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  • Author or Editor: S. Fernández-Valverde x
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Abstract  

For a better understanding of isotope exchange in solid Tl4Cl6 the effects of crushing the crystals were investigated by conductivity and by positron annihilation lifetime measurements. As in untreated Tl4Cl6, the conductivity variation with temperature shows a break at about 450 K. The activation energies, 0.53 eV and 0.70 eV, respectively, below and above 450 K, are very close to those in the untreated material but the absolute values of conductivity are lower after crushing which is attributed to the trapping of the mobile defects of dislocation. The positron lifetime variation with increasing temperature shows some contribution of extrinsic defects, annealing at about 413 K. On further heating or cooling cycles, the lifetime changes are controlled by the production of intrinsic cation vacancies, whose formation enthalpy, 0.39 eV, is close to that derived for untreated Tl4Cl6. The shape of the initial part of the curves would indicate that crushing does not directly create appreciable concentrations of cation vacancies but would rather produce annealable defects, possibly dislocations, favouring the formation and/or trapping of such vacancies.

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Abstract  

A study of the isotope exchange reaction U(III)org/U(IV)aq in the extraction system: 7M HCl — tributyl phosphate (TBP) — toluene has been performed. For 20 s of contact the results show a separation factor235U/238U of 1.014. This large separation factor is explained by the oxidation reaction of235U(III) and238U(III).

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Abstract  

Several attempts have been made to synthesize mixed-valence compound of antimony at low temperature in view of studying the isotopic exchange in solids. The experimental procedure for the synthesis, at 263 K of Cs4(SbIIICl6)(SbVCl6) has been established. This compound seems very promising for isotopic exchange studies in the solid state.

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Abstract  

The sorption of uranium(VI) on the perovskite structure of strontium titanate in a 0.5M KNO3 solution is studied. SrTiO3 commercial material was characterized by XRD showing a tausonite face, with a specific area of 2.42 m2.g−1. The electrical surface characterization of the compound was performed by mass and potentiometric titrations. pHpzc in water was 8.5±0.3 and 9.1±0.2 in 0.5M KNO3 solution, showing a positively charged surface. FITEQL 4.0 program was used to calculate the sorption curves and the surface acidity constants by the constant capacitance model obtaining: log K 1 = 8.67 and log K 2 = −9.43. The sorption edge was fitted with two different uranium(VI) species sorbed, corresponding to bidentate complexes of UO2 2+ and UO2(OH)2H2O on the surface of strontium titanate.

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