Synthesis, Crystal Structure, Magnetic Properties, and Stability of the Manganese-Rich “Mn<sub>3</sub>AlC” 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 “Mn<sub>3</sub>AlC” 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 <i>a</i> = 3.875(2) Å. We find a mixed occupation of the 1<i>a</i> position by Al and Mn, which, together with the C position being fully occupied, leads to the actual composition Mn<sub>3.1</sub>Al<sub>0.9</sub>C. 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 1<i>a</i> 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.