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Well-cristallized Tl 2 Ba 2 CaCu 2 O 8 phases have been observed superconducting or normal below 108 K depending on their stoichiometry. This observation is an evidence that a (Cu 11 -O-) mixed valence induced by vacancies or substitutions on different sites gives rise to superconductivity in this phase. The new phase TlBa 2 Ca 2 Cu 3 O 9 which intrinsically contains a mixed valence has been observed as having a sharp transition to bulk superconductivity in the Meissner effect at a critical temperature of 120 K Des echantillons de la phase Tl 2 Ba 2 CaCu 2 O 8 bien cristallisee ont ete observes supraconducteurs ou normaux en dessous de 108 K suivant leur stoechiometrie. Cette observation montre que la valence mixte (Cu 11- O - ) induite par les lacunes ou les substitutions sur les differents sites accroit la supraconductivite de cette phase. La nouvelle phase TlBa 2 Ca 2 Cu 3 O 9 , qui contient intrinsequement une valence mixte, a ete observee, en effet Meissner, supraconductrice en volume avec une transition etroite a 120 K

A method, based on X-ray absorption spectroscopy (XAS) in the range 0.8–1.5 keV, to determine the relative amounts of rare earth ions in different valencies is explained and tested for the case of terbium mixed oxides. The results are in agreement with those obtained by existing analytical methods. The XAS method is advantageous in that it can be applied where other, conventional, methods break down.

In this report we describe the activation volumes associated with the heme-heme electron transfer (ET) and CO rebinding to the binuclear center subsequent to photolysis of the CO-mixed-valence derivative of Escherichia coli cytochrome bo(3) (Cbo). The activation volumes associated with the heme-heme ET (k=1.2 x 10(5) s(-1)), and CO rebinding (k=57 s(-1)) are found to be +27.4 ml/mol and -2.6 ml/mol, respectively. The activation volume associated with the rebinding of CO is consistent with previous Cu X-ray absorption studies of Cbo where a structural change was observed at the Cu(B) site (loss of a histidine ligand) due to a change in the redox state of the binuclear center. In addition, the volume of activation for the heme-heme ET was found to be quite distinct from the activation volumes obtained for heme-heme ET in bovine heart Cytochrome c oxidase. Differences in mechanisms/pathways for heme b/heme o(3) and heme a/heme a(3) ET are suggested based on the associated activation volumes and previously obtained Marcus parameters.

Les oxydes d'éléments de transition présentent une grande diversité de comportement allant du diélectrique au métal et même au supraconducteur. Ce phénomène a donné lieu à un grand nombre de théorie qui n'ont cessé de se développer au cours des quarantes dernières années. Ces théories dont un rappel est proposé au début du mémoire ont été appliquées à des systèmes d'oxydes de tungstène à valences mixtes tels que les bronzes, comme NaxWO3 et CsxWO3, purs ou dopés, ou des solutions solides de structures dérivées du rutile, comme Cr1-xW1+xO4. L'étude de ces systèmes dans lesquels la localisation d'Anderson semble jouer un rôle important a permis de mettre en évidence le rôle du libre parcours moyen inélastique et à basse température celui des corrélations électroniques.; non disponible

With the advent of the very intense 3rd generation synchrotron sources of X rays, new powerful spectroscopic techniques, requiring the detection of very low cross section phenomena, have come of age. Among them Resonant Inelastic X-Ray Scattering (RIXS) is rapidly becoming a major probe of the electronic structure of solids. RIXS measures the decay radiation that follows the excitation of core electrons into states located in a narrow region above the Fermi level. It shares with other core-level spectroscopies a remarkable chemical sensitivity and provides specific information on selected elements in a compound. Since photons are used to excite the process and photons are detected, truly bulk-sensitive information can be obtained. RIXS is sensitive to the nature of the ground state and of the characteristic excitations of a material, and can separately measure their energies, because exciting atoms with different atomic configuration leaves the solid in final states of different total energy. Therefore, it can discriminate different chemical environments and valence states of the same element. In this paper, we outline the basic principles of RIXS, with special emphasis on electronic excitations, and discuss the relevance of this information for studies of hybridization and valence in materials science, by referring to recent experiments.

We used X-ray absorption in the partial fluorescence yield mode (PFY-XAS), and resonant X-ray emission (RXES) to study the pressure dependence of Yb valence in the intermediate valence compound YbAl2. We find that the Yb valence changes from 2.25 at ambient pressure to 2.95 at 385 kbar, and that the valence transition is completed around 200 kbar. The combined use of complementary spectroscopic techniques considerably reduces the uncertainties in the extracted valence values. (C) 2004 Elsevier B.V. All rights reserved.