of about 295 ± 13 K and a soft magnetic behavior. The small but significant nonstoichiometry on 1a leads to a slightly larger lattice parameter, a higher electron count, and, thus, a lowered density of states at the Fermi level, indicative of increased phase stability.