Structure, transport and magnetic properties of MgNi3B2

The structural and physical characterization of the ternary magnesium nickel boride MgNi3B2 is reported. The crystal structure, studied by single-crystal and powder X-ray diffraction techniques, is hexagonal, hP18-P6422 (no. 181), lattice constants a = 4.880(1) Å, c = 8.787(1) Å, with complete ordering of the three atomic species over different crystallographic sites. The present results remove the uncertainty as regards the true composition of the ternary compound reported previously in the literature as ‘Mg2Ni5B4’, ‘MgNiB’ or ‘MgNi3B2’, whose relevance is due to the observation that it is found as a parasitic phase in the preparation of superconducting MgB2 wires, when using Ni or Ni-alloy tubes.The electrical resistivity of MgNi3B2 shows metallic behaviour with no evidence of superconductivity down to at least 1.8 K. High values of the Debye temperature θD (>600 K) are obtained both from the resistivity and heat capacity data, indicating a relatively rigid lattice. The susceptibility is diamagnetic at room temperature, which shows that the Ni-3d bands are completely filled in this compound.