The specific crystallographic features of Ni1/3TiS2 and Ni1/4TiS2 led us to carry out ultrahigh vacuum-scanning tunneling microscopy (UHV-STM) studies, in order to bring to light the different atom chemical environments of the structure and the host–guest interactions. These compounds present specific structural reorganizations that involve, in particular, different chemical surroundings for chalcogen atoms (the outer layer of the compounds). We imaged the top sulfur planes (001) for both compounds and propose an explanation of the two well-differentiated contrasts observed.

The results of a systematic study on two misfit layer compounds: (PbS)1.18(TiS2) and (PbS)1.18(TiS2)2 by X-ray Photoelectron Spectroscopy are reported. The core levels and the valence band spectra have been analysed and compared with the non-intergrowth samples (PbS) and (TiS2). In order to shed light on the stabilization of misfit layer compounds, we performed a comparison between theoretical (EHT-TB method) and experimental results on the mono-layer misfit compound. The chemical shifts or/and the broadenning of the core level peaks have been discussed in terms of initial and final states, the former based on the possible charge transfer between PbS and TiS2. We have shown how the XPS valence band is modified by the PbS–TiS2 interactions by using calculated densities of states.

Scanning tunneling microscopy, atomic force microscopy and X-ray photoelectron spectroscopy were used to study the surface of two misfit layer compounds, (PbS)1.18(TiS2) and (PbS)1.18(TiS2)2. For the cleaved compounds, STM and AFM images in air and at room temperature revealed the trigonal symmetry of the transition metal dichalcogenide TiS2-1T. After a few hours of exposure to air, AFM and XPS show that the surfaces are strongly modified, with different features according to the mono or the bilayer misfit compounds considered. Depending on the time exposure to air, these results were interpreted in terms of hydrolysis and oxidation reactions.

The effect of metal intercalation (silver and iron) into lT-Cdl2-type TiS2 layered crystals, expressed as MxTiS2, has been studied by X-ray photoelectron spectroscopy (XPS) and self-consistent electronic calculations (augmented sphere wave method). The spectra are found to depend strongly on the guest metals, and we have shown how the XPS valence bands are modified by the ‘host-guest’ interactions by using calculated densities of states. In the first approximation, the chemical shift of the core peaks are correlated with Mulliken population analysis.

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The bulk and surface electronic structure of α-SnS is investigated by X-ray photoelectron spectroscopy and band structure calculations, using ab-initio Hartree-Fock program CRYSTAL. Calculated band structure and density of states are consistent with recent calculations using other methods. Scanning tunnelling microscopy (STM) and Atomic Force microscopy (AFM) images were examined by calculating the partial electron density ϱ(r, Ef) and total electron density ϱ(r) of a slab which consists of height (100) α-SnS layers. It was found that bright spots in experimental STM images correspond to sulphur and tin atoms in positive bias voltage and to the outermost layer (tin atoms only) in negative bias voltage. However, the simulated image related to filled states cannot explain the new periodicity of the experimental pattern in low negative polarisation, which suggests the appearance of a surface charge density wave. The performed AFM image show a periodicity which can be simply explained by the atomic corrugation at the surface.

Scanning tunnelling microscopy, atomic force microscopy and X-ray photoelectron spectroscopy were used to study the surface of TiS2, a layered material. STM and AFM images in air and at room temperature revealed the trigonal symmetry of the lattice. After a few minutes exposure to air, we have shown by AFM and XPS the growth of a thin layer of TiO2 on the surface.

Electron probe microanalysis of incommensurate sandwiched layered compounds of the general formula [(Pb,Sn)S](1+x)[(Nb,Ti)S-2](m) shows a systematic (Pb,Sn) depletion, interpreted as a partial (Ti,Nb) substitution into the pseudoquadratic (''Q'') layer (Pb,Sn)S. (Ti,Nb) substituting for (Pb,Sn)(2+) would have the +3 oxidation state, with an equal vis-a-vis transformation of(Ti,Nb)(4+) into (Ti,Nb)(3+) in the (Nb,Ti)S-2 adjacent layer. This will contribute to increase the global electronic population of the Ti t(2g), or Nb d(z)(2) band, without electron donation from the Q layer. This local heterovalent substitution induces a cationic coupling between the two kinds of layers, which permits the understanding of the stabilization of these sandwiched structures, as initially pointed out for the mineral cylindrite (Makovicky, 1974). According to this model, the substitution in the Q layer must be ordered at short distance (vernier rule, Makovicky and Hyde, 1981), and at the modulation scale (for 2n Q cations) the coefficient of substitution of(Ti,Nb) must be an integer. True incommensurability of these structures can be understood as long-distance fluctuations between two vernier. ratios. The new chemical data prompt us to reexamine some of their physical properties, i.e., superconductivity. The presence of various local chemical disruptions would determine the existence of all layered misfit chalcogenides.

The misfit layer compounds (MX)(1+x)(TX(2))(m) (with M = Sn, Pb; T = Ti, Nb; X = S, Se; 0.08<x<0.3; m = 1, 2...) which form a family of 2D electronic systems are investigated by resistivity, Hall and Seebeck measurements. They all exhibit a highly anisotropic metallic behavior; some of them undergo a superconducting transition at low temperatures (T-c < 6K). Comparison between monolayer (m = 1) and bilayer (m = 2) derivatives gives evidence of the role of polytypism (2H or 3R type for the TX(2) part) in the occurrence of a superconducting transition. This polytypism is likely connected with non-stoichiometry (T(1+x)X(2)) as already mentioned for the patent binary TX(2) compounds.

We have synthetized new compounds of the misfit layer family of general formula (MX)n(NbX2)m with X : S, Se, M : Pb, Sn, rare earth metals, 1.1 < n < 1.2, m = 1, 2. Some of them exhibit a superconducting transition. In particular we compare the superconducting properties of polytypes as "PbNbS3"-"PbNb2S5"-"LaNbSe3"-"LaNb2Se5". We show that the stacking mode of the (NbX2)n is of prime importance for the occurrence of superconductivity. Anisotropy in the superconducting state has been measured by angular magnetoresistance. Superconductivity in the compound "LaNb2Se5" has an extreme anisotropic character that in addition with the structural incommensurability allows a direct comparison with high T(c) oxides.

The crystal structure of the new misfit layer compound (PbSe)1.12(NbSe2)2 has been determined. The structure which is analysed in terms of a composite crystal consists of \PbSe\, 2x \NbSe2\ and common parts. It corresponds to an alternating stacking sequence of \PbSe\ and 2x\NbSe2\ layers along the c direction. The misfit between the two layers occurs along the a direction : a(PbSe) = 6.143 angstrom and a(NbSe2) = 3.429 angstrom. This yields a ratio of 1.791 which is irrational but close to 9/5.

The crystal structure of the monoclinic (PbS)1.14(NbS2)2 polytype has been determined. The structure which is analysed in terms of a composite crystal consists of (PbS), 2x(NbS2) and common parts. PbS and (NbS2)2 slabs altemate along the c direction. The misfit between the two types of slabs occurs along the a direction. Some comments about polytypism and the presence of additional Nb atoms within the VdW gap are given.

The structure of the misfit layer compound (PbSe)1.10NbSe2 has been determined by single crystal X.ray diffraction. PbSe and NbSe2 layers alternating in the stacking direction c make up the structure. The PbSe layer forms approximately an NaCl structure type, whereas in NbSe2 the Nb atoms are located within slightly distorted trigonal prisms of selenium. Refinements were performed in the orthorhombic space groups Fm2m for PbSe and Cm2m for NbSe2 [PbSe : a1 = 6.202(3)Å, b = 6.058(1)Å, c = 24.952(9)Å; NbSe2 : a2 = 3.422(1)Å, b = 6.056(1)Å, c = 24.940(8)Å] and converged to Rf1 = 0.073 and Rf2 = 0.071, respectively.

(PbS)1.18(TiS2)2 is a misfit layer compound in which two types of slabs, PbS and 2(TiS2), alternate along the c direction. The \TiS2\ blocks (about 11.39 angstrom thick) are interleaved by \PbS\ layers, thus leading to a value for c of 17.46 angstrom. The misfit between the two slab types occurs along the a direction; the parameter values being a(PbS)=5.761 angstrom and a(TiS2) = 3.390 angstrom respectively. This yields a ratio of approximately 1.699, which is irrational but close to 5/3. The common in-plane b parameter is equal to 5.873 angstrom. The PbS unit consists of a {001} slice (half an edge thick) of an NaCl-type f.c.c. cell.

The crystal structure of the new misfit layer compound has been determined. The structure which is analysed in terms of a composite crystal consists of (PbSe), 2x (NbSe[sub 2]) and common parts. It corresponds to an alternating stacking sequence of (PbSe) and 2x(NbSe[sub 2]) layers along the c direction. The misfit between the two layers occurs along the a direction: a(PbSe) = 6.143 A and a(NbSe[sub 2]) = 3.429 A. This yields a ratio of 1.791 which is irrational but close to 9/5.

Crystal structure of a new misfit layer compound (SmS)1.19NbS2 has been determined. A composite crystal approach has been chosen which leads to average structure for each separate sublattices, \SmS\-\NbS2\ and their common part. The building principle of this compound consists of an alternate stacking of \SmS\ and \NbS2\ sublattices along the c axis; both sublattices have F centered orthorhombic symmetries.A brief report on electrical and magnetic properties is given.

Considering "PbNb2S5" as an example, we show that lithium intercalation (chemically with the n-butyllithium or electrochemically) is possible in this kind of material with a misfit layered structure. Lithium atoms likely take place at first an vacant octahedral sites within the Van der Waals gap between adjacent NbS2 slabs. However the amount of intercalation exceeds what could be deduced on the basis of simple geometrical considerations.

AbstractThe title compound is prepared by reaction of the elements at 900 °C (Pb:Nb:S = 1:2:5, sealed quartz tube, 2 weeks).