ON THE PHASE-RELATIONS AND STRUCTURAL AND MAGNETIC-PROPERTIES OF THE STABLE MANGANESE CARBIDES MN23C6, MN5C2 AND MN7C3

Single phase samples of the manganese carbides Mn23C6, Mn5C2 and Mn7C3 were prepared, and examined by powder X-ray and neutron diffraction at various temperatures. In the case of Mn7C3, a chemical transport technique was developed to achieve well crystallized samples of at least 99% phase purity. The thermal stability of Mn23C6 (and Mn15C4) is found to be higher than earlier reported, since Mn4C1+x could not be obtained by quenching from temperatures up to 1250-degrees-C. Improved unit cell parameters and thermal expansion data are given for the carbides. Refined atomic coordinates from powder neutron diffraction data permit a confirmation of isostructurality with the structure prototypes Cr23C6, Pd5B2, Cr7C3 and comparisons of structural and bonding features for the three carbide phases. Owing to the small atomic size of Mn, as compared with Pd and Cr, carbon exhibits higher coordination numbers in Mn5C2 and Mn7C3 than in the prototypes. Monocapped trigonal prisms are found in Mn5C2, and coordination numbers 7 and 8 can be assigned to the carbon coordination polyhedra in the Mn7C3 structure. As the carbon content increases in the sequence Mn23C6, Mn5C2 and Mn7C3, the Mn-C bond order increases (average Mn-C distance 211, 208 and 208 (206) pm, respectively) and the Mn-Mn bond order decreases (average Mn-Mn distance 261.6, 263.8, 264.5 pm, respectively). The closest C-C separation (335, 312 and 295 pm, respectively) is believed to be caused by the geometry of the structures, and consequently the possibility of direct C-C bonding interaction is excluded. The three carbides are weak paramagnets at ambient temperature. For Mn23C6, Curie-Weiss behaviour is observed with THETA-p = +38 +/- 3 K, and mu-eff = 1.46 +/- 0.02-mu-B. Neutron diffraction data show that Mn23C6 is magnetically ordered below T(N) = 104 +/- 2 K. No indications of magnetic order were found for Mn5C2 and Mn7C3 at temperatures above 16 K.