The structures of the η-carbides Ni 6Mo 6C, Co 6Mo 6C, and Co 6Mo 6C 2

Abstract

The structures of the three η-carbides Ni 6Mo 6C, Co 6Mo 6C, and Co 6Mo 6C 2, 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 Fd 3m (No. 227) and a = 10.8932(3) Å, a = 10.8998(2) Å, and a = 11.0709(3) Å, respectively. In the η-12 carbides, Ni 6Mo 6C and Co 6Mo 6C, the carbon atoms are exclusively located on the 0 0 0 (8a) sites, the 1 8 1 8 1 8 (16c) sites being vacant. In the η-6 carbide, Co 6Mo 6C 2, 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 Å 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 Co 5.87(2)Mo 6.13(2)C 2, reflecting partial Mo occupancy of the Co 2(16d) site. Overall sample stoichiometry (Mo : Co = 1.00) is maintained by the presence of Co 3Mo.