Anomaly in the dielectric response at the charge orbital ordering transition of crystalline Pr0.67Ca0.33MnO3

The complex impedance of a Pr0.67Ca0.33MnO3 crystal has been measured. The frequency dependence is studied for a wide range of temperatures (50K-403K) and is found to be characteristic of relaxation process with a single Debye time relaxation constant, which is interpreted as a dielectric constant of the material. A strong peak is observed in this dielectric constant (up to two millions) at the charge ordering transition suggesting an interpretation in terms of ordering of electric dipoles at TCO or in term of phase separation. Comparison with Pr0.63Ca0.37MnO3 - in which the phase separation is much smaller and the peak in the dielectric constant is absent - suggests an interpretation in term of phase separation between insulating and metallic states.Comment: pdf fil

Spin state diagrams of 3dn cations in tetragonally distorted octahedral sites

The possible existence of an intermediate spin state is now a few decades fascinating solid state chemists and physicists, experimentalists, and theoreticians. In this article, we revise some recent results on the stability diagrams of spin states, low, high, or intermediate, in distorted environments and extend their approach to redraw more realistic diagrams for d(4), d(5), and d(6) ions in a tetragonally distorted 6-fold oxygen environment (D4h, D2d, and C4ν). The model relies on a point charge model and further uses effective parameters to account for the cubic field drift on spin state change and for suitable values for solid state of the expectation values of the 3d-radial wave functions; additionally the model uses rational parameters to characterize the distortion; finally, we also consider the possible existence of states' combinations to propose reliable stability diagrams. Whatever the representation involved in the distortion, the existence domain of the intermediate spin state appears very small and more likely replaced with mixtures of cations in low and high spin states; the opportunity of induced distortive ordering is discussed

Synthèse et caractérisation de matériaux diélectriques pour la conception de condensateurs multicouches à électrodes internes en métaux non nobles

CAEN-BU Sciences et STAPS (141182103) / SudocLIMOGES-ENSCI (870852305) / SudocSudocFranceF

Low temperature sintering of CaZrO3 using lithium fluoride addition

International audienceThe effects of lithium fluoride on the sinterability, the microstructure and the low frequency dielectric properties of acceptor/donor-doped CaZrO3 ceramics were investigated. The acceptor(Mn)/donor(W) doping was used to avoid CaZrO3 reduction. Lithium fluoride was selected as a liquid phase sintering aid to lower the sintering temperature. The dielectric properties of CaZrO3 ceramics with LiF additions are strongly dependent on the densification, the microstructure, and the reaction with LiF. CaZrO3 ceramics without LiF addition sintered at high temperature display a high permittivity, low losses and a good behavior under an electrical field as a function of temperature. Using LiF, this dielectric can be sintered at 1000 °C to achieve theoretical densities of 91%, εr values of 31 and losses close to 0.4%. This low sintering temperature allows co-sintering with base metal, nickel and copper, for which scanning electron microscopy and energy dispersive spectroscopy results are presented

High decrease in CaZrO3 sintering temperature using complex fluoride fluxes

International audienc

High decrease in CaZrO3 sintering temperature using complex fluoride fluxes

International audienc

Experimental investigation of structure - property relationships and dimensionality aspects in some cobalt and vanadium oxides

Les oxydes doubles lamellaires de cobalt et les oxydes de vanadium ont récemment suscité un vifintérêt suite à la découverte de leurs propriétés thermoélectriques prometteuses. Nos efforts visentà synthétiser de nouveaux composés dérivés de ces systèmes en utilisant la synthèse à l'étatsolide, l'échange d'ions et/ou les techniques hydrothermales. Afin de moduler et d'optimiser leurscaractéristiques thermoélectriques, nous avons ajusté la composition des oxydes de cobalt grâce àdes substitutions appropriées. Au sein du dioxyde de vanadium quasi-1D, nous avons considérétrois différentes structures polymorphes : M1, A et B. Nous avons exploré les différents systèmesen construisant l ensemble des diagrammes de phases. Nous avons également étudié l'effet de lasubstitution du vanadium par le molybdène et le chrome sur la stabilité de ces structurespolymorphes et caractérisé leurs propriétés électroniques en relation avec les mécanismes detransition de phase.Lamellar cobalt double oxides and vanadium oxides have recently attracted tremendous interestafter the discovery of their interesting thermoelectric properties. Our efforts aimed at synthesizingnovel related compounds using standard solid state, ion exchange and/or hydrothermaltechniques. In order to modulate and optimize their thermoelectric characteristics, we have tunedthe composition of cobalt double oxides by appropriate substitutions. In quasi 1D vanadiumoxides, the interplay between spin, charge and orbital degrees of freedom often leads toremarkable properties. Here we dealt with three different polymorphs of vanadium dioxide,namely M1, A and B. We explored the several novel systems by constructing systematic phasediagrams. We also studied the effect of Mo and Cr:V substitution on the stability of thesepolymorphs and characterized their electronic properties in relation with the structural phasetransition mechanisms. Finally, we explored their potentiality for thermoelectric applications.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Reinvestigation of the magnetic behavior of O3–LiCoO₂

Stoichiometric high temperature LiCoO2 obtained by long annealing in oxygen was characterized by electron spin resonance and magnetization measurements. Both methods allow identifying unambiguously not only the presence of traces of cobalt oxides in the material but also paramagnetic defects in lithium cobaltite itself. We report on the presence of surface Li+-O- centers in pure LiCoO2, which has not been observed before in this material, and on the presence of Co2+ related centers in argon- and subsequent oxygen-annealed samples

(Li/Ag)CoO(2): A New Intergrowth Cobalt Oxide Composed of Rock Salt and Delafossite Layers.

A new ordered (Li/Ag)CoO(2) layered compound with an unusual oxygen packing combining rock salt and delafossite layers is obtained during the (Li(+), Na(+))/Ag(+) ionic exchange from the OP4-(Li/Na)CoO(2) precursor. This compound is actually an intermediate step to the final D4-AgCoO(2) delafossite and can be isolated thanks to the kinetics difference between the Li(+)/Ag(+) and Na(+)/Ag(+) exchange processes. It crystallizes in the P6(3)/mmc space group with cell parameters a(hex.) = 2.848(3) Å and c(hex.) = 21.607(7) Å. The details of the structure as well as its thermal stability and transport properties are presented and discussed

First Experimental Evidence of a New D4-AgCoO(2) Delafossite Stacking.

A new polytype of AgCoO(2) delafossite has been prepared from the ordered OP4-(Li/Na)CoO(2) layered compound using ion-exchange reaction in molten salts. As expected from the structural model assuming a topotactic process, the lamellar structure of this new polytype is an alternate combination of the already known 2H and 3R delafossite polytypes. It crystallizes in the P6(3)/mmc space group with cell parameters a(hex.) = 2.871(1) Å and c(hex.) = 24.448(1) Å. Thermal stability, morphology characterization, and electrical properties are reported here and compared with those of the 2H and 3R AgCoO(2) polytypes