JOURNAL of the Tunisian Chemical Society

Mixed barium-lead fluoroapatites Ba_(10-x)Pb_x(PO₄)₆F₂ with 0 ≤ x ≤ 10 were prepared by precipitation in aqueous medium using double decomposition method. They were investigated by X-ray powder diffraction, infrared spectroscopy and chemical analysis. The structural refinement, by X-ray powder pattern fitting using Rietveld method indicate that the progressive replacement of barium by lead in the apatitic structure, leads to a slight decrease in the crystallographic parameters a and c following the Vegard's law and a shift of F^- ions outside the center of triangle formed by M(2) atoms. The cationic repartition between the two crystallographic sites indicate a clear preference of lead for the S(1) site, in agreement with its smaller ionic radii. Through the substitution, a slight-wavenumber shift of the IR absorption bands characteristic of PO₄ internal modes toward lower frequencies is observed, in agreement with the growth of covalent character of M-O bonds. The low wavenumber bands around 300 cm^-1 may be assigned to a "v₃ type stretching" of the M₃-F unit.

A. Aissa, B. Badraouia, N. Fatteh, M. Debbabia

Fluoroapatites, Rietveld method, crystallographic sites, IR spectroscopy

Pages 203-211

This paper deals with the modelling of the thermodynamic properties of the water-ammonia mixture. At first, the Gibbs enthalpy function of each of the pure fluids is established. Then, a model of the excess free enthalpy of the mixture in the liquid phase is formulated, while the vapor phase is considered as an ideal solution. This model describes with very good accuracy the thermodynamic surface of the water-ammonia mixture in the liquid phase, in the vapor phase as well as at the vapor-liquid equilibrium, for temperatures going from 200 K up to 500 K and pressures to 60 bar.

K. Mejbri, N. Ben Ezzine, M. Barhoumi, A. Bellagi

ammonia-water mixture, thermodynamic properties, Gibbs enthalpy function, excess properties

Pages 213-228