The layered lanthanum carbide halide superconductors La2C2(X,X ')(2) (X,X ' = Cl, Br, I): Neutron powder diffraction characterization and electronic properties

The superconducting properties of La2C2(X, X')(2)(X, X' = Cl, Br, I) are in vestigated. Three different samples of La2C2Br2, which are obtained under d ifferent synthesis conditions, show superconductivity at 7.03(5) K. La2C2I2 (T-c approximate to 1.7 K) exhibits slightly different transition temperat ures for differently prepared samples. A total of five samples of La2C2(X, X')(2) were investigated by high resolution neutron powder diffraction from room temperature to 1.5 K. Rietveld refinements yield the structural param eters, in particular of the C atoms. These allow us to follow in detail the variation of the interatomic distance in the C-2 group. On the basis of th e low temperature structural parameters, extended Huckel band structure cal culations were performed and a band of low dispersion along the Gamma-N dir ection in the Brillouin zone was identified. We discuss correlations of str uctural and electronic properties with the superconducting transition tempe ratures of the La2C2(X, X')(2) phases.