The structures of the η-carbides Ni6Mo6C, Co6Mo6C, and Co6Mo6C2

The structures of the three η-carbides Ni6Mo6C, Co6Mo6C, and Co6Mo6C2, prepared by a new synthetic route, have been determined by Rietveld analyses of monochromatic neutron powder diffraction data. All three materials share the η-carbide structure-type with space group Fd3m (No. 227) and a = 10.8932(3) A, a = 10.8998(2) A, and a = 11.0709(3) A, respectively. In the η-12 carbides, Ni6Mo6C and Co6Mo6C, the carbon atoms are exclusively located on the 0 0 0 (8a) sites, the 18 18 18 (16c) sites being vacant. In the η-6 carbide, Co6Mo6C2, on the other hand the 16c sites are completely filled while the 8a sites are vacant. These studies both demonstrate that the compound precursor synthesis route used here yields high-quality, homogeneous materials and provide precise structural details for the η-12 and η-6 carbide phases. The differences between these two structures are discussed. The complete absence of solid solubility between the η-12 and η-6 phases is considered to reflect the limited separation of only 2.4 A between the 8a and 16c carbon atom sites. The Mo : Co ratio of the η-6 phase is observed to be slightly greater than 1.0, with a refined formula Co5.87(2)Mo6.13(2)C2, reflecting partial Mo occupancy of the Co2(16d) site. Overall sample stoichiometry (Mo : Co = 1.00) is maintained by the presence of Co3Mo.