44 research outputs found
Electronic structure of the strongly hybridized ferromagnet CeFe2
We report on results from high-energy spectroscopic measurements on CeFe2, a
system of particular interest due to its anomalous ferromagnetism with an
unusually low Curie temperature and small magnetization compared to the other
rare earth-iron Laves phase compounds. Our experimental results indicate very
strong hybridization of the Ce 4f states with the delocalized band states,
mainly the Fe 3d states. In the interpretation and analysis of our measured
spectra, we have made use of two different theoretical approaches: The first
one is based on the Anderson impurity model, with surface contributions
explicitly taken into account. The second method consists of band-structure
calculations for bulk CeFe2. The analysis based on the Anderson impurity model
gives calculated spectra in good agreement with the whole range of measured
spectra, and reveals that the Ce 4f -- Fe 3d hybridization is considerably
reduced at the surface, resulting in even stronger hybridization in the bulk
than previously thought. The band-structure calculations are ab initio
full-potential linear muffin-tin orbital calculations within the
local-spin-density approximation of the density functional. The Ce 4f electrons
were treated as itinerant band electrons. Interestingly, the Ce 4f partial
density of states obtained from the band-structure calculations also agree well
with the experimental spectra concerning both the 4f peak position and the 4f
bandwidth, if the surface effects are properly taken into account. In addition,
results, notably the partial spin magnetic moments, from the band-structure
calculations are discussed in some detail and compared to experimental findings
and earlier calculations.Comment: 10 pages, 8 figures, to appear in Phys. Rev. B in December 200
Laboratory measurements of electrical conductivities of hydrous and dry Mt. Vesuvius melts under pressure
International audienceQuantitative interpretation of MT anomalies in volcanic regions requires laboratory measurements of electrical conductivities of natural magma compositions. The electrical conductivities of three lava compositions from Mt. Vesuvius (Italy) have been measured using an impedance spectrometer. Experiments were conducted on both glasses and melts between 400 and 1300°C, and both at ambient pressure in air and at high pressures (up to 400 MPa). Both dry and hydrous (up to 5.6 wt% H2O) melt compositions were investigated. A change of the conduction mechanism corresponding to the glass transition was systematically observed. The conductivity data were fitted by sample-specific Arrhenius laws on either side of Tg. The electrical conductivity increases with temperature and is higher in the order tephrite, phonotephrite to phonolite. For the three compositions investigated, increasing pressure decreases the conductivity, although the effect of pressure is relatively small. The three compositions investigated have similar activation volumes (ÎV=16-24 cm3/mol). Increasing the water content of the melt increases the conductivity. Comparison of activation energies (Ea) from conductivity and sodium diffusion, and use of the Nernst-Einstein relation allow sodium to be identified as the main charge carrier in our melts and presumably also in the corresponding glasses. Our data and those of previous studies highlight the correlation between the Arrhenius parameters Ea and Ï0. A semi-empirical method allowing the determination of the electrical conductivity of natural magmatic liquids is proposed, in which the activation energy is modelled on the basis of the Anderson-Stuart model, Ï0 being obtained from the compensation law and ÎV fitted from our experimental data. The model enables the electrical conductivity to be calculated for the entire range of melt compositions at Mt. Vesuvius and also predicts satisfactorily the electrical response of other melt compositions. Electrical conductivity data for Mt. Vesuvius melts and magmas are slightly lower than the electrical anomaly revealed by MT studies
The model case of an oxygen storage catalyst â non-stoichiometry, point defects and electrical conductivity of single crystalline CeO 2
Ecological risk assessment of organic waste amendments using the species sensitivity distribution from a soil organisms test battery
Conductibilité électrique et défauts de structure de HfO2 monoclinique à haute température
La conductibilitĂ© Ă©lectrique du dioxyde de hafnium HfO2 a Ă©tĂ© Ă©tudiĂ©e en fonction de la tempĂ©rature et de la pression partielle dâoxygĂšne entre 1 000 et 1 600° C et dans lâintervalle de pressions partielles dâoxygĂšne de 1 Ă 10â4 atm.De plus, les parts de conduction ionique et Ă©lectronique ont Ă©tĂ© dĂ©terminĂ©es par la mĂ©thode de la f.Ă©.m. de piles dans lesquelles la pression partielle dâoxygĂšne aux Ă©lectrodes est fixĂ©e par lâintermĂ©diaire soit de mĂ©langes gazeux (entre 1 et 10â3 atm.), soit dâĂ©quilibres hĂ©tĂ©rogĂšnes mĂ©tal-oxyde.Les rĂ©sultats montrent que le nombre de transport ionique :â croĂźt avec la tempĂ©rature;â dĂ©croĂźt quand la pression partielle dâoxygĂšne sâĂ©lĂšve.Des rĂ©sultats obtenus on dĂ©duit les lois de variation des conductivitĂ©s Ă©lectronique et ionique en fonction de la tempĂ©rature et de la pression partielle dâoxygĂšne.Une interprĂ©tation quantitative des rĂ©sultats obtenus est proposĂ©e Elle suppose lâexistence dâun dĂ©sordre de type Schottky avec prĂ©dominance de lacunes cationiques, sans associations de dĂ©fauts.Lâaccord avec lâexpĂ©rience est satisfaisant
Applications selon BauerlĂ© du tracĂ© des diagrammes dâadmittance complexe en Ă©lectrochimie des solides
La mĂ©thode du tracĂ© des diagrammes dâadmittance complexe Ă Ă©tĂ© appliquĂ©e Ă lâĂ©tude des cellules O2, mĂ©tal/(ZrO2)0,91 â (Y2O3)0,09/mĂ©taI, O2. Trois domaines de frĂ©quence distincts ont Ă©tĂ© mis en Ă©vidence. Les rĂ©sultats obtenus dans le domaine des basses frĂ©quences sont plus particuliĂšrement caractĂ©ristiques des rĂ©actions dâĂ©lectrodes. Les mesures effectuĂ©es dans ce domaine permettent de dĂ©terminer les paramĂštres de la rĂ©action dâĂ©lectrode Ă oxygĂšne en sâaffranchissant des effets diĂ©lectriques dus Ă lâĂ©lectrolyte. La mĂ©thode a Ă©tĂ© appliquĂ©e Ă lâĂ©tude des Ă©lectrodes O2, Ag/zircone stabilisĂ©e et O2, Pt/zircone stabilisĂ©e.
La surtension de l'Ă©lectrode en argent est nĂ©gligeable pour les pressions dâoxygĂšne voisines de lâatmosphĂšre, pour les pressions plus faibles la composante rĂ©elle de lâimpĂ©dance dâĂ©lectrode est proportionnelle Ă [math] et son Ă©nergie dâactivation voisine de 2 eV. Dans le cas de lâĂ©lectrode de platine les surtensions sont notables mĂȘme sous pression dâoxygĂšne Ă©levĂ©e, leurs variations sont beaucoup plus complexes que dans le cas de lâargent et semblent rĂ©sulter de la formation dâun oxyde superficiel de platine.
Les effets capacitifs observĂ©s rĂ©sulteraient principalement dâune surtension de concentration en oxygĂšne neutre. La capacitĂ© de double couche des Ă©lectrodes est petite par rapport aux capacitĂ©s mesurĂ©es el est donc infĂ©rieure Ă 10 ”.F/cm2
Conduction électrique et défauts de structure dans le dioxyde de thorium a haute température.
La conductivitĂ© Ă©lectrique totale dâĂ©chantillons frittĂ©s de dioxyde de thorium de compacitĂ© Ă©levĂ©e (voisine de 99 %) a Ă©tĂ© mesurĂ©e dans les domaines de tempĂ©rature (1 000- 1 500 °C) et de pression partielle dâoxygĂšne (1-10-29 atm.).La poudre de dioxyde de Thorium pur est prĂ©parĂ©e par dĂ©composition thermique de lâoxalate prĂ©cipitĂ©.Des conditions de frittage diffĂ©rentes (atmosphĂšre, tempĂ©rature) sont sans effet sur les valeurs de conductivitĂ© mesurĂ©es.Les rĂ©sultats obtenus sont comparĂ©s Ă lâensemble des donnĂ©es de la littĂ©rature. Ils confirment le caractĂšre mixte (ionique et Ă©lectronique) de la conduction, dĂ©jĂ observĂ© par diffĂ©rents auteurs
ActivitĂ©s des constituants dâun silicate binaire
LâinterprĂ©tation de rĂ©sultats expĂ©rimentaux fournis par diffĂ©rentes Ă©tudes consacrĂ©es aux silicates binaires liquides a conduit certains auteurs Ă supposer un Ă©quilibre thermodynamique entre trois Ă©tats de liaisons diffĂ©rents de lâatome dâoxygĂšne (modĂšle de FINCHAM et RICHARDSON).A partir de lâhypothĂšse dĂ©jĂ Ă©mise par TOOP et SAMIS, de lâidĂ©alitĂ© du mĂ©lange de ces trois Ă©tats de liaison dâoxygĂšne, nous avons calculĂ© la concentration de ces derniers en fonction de la composition du silicate.En limitant notre Ă©tude aux silicates alcalins, nous avons comparĂ© les courbes dâactivitĂ©s calculĂ©es Ă partir de diffĂ©rentes donnĂ©es expĂ©rimentales
Magnetic structures and interactions in RZn12 and RCu4Al8 compounds
Les propriĂ©tĂ©s et les structures magnĂ©tiques des composĂ©s RCu4Al8 et TZn12 sont dĂ©duites des mesures d'aimantation et des diagrammes de diffraction de neutrons. Pour rendre compte des structures magnĂ©tiques dans un modĂšle d'Heisenberg, des interactions Ă longue distance (> 10 Ă
) doivent ĂȘtre considĂ©rĂ©es.The magnetic properties and structures of RCu4Al8, and RZn12 compounds are deduced from magnetic measurements and neutron diffraction experiments. To explain the observed structures in the Heisenberg exchange model, long range interactions (> 10 Ă
) must be considered