SUPERCONDUCTIVITY IN RARE-EARTH-METAL CAR BIDE HALIDES OF THE TYPE SE(2)X(2)C(2)

The metallic nature of the carbide halides Y(2)X(2)C(2) is due to Y-C covalency. The superconductivity of the compounds is attributed to a p airwise attraction of conduction electrons by C-2-pi states at the Fe rmi level. The hypothesis is followed by experiments and band structur e calculations. - Neutron powder diffraction reveals d(C-C) = 128(1)pm for YZBr2C2. X-ray single crystal investigations on Y2Br2C2 and Y2I1. 5Br0.5C2 show a characteristic variation of the coordination of the Ct unit. Systematic changes of the average halide radius in Y-2(X,X')(2) C-2 (X,X' = Br, Cl I, Cl and I, Br) lead to a monotonic increase of T- c = 2.3 K (X = Cl) via T-c = 5.05 K (X = Br) to a maximum T-c = 11.2 K for Y2I1.6Br0.4C2. NO isotope effect for C-12/C-13 could be detected. Photoelectron spectra of Y2Br2C2 (excitation energies between 40 and 140 eV) are compared with the results of band structure calculations ( LMTO, E.H.). The electronic structure reveals two bands crossing the F ermi level. One of them has C-2-pi-Y-d(xz,yz) character and exhibits a saddle-point at EF The other intersects the Fermi level with large d ispersion and has exclusively Y-d(x2-y2) character at the crossing poi nt. The results are discussed with respect to theoretical models (van Hove singularity, local pairs and itinerant electrons).