Spectroscopic study of BixEu1−xVO4 and BiyGd1−yVO4 mixed oxides

The mixed oxides Bi_xEu_1−xVO₄ and Bi_yGd_1−yVO₄ crystallize in a zircon-type structure, for 0 <x < 0.6 and 0 < y < 0.64, and in a fergusonite-type structure, for 0.94 < x < 1 and 0.93 < y < 1. A process of competition between the dominant and the constrained effects of the lone-pair 6s² of Bi³⁺ is discussed. The diffuse reflectance spectroscopic studies of these mixed oxides are presented. The observed broad bands are attributed to charge transfer processes and the sharp peaks in the Bi_xEu_1−xVO₄ spectra are ascribed to intra-configurational 4f – 4ftransitions of the Eu³⁺ ion. The broad absorption shift in BiLnVO₄ (Ln : Eu and Gd) compounds to the longer wavelengths range, when Bi is introduced in the LnVO₄ lattice, is ascribed to charge transfer processes in a Bi-VO₄ center and are interpreted assuming a Jahn-Teller effect in the excited state of Bi³⁺. The concept of an internal pressure of Bi³⁺ ions is also used to explain the broad A-band shifts.     

Influence of sorption and diffusion of aroma compounds in silicone rubber on their extraction by pervaporation

Pervaporation was used to extract aroma compounds from dilute aqueous solutions. The use of a stirred cell allowed the reduction of the boundary layer effect which can be estimated using a mass transfer correlation. A resistance-in-series model was used to describe the flux of organic compounds through a silicone membrane. Calculation of the membrane resistance required the determination of the solubility and diffusion coefficients of the aroma substances in the polymer. These two characteristics were obtained by measuring the quantities of the aroma sorbed on the silicone at equilibrium and by studying the kinetics of sorption. At low concentration ranges, both characteristics were constant. They can be related to the hydrophobicity, the molar volume and the flux of the substances through the silicone membrane during pervaporation.