Analysis of the new ternary phase with C6Cr23 structure in Mg-Co-B system by Rietveld method and physical properties of its Ni-substituting effect

Abstract A new ternary Mg1.4Co21.6B6 compound in the Mg-Co-B system was synthesized via a conventional solid-state reaction method and the effect of Ni-substitution on its crystal structure, thermal stability, solid solubility and physical properties were systematically investigated. The crystal structure of the Mg1.4Co21.6B6 compound was fully determined by the X-ray diffraction technique with Rietveld refinement method. It is found that Mg1.4Co21.6B6 crystallizes in the form of C6Cr23 structure type (space group: Fm-3m (No.225), a = 10.556(1)A, Z = 4). The results showed that the 4a sites have been occupied completely by Co atoms in present compound which with M2-xNi21+xB6 form belonging to the W2Cr21C6-type. When Mg1.4Co21.6B6 is repeatedly sintered at elevated temperatures, it becomes unstable and decomposes into Co3B and Mg. The lattice parameters of the Mg1.4Co21.6B6 solid solution alters dramtically with increasing Ni substitution, with no regular trend being observed. The electrical and magnetic performances of the 3.6Mg:3Co:17Ni:6B and 3.6Mg:3Co:18Ni:6B (nominal compositions) samples suggest that both samples are typical ferromagnetic materials. The temperature in the maximum drop of the ρ(T) curve decreases as a function of the Ni content. Base on the correlation between the critical temperature and Ni content, a linear fitting equation is obtained and the critical temperature of Mg1.4Co21.6B6 calculated utilizing the linear fitting equation. The findings in this work may provide certain reference values for material science on electrical magnetic properties and other references for researching the material further.