Lamellar and filament-like crystals of misfit-layer compounds containing (Sm, Ta, S) and (Pb, Bi, Nb, S) elements

This paper presents the preparation and characterization of filament-like crystals as well as lamellar crystals for misfit-layer compounds containing (Sm, Ta, S) and (Pb, Bi, Nb, S) elements. They were grown by chemical-vapor transport reaction. The crystal structure and morphologies are investigated by transmission and reflection X-ray diffraction (XRD) and scanning electron microscopy (SEM). Filament-like crystals of (SmS)1.19TaS2 and {(Nb1-y-zPbyBizS)1.5}1+xNbS2 are grown abundantly. They belong to a large family of 1Q/1H and 1.5Q/1H types of misfit-layer compounds. They have an asymmetric layered structure, constructed of Q (SmS and Nb1-y-zPbyBizS) and H (NbS2) layers alternately stacked along the c axis. The lattice parameters of {(Nb1-y-zPbyBizS)1.5}1+xNbS2 are scarcely dependent on the amounts of Pb and Bi in raw materials, excluding a slight distortion in a crystal structure from the orthorhombic to the monoclinic. A comparison with the XRD results of the lamellar crystals of (Pb1-y'Biy'S)1+x'(NbS2)n (n=1 and 2) shows that Bi is in a higher oxidation state than Pb. However, both elements are contained only slightly in the filament-like crystals. The preliminary two-probe measurements of the temperature-dependent resistance have been made. The results indicate that there are two distinct types with positive and negative temperature coefficients