The antipressure effect of Cu on Tc in superconducting La(Ni1-x Cux) C2 (0≤x≤0.25)

As observed with x-ray powder diffraction, the orthorhombic structure of the parent compound LaNiC2 is retained in La(Ni1-xCux) C-2 up to the solubility limit near x = 0.2. The refined lattice parameters show that both the c-axis and the volume of the unit cell v expand clearly, while the a-and b-axes show less percentage change due to doping with copper. These results are inconsistent with what one would expect from a chemical pressure effect. The change in T-c with x is similar to the change in the lattice parameters (c and v) with x and is consistent with the assumption that Cu has finite solubility in this system. Magnetic measurements indicate that the increase in Tc is initially rapid (dT(c)/dx = 12 K) but slows down (dTc/dx = 1 K) for x >= 0.2 in the system La(Ni1-xCux) C2. As expected from the BCS theory, the Cu doping in this system may affect not only the density of state N(E-F), but also the electron - phonon coupling parameter.