Microstructural features of ${\mathrm{Mg}}_x{\mathrm{C}}_y{\mathrm{Ni}}_3$ superconducting materials

Structural investigations on a series of superconducting materials with nominal compositions of ${\mathrm{Mg}}_x{\mathrm{C}}_y{\mathrm{Ni}}_3$ $\mathrm{}(1.0<x<\mathrm{}1.3,1.0<y<\mathrm{}1.55\mathrm{})\mathrm{}$ have revealed evident structural inhomogeneity. Regular domains, with an average size as small as ∼4 nm, appear commonly in the superconducting materials with $T_c>6\mathrm{}\mathrm{K}.$ This structural phenomenon can be qualitatively explained in terms of the perovskite cubic structure ${\mathrm{MgCNi}}_3$ modulated locally by the variable stoichiometry on the C sites. The presence of the local C deficiency could be a dominant factor affecting the crystal structure and superconductivity.