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

The crystals of the new misfit-layer compound (YbS)1.25CrS2 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 a1 = 5.31 A, b1 = 5.75 A, and c1 = 10.63 A for the YbS sublattice and a2 = 3.32 A, b2 = 5.75 A, and c2 = 10.63 A for the CrS2 sublattice. Both layers are incommensurate along the a-axis and the incommensurability parameter, which is defined by α = a2/a1, 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)1.173NbS2