Preparation and characterization of intercalated misfit-layer compounds and natural superlattices

This paper presents the preparation and characterization of the new types of misfit-layer compounds and natural superlattices consisting of Ce(x)Nb(1-x-y)square(y)S (Q') and Ni2NbS2 (H) layers, where square is an atomic deficit at a metal site. A Q' layer is larger by about 1 Angstrom in thickness than a CeS (Q) layer in (CeS)(1.16)NbS2. They are prepared by chemical vapor transport reaction in a closed silica tube under quasi-equilibrium conditions. The 1Q'/3H type of compound are grown as a single crystal while the 1Q'/4H type of compound is grown as composite crystals with the 1Q'/3H and 1Q/2H compounds. Natural superlattices which have a long period in a direction perpendicular to layers are found. Their chemical formulae are given by (Ce(x)Nb(1-x-y)square(y)S)(m)(Ni-z)(n-m)(NbS2)(n), where m and n are integers. It is found from an X-ray-photoelectron spectroscopy (XPS) study that Nb affects the valence and the bonding of Ce in the Q' layers. It is in a higher oxidation state than Nb in NbS2 layers. A scanning tunneling microscope (STM) study shows that some of the superlattices form a hexagonal root3a(0) x root3a(0)30degrees supercell in the (a, b)-plane and behave as a narrow-gap semiconductor so that no STM images are obtained at bias voltage less than 0.3 eV. (C) 2004 Elsevier Inc. All rights reserved.