Mechanism of the Charge Transport in Intercalated Compounds Application of Optical and Potential Electrochemical Spectro
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MECHANISM OF THE CHARGE TRANSPORT IN INTERCALATED COMPOUNDS APPLICATION OF OPTICAL AND POTENTIAL ELECTROCHEMICAL SPECTROSCOPIES C. JULIEN Laboratoire de Physique des Solides, associ6 au CNRS Universit6 P. et M. Curie, 4 place Jussieu, 75252 Paris Cedex 05, France
ABSTRACT Layered compounds are known to be among the best host structures to practice lithium intercalation chemistry. Besides geometrical aspects which play an important role but are now quite well understood, this paper emphasizes the mechanism of the charge transfer upon intercalation. Electrochemical and optical spectroscopies associated to transport measurements are used for the investigations of the electronic modifications involved in the charge transfer in lithium intercalated compounds. A selection of appropriate materials is discussed. Optical spectroscopies such as photoluminescence, Raman scattering and far-infrared reflectivity show that free-carrier density and electron mobility change drastically. A good agreement is obtained with the data measurements carried out by transport experiments. 1. INTRODUCTION Intercalation compounds are solids which have inner atoms providing a structure into which guest atoms or molecules can be inserted. The intercalation of cations in a given host-structure is the result of a reversible ion-electron transfer reaction which is classically represented by the scheme xA+ + xe" + Hs 4* Ax+ Hsx-,
(1)
where Hs denotes the host structure. In the usual case A+ is an alkali metal. Generally two main effects occur during intercalation : (i) a change of preferential crystallographic parameters without destruction of the original structure and (ii) a charge transfer which can affect strongly the electronic properties of the host. The A+ intercalated cations have been associated various structural arrangements [1] and numerous studies have also dealt with their mobility inside the framework [2-4]. These aspects are quite well understood at present. The electronic transfer, generally not considered in many papers, plays an essentiel role towards the formation of intercalation compounds, and it also largely governs the phase transitions. Charge is transferred between the outer orbitals of the guest atoms and the electron bands of the host during the intercalation process. Two prototypes can be cited here. In the transition-metal dichalcogenides, the guest gives up charge to become a positive ion, and the electrons donated by the guests enter the host band structure [5]. In graphite, which is one of the oldest known intercalation system guest can act as donors or acceptors as well [6]. Electrochemical and optical spectroscopies associated with transport measurements are tools for the investigations of the charge transfer in lithium intercalated layered compounds. Electrochemical potential spectroscopy is powerful for the study of the variation of free-energy and the determination of induced phase or superlattice formation which can be confirmed by X-ray or diffraction microscopy [7]. Optical spectroscopies such as photolum
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