Transport and Magnetic Properties of R1-xAxCoO3 (R=La, Pr and Nd; A=Ba, Sr and Ca)

Transport and magnetic measurements have been carried out on perovskite Co-oxides R1-xAxCoO3 (R=La, Pr, and Nd; A=Ba, Sr and Ca; 0<x<0.5: All sets of the R and A species except Nd1-xBaxCoO3 have been studied.). With increasing the Sr- or Ba-concentration x, the system becomes metallic ferromagnet with rather large magnetic moments. For R=Pr and Nd and A=Ca, the system approaches the metal- insulator phase boundary but does not become metallic. The magnetic moments of the Ca-doped systems measured with the magnetic field H=0.1 T are much smaller than those of the Ba- and Sr-doped systems. The thermoelectric powers of the Ba- and Sr-doped systems decrease from large positive values of lightly doped samples to negative ones with increasing doping level, while those of Ca-doped systems remain positive. These results can be understood by considering the relationship between the average ionic radius of R1-xAx and the energy difference between the low spin and intermediate spin states. We have found the resistivity-anomaly in the measurements of Pr1-xCaxCoO3 under pressure in the wide region of x, which indicates the existence of a phase transition different from the one reported in the very restricted region of x~0.5 at ambient pressure [Tsubouchi et al. Phys. Rev. B 66 (2002) 052418.]. No indication of this kind of transition has been observed in other species of R.Comment: 9 pages, 8 figures. J. Phys. Soc. Jpn. 72 (2003) No.

Transition isolant-métal dans les oxyfluorures V

L’évolution des propriétés structurales, électriques, magnétiques et thermiques des phases V1-xCoxO2-2xF2x, étudiée dans un domaine de compositions proche de VO2 (x ≤ 0,10), révèle l’existence, aux faibles concentrations, d’une transition isolant-métal du type de celles des phases V1-xNbxO2 et VO2-x Fx. La température de transition Tt décroit lorsqu’augmente le taux en substitution en fluorure. L’introduction de fluor provoque la création de niveaux localisés en-dessous de la bande de conduction de la phase monoclinique ainsi qu’une localisation d’Anderson dans la phase rutile qui existent dans VO2 pur

Photocourant dans des monocristaux de dichalcogénures lamellaires MSe

Des monocristaux de diséléniures de molybdène et de tungstène ont été utilisés comme électrode de travail pour des mesures de photocourant réalisées sous illumination à l'aide d'un montage potentiométrique classique. Leur gain en photocourant et leur caractère n ou p ont ainsi été comparés.Ils dépendent fortement de l'agent de transport (nature, concentration), du type d'électrolyte et des états de surface. Le maximum de densité de courant à saturation (Imax = 440 A.m-2) a été obtenu, en milieu I3-/I-. avec des matériaux transportés par des tétrachlorures (SeCl4 pour MoSe2. TeCl4 pour WSe2)

Methanogenic octadecene degradation by syntrophic enrichment culture from brackish sediments

A microbial enrichment culture from brackish sediments was able to grow on octadec-1-ene (an unsaturated aliphatic hydrocarbon) as sole source of carbon and energy, under methanogenic conditions. Octadecene degradation is stopped either when bromoethanesulfonic acid, a selective inhibitor of methanogenesis is introduced, or when hydrogen is introduced. In the presence of bromoethanesulfonic acid, the degradation is restored by the addition of a hydrogenotrophic sulfate-reducing microorganism with sulfate. Results of molecular biodiversity, which revealed the presence of bacteria as well as of acetoclastic and hydrogenotrophic methanogens, are consistent with a syntrophic degradation involving Bacteria and Archaea. This is the first demonstration of syntrophic alkene degradation by microbial communities, showing that syntrophy is more widespread than we could have thought so far. These results highlight the need for a better understanding of microbial interactions and their role in the organic-matter degradation in polluted environments

Kinetics of aerobic phenol biodegradation by the acidophilic and hyperthermophilic archaeon Sulfolobus solfataricus 98/2

Biodegradation of 51-745 mg l(-1) of phenol by a well-acclimatized strain of Sulfolobus solfataricus, a thermoacidophilic archaeon, was studied in batch experiments at 80 degrees C and pH 3.2. Phenol inhibited growth and specific degradation rates (mu and qs). Fitting the experimental growth data with the Haldane model gave the following kinetic parameters: mu* = 0.094 h(-1), K-s = 77.7 mg l(-1), K-i = 319.4 mg l(-1) (R-2= 0.950). The true mu(max), calculated from mu*, was 0.047 h(-1). A volumetric degradation rate (V-max) was calculated by fitting the phenol consumption data with the Gompertz model. The value of V-max increased with initial phenol concentration (S-i) up to 14.4 mg l(-1) h(-1). The qs values, calculated from V-max. were fitted with the Haldane equation, yielding qs(max) of 0.110 gg(-1) h(-1). The yield factor (Y-x/s) depends on S-i and reached a maximum of 0.83 gg(-1) at S-i =93 mg l(-1). S. solfataricus 98/2 displayed low mu(max) and qs(max) but a good tolerance to phenol (fairly high K-i, K'(j), high Y-x/s). This ability to grow on and degrade phenol (93 mg l(-1) < optimal S-i < 175 mg l(-1)) at high temperature and low pH is unique and may be useful for removing phenol from hot acidic contaminated effluents. Other possible application could lie in the production of the enzymes involved in the key steps of phenol degradation provided the cloning of the enzymes-related genes in fast-growing mesophiles