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 investigated. Three different samples of La2C2Br2, which are obtained under different synthesis conditions, show superconductivity at 7.03(5) K. La2C2I2 (T-c approximate to 1.7 K) exhibits slightly different transition temperatures 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 parameters, 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 the low temperature structural parameters, extended Huckel band structure calculations were performed and a band of low dispersion along the Gamma-N direction in the Brillouin zone was identified. We discuss correlations of structural and electronic properties with the superconducting transition temperatures of the La2C2(X, X')(2) phases.