Guy Ouvrard

Electronic structures and charge transfer in lithium and mercury intercalated titanium disulfides

Mercury can be intercalated into TiS2 by a reaction between elemental mercury and TiS2 at room temperature. The structure of the obtained compound Hg1.24TiS2 can be described as two non-commensurate monoclinic sublattices. The mercury atoms form metal chains inserted into trigonal prismatic channels created by the expanded TiS2 host lattice. The structural arrangement and interatomic distances for this compound indicate the presence of primarily neutral mercury, with very low charge transfer, and relatively weak Hgue5f8S interactions. In order to understand this peculiar behaviour, electron band structure calculations have been made using the extended Huckel method and compared with experimental data from different spectroscopies: XAFS, EELS and XPS. Pristine TiS2 and its lithium and mercury intercalated compounds have been investigated. The experimental data are in good agreement with the calculated electronic structures. The main conclusion is that 0.24 electrons are transferred by lithium atom to the TiS2 host structure for the composition Li1TiS2. This transfer is almost equivalent on titanium (0.09 electrons) and sulphur (0.075 electrons per atom). For the mercury intercalated TiS2 phase, both calculations and experimental data show an electronic transfer from mercury to TiS2 very close to zero.

Coevaporated KInSe 2 : A Fast Alternative to KF Postdeposition Treatment in High-Efficiency Cu(In,Ga)Se 2 Thin Film Solar Cells

Cu(In,Ga)Se2-based thin film solar cells have reached 22.3% energy conversion efficiency. This outstanding level of performance has been made possible by the use of a so-called potassium fluoride postdeposition treatment (KF-PDT) after the absorber synthesis. Such a treatment, consisting of evaporating KF under Se atmosphere, has been suggested to enhance the formation of a KInSe2 surface layer. In this paper, we propose an alternative to the standard KF-PDT, in which we simultaneously evaporate KF and In under Se atmosphere. We compare by X-ray photoemission spectroscopy the modified absorber surfaces in both cases and discuss the advantages of this alternative in terms of robustness and rapidity of the process.

Electrochemical Mechanisms during Lithium Insertion into TiO0.6 S 2.8 Thin Film Positive Electrode in Lithium Microbatteries

Amorphous thin films were prepared by radio frequency magnetron sputtering from TiS 2 target under pure argon atmosphere. Two complementary techniques, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, were used to investigate the local and electronic structure of the as-prepared sample and to study the electrochemical mechanisms occurring during the first cycle. The results exhibit that the as-prepared TiO 0 . 6 S 2 . 8 thin film is close to bulk TiS 3 [which may be viewed as Ti 4 + (S 2 - 2 )S 2 - ] with regard to the local and electronic structure. An electrochemical characterization of the film was performed. Upon lithium insertion, sulfur is first involved in the electrochemical reduction, followed by titanium. The reduction of S 2 - 2 to S 2 - is concomitant with the elongation of the distance between the two sulfur ions of the disulfide pair. At the end of the charge, the electrochemical processes appear to be reversible. This study clearly evidences the major contribution of sulfur atoms beside titanium.

Mercury sublattice melting transition in the misfit intercalation compound H1.24TiS2

Abstract Mercury can be intercalated into TiS 2 to form a compound, Hg 1.24 TiS 2 , which exhibits novel behavior, including superstoichiometric mercury uptake, no, or at most a very small degree of, guest-host charge transfer, and the formation of incommensurate Hg chains in the guest galleries. Herein, X-ray powder diffraction (XPD) and differential scanning calorimetry (DSC) have been used to determine the effects of temperature on the Hg 1.24 TiS 2 structure from ambient temperature to 500 K. DSC studies reveal the presence of a reversible thermal transition near 473 K. The XPD patterns taken below the transition temperature are all characteristic of the ambient temperature structure together with modest sublattice thermal expansion with increasing temperature. However, above the transition temperature, all of the reflections uniquely associated with the Hg sublattice disappear, while the positions and intensities of the (001) reflections confirm the Hg remains intercalated. Thus, above the 473 K transition the in-plane Hg-sublattice structure and the associated intercalant Hg chains have melted to form guest layers with liquid-like disorder. The evolution of the host and Hg sublattice cell parameters as a function of temperature exhibits the expected discontinuous behavior associated with such a first-order transition.

XAFS STUDY OF CHARGE TRANSFER IN INTERCALATION COMPOUNDS

Abstract An intercalation process is a reversible topotactic reaction in which a guest species occupies empty sites in a solid structure. A charge transfer is always observed between the guest and the host. An accurate knowledge of this electronic exchange, i.e. how many electrons are transferred and on which electronic level, can only be obtained from precise electronic band structure calculations. Such calculations have to be supported by experimental data. For this purpose we have compared X-ray absorption spectroscopy (XAS) data with edge simulations in the multiple scattering formalism and band structure calculations in using the Tight Binding Linear Muffin Tin Orbitals method in the Atomic Spheres Approximation (TB-LMTO-ASA) method. This approach is first illustrated by a study of sulfur K edge on the two modifications (1T and 2H) of tantalum disulfide TaS 2 . It is then applied for an accurate characterization of the charge transfer in the case of lithium intercalation into TiS 2 , from XAFS experiments at the sulfur K edge. It is concluded that sulfur atoms are largely taking part in the intercalation process.

A Unique Tool to Study the Intercalation Process

Abstract We applied X-ray Absorption Spectroscopy, in combination with electronic band structure calculations, to probe the electronic structure before and after lithium intercalation into TiS2, TiSe2 and 2H-NbSe2. We are then able to precisely determine the nature of the charge transfer from intercalated lithium to the host It is shown that lithium is not fully ionized and that chalcogen atoms are deeply involved in the charge transfer process.

Olivine-spinel phase transformations in Mn2SiSe4 at high pressure

High pressure behavior of MnzSiSe,+ with the olivine structure is investigated using X-ray diffraction. The pressureinduced olivine-spine1 phase transition in the 2040 kbar range is accompanied by formation of intermediate spinelloid compounds. 0 1997 Published by Elsevier Science B.V. The olivine structure (Puma space group) is a common one for materials with the M1M2TX4 stoichiometry, where T = Si. Ge, Sn (a tetrahedral site); X = 0, S, Se, and Mf+ and Mi+ are cations at octahedral sites [l]. Many oxides with the olivine structure are known to transform to the spine1 structure (F&KY space group) at high pressure, with no change in the cation coordination [2]. In olivines, oxygen atoms form a distorted hexagonal close packing whereas spinels have a face-centered oxygen packing [3]. Both olivine and spine1 structures are taken at ambient pressure by many chalcogenides X = S, Se, Te [4-61. However, the olivine-spine1 transition at high pressure has not yet been demonstrated for sulfide or selenide compounds.

Mercury Intercalation into Lamellar Transition Metal Disulfides

Abstract Mercury can be intercalated at room or moderate temperature into lamellar titanium and tantalum disulfides. The structure of intercalated compounds is described by two incommensurate sublattices corresponding to the disulfide host layers and the intercalated mercury, respectively. Depending on the synthetic procedure two different phases are obtained for mercury intercalated tantalum disulfide, which mainly differ in the relative orientation of disulfide and mercury networks. Mercury intercalation of 1T-TaS2 results in a tantalum coordination change from octahedral to trigonal prismatic. The mercury-to-host charge transfer is very low and more heavily involves the sulfur orbitals than previously observed for lithium intercalated compounds.

Mercury Intercalation in Titanium and Tantalum Disulfides

Mercury intercalation in TiS2 and TaS2 can be carried out using “soft chemistry”, techniques, i.e at room or low (T<200°C) temperatures.