Phase transition in Cu-intercalated misfit-layer compounds

The Cu-intercalated misfit-layer compounds Cu-x(MS)(1+y) (NbS2)(2) (M = Ce and Sm), which consist of regularly stacked Cu, MS, and NbS2 layers with incommensurate intergrowth structure, are studied by means of X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), The Nb N-2,N-3 inner-shell electron energy loss spectroscopy (ISEELS) spectra and the valence-band XPS spectra show charge transfer from Cu and MS layers to NbS2 layers, A strong Cu-Nb interaction and a drastic change of the electronic structure are found, which may result from the change of the crystal structure within a NbS2 sublattice from the 3R polytype at lower Cu concentration to the 2H and/or the 4H polytypes at higher Cu concentration, In the 3R polytype, Nb atoms sit in the center of a trigonal prism with six S atoms at the corners whereas in the 2H and/or the 4H polytypes, all or half of the Nb atoms are surrounded octahedrally by six S atoms. An intercalated Cu layer donates a larger number of electrons to NbS2 layers than a MS layer at the maximum uptake of x = 0.67, Finally, the Nb d(z)2 band is completely filled with two electrons per Nb atom and the rigid-band model is no longer valid. (C) 1997 Academic Press.