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

Texturation de YBa2_2Cu3_3O7−x_{7-x} par la méthode de l'échangeur thermique

The Heat Exchanger Method (HEM) has been used to prepare bulk textured YBa$_2$Cu$_3$O$_{7-x}$ ceramics. The experimental process and the thermal parameters used for texturing are described. The resulting bulk textured ceramics have been investigated by X-ray diffraction, optical and scanning electronic microscopies. A 92 K temperature transition and a $J_c$ of 5 000 A/cm$^2$ at 77 K in the absence of magnetic field have been shown by ac susceptibility measurements.Un dispositif de croissance cristalline basé sur la technique de l'échangeur thermique a été développé au laboratoire. Cette méthode a été appliquée à l'obtention de céramiques orientées de YBa$_2$Cu$_3$O$_{7-x}$. Les conditions expérimentales ainsi que les différents paramètres thermiques afférents à la méthode de texturation utilisée sont précisés. La texturation des échantillons obtenus a été caractérisée par diffraction des rayons X, par microscopie optique et par microscopie électronique à balayage ; la susceptibilité alternative a révélé une transition supraconductrice à 92 K et une densité de courant critique de 5 000 A/cm$^2$ à 77 K et sans champ magnétique

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

ANISOTROPY OF ELECTRICAL-PROPERTIES IN ALPHA-FE2O3 CERAMICS

Electrical conductivity and thermoelectric power measurements carried out in a heamatite ceramic showed a strong anisotropy in directions normal and parallel to the uniaxial pressing direction. This behaviour is similar to that verified in alpha-Fe2O3 single crystal. The results suggest that the extended structural defects, generated during sintering, disturb the magnetic order on the (001) planes of alpha-Fe2O3 and limit the mobility of n type carriers