Electrical and optical properties of the mixed-valence misfit-layer compound (YbS)1.25CrS2

The crystals of the new misfit-layer compound (YbS)(1.25)CrS2 were grown by chemical transport reaction in a closed silica ampoule. This compound is built up of alternately stacking of YbS and CrS2 layers. The dimensions of a unit cell are a(1) = 5.31 angstrom, b(1) = 5.75 angstrom, and c(1) = 10.63 angstrom for the YbS sublattice and a(2) = 3.32 angstrom, b(2) = 5.75 angstrom, and c(2) = 10.63 angstrom for the CrS2 sublattice. Both layers are incommensurate along the a-axis and the incommensurability parameter, which is defined by alpha = a(2)/a(1), is about 0.625. It is found from the comparison of lattice parameters among a series of lanthanide misfit-layer compounds and lanthanide monosulfides that Yb is in an intermediate valence state close to trivalency rather than divalency due to charge transfer to CrS2 layers. Temperature-dependent electrical resistivity shows that the compound is a semiconductor with the activation energy of about 0.31 eV. This activation energy almost agrees with that of the acceptor levels in nonstoichiometric YbS samples. A reflectivity spectrum reveals small peaks at 0.8 and 1.1 eV, whereas an absorption spectrum exhibits a large peak at 1.1 eV The results obtained suggest that valence fluctuation happens for Yb-like Eu in (EuS)(1.173)NbS2. (c) 2007 Elsevier B.V. All rights reserved.