Interplay between the Charge Transport Phenomena and the Charge-Transfer Phase Transition in RbxMn[Fe(CN)6]y · zH2O

Charge transport and dielectric measurements were carried out on compacted powder and single-crystal samples of bistable RbxMn[Fe(CN)6]y · zH2O in the two valence-tautomeric forms (MnIIFeIII and MnIIIFeII) as a function of temperature (120-350 K) and frequency (10-2-106 Hz). The complex conductivity data reveal universal conductivity behavior and obey the Barton-Nakajima-Namikawa relationship. The charge transport is accompanied by dielectric relaxation that displays the same thermal activation energy as the conductivity. Surprisingly, the activation energy of the conductivity was found very similar in the two valence-tautomeric forms (∼0.55 eV), and the conductivity change between the two phases is governed mainly by the variation of the preexponential factor in each sample. The phase transition is accompanied by a large thermal hysteresis of the conductivity and the dielectric constant. In the hysteresis region, however, a crossover occurs in the charge transport mechanism at T < ∼220 K from an Arrhenius-type to a varying activation energy behavior, conferring an unusual “double-loop” shape to the hysteresis

Dispositif automatique de mesures de résistivité entre 4 et 1 100 K

Conductivity measurements, using Ohm's law, are obtained by a current injection and the evaluation of the associated voltage. A computer controlled d.c. resistivity equipment is described in which the injected current is optimized taking into account the non-linearity of the I versus V plot and also the parasitic contact potentials. The application of this automatic system and validity of the algorithm have been tested from 4.2 to 1 100 K for various samples with resistivity extending from 10-4 to 10 5 Ω.m.La mesure de la conductivité d'un échantillon par application de la loi l'Ohm impose l'injection d'un courant et la mesure de la tension correspondante. Une méthode générale de mesures automatisée de la résistivité est proposée, dans laquelle le courant injecté dans l'échantillon est optimisé en tenant compte de la non-linéarité éventuelle de la caractéristique I( V) du matériau testé et de l'existence de potentiels de contact. Le domaine d'application de ce système automatisé ainsi que la validité de l'algorithme présenté sont testés de 4,2 à 1 100 K pour des échantillons dont la résistivité varie de 10-4 à 105 Q.m

Transport and Magnetic-properties of U(2)m(3)si(5) Silicides (m=co,Rh,Ru)

We report and discuss experimental data on the electrical resistivity p, thermoelectric power S, thermal conductivity kappa, specific heat C and magnetic susceptibility chi of polycrystalline U(2)M(3)Si(5) silicides (M = Co, Rh, Ru). The magnetic and transport properties of these compounds depend strongly on the nature of the transition metal M. In U2Co3Si5, p(T) shows features that could be identified to Kondo systems while S(T) shows a two-peak structure. U2Rh3Si5 shows an antiferromagnetic ordering at T-N similar to 25 K, which is accompanied by steep changes in both p(T) and S(T). U2Ru3Si5 behaves like a nonmagnetic compound and displays distinct anomalies in the low-temperature domains of p(T, H) and S(T, H). The lattice thermal conductivity varies as T-2 at low temperatures, suggesting a dominant scattering of the phonons by the conduction electrons

Correlations Between Fluorination and Transport-properties in the La1.85-xndxsr0.15cuo4 System

The influence of fluorination on ceramic samples of La1.85-xNdxSr0.15CuO4 solid solutions has been studied. The presence of small amounts of fluorine species in the interior of the grains which has been evidenced by Auger electron spectroscopy significantly influences the structural and transport properties. It has been shown that the intercalation of fluorine into the structure reinforces the metallic-like behaviour and superconducting properties by hindering the tendency to the low-temperature structural transition and by increasing the charge carrier density

Synthesis and thermostructural studies of a CuFe1-xCrxO2 delafossite solid solution with 0<=X<=1

In this work, different CuFe1-xCrxO2 compositions with 0<=x<=1 were prepared by a standard solid-state reaction. These oxides crystallize with the delafossite structure. The phase stability and thermal behavior of the complete CuFe1-xCrxO2 solid solution was studied by thermogravimetric analysis and high-temperature X-ray diffraction experiments under an air atmosphere up to 1000 C. For x = 0, CuFeO2 is oxidized into the spinel (CuFe2O4) and copper monoxide (CuO) phases, whereas for x = 1, CuCrO2 is thermally stable. For all of the intermediate compositions (0 < x < 1), complex oxidation, reduction, and phase transitions between delafossite and spinel have been observed. chromium tends to stabilize the stoichiometric delafossite phase, while iron favors the delafossite-to-spinel phase transition