Synthesis and characterization of superconducting MgCNi₃

The intermetallic perovskite MgCNi₃, superconducting with a T_c of 8 K, is particularly interesting from several points of view: in spite of the large amount of nickel present the compound is not ferromagnetic; nevertheless band structure calculations put in evidence a Van Hove singularity just above the Fermi level suggesting that MgCNi₃ is on the border of magnetic instability and that may be driven to ferromagnetism by partial substitution of Mg with a monovalent metal as Li or Na. The material is also interesting because of the presence of two kinds of pinning centres, grain boundaries and nanometer-scale precipitates inside the grains, combination which is not found in other metallic superconductors. We have synthesized polycrystalline samples with composition MgCNi₃ by a solid state route. Both the resistive and the magnetic measurements show a double superconductive transition, the first one occurring at a temperature of 10 K, higher than that reported in literature (8 K) and a second one at 6 K. High temperature resistivity shows a metallic behaviour.