Chemistry of interstitial molybdenum ternary nitrides MnMo3N (M=Fe, Co, n=3; M=Ni, n=2)

Interstitial molybdenum ternary nitrides, MnMo3N (M=Fe and Co, n=3; M=Ni, n=2), can be obtained by ammonolysis of molybdate precursors, MMoO4nH2O. A study of the influence of the preparative variables on the outcomes from this procedure is presented. Fe3Mo3N and Co3Mo3N are prepared as nearly single phases at temperatures as low as 973K, but higher temperatures are required to obtain pure samples (1073 and 1173K for Fe and Co compounds, respectively). In the case of Fe3Mo3N, moreover, a slow cooling of the samples results in segregation of impurities. The nickel nitride derivative shows a different stoichiometry, Ni2Mo3N, which results in the systematic presence of impurities. The structures of M3Mo3N (M=Fe, Co) have been refined from X-ray powder diffraction data. These nitrides crystallize in the cubic system, space group Fd3m [a=11.07633(8) and 11.02396(8) for M=Fe and Co, respectively]. The structure of Ni2Mo3N has been determined in an ab initio manner from X-ray powder diffraction data. The cell is cubic, space group P4132 [a=6.63422(4)]. Starting positional parameters were obtained by direct methods, and the structure was refined by Rietveld analysis of the data. All three nitrides are prepared as phases constituted by submicrometer homogeneous particles. They show metallic behavior, and temperature programmed oxidation studies reveal an enhanced stability for the nickel derivative in an oxygen atmosphere.