Standard
enthalpy of formation of lanthanum oxybritholites |
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Authors: | K Ardhaoui J Rogez A Ben Chérifa M Jemal P Satre |
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Institution: | (1) Faculté des Sciences de Tunis, Département de Chimie, Campus Universitaire, Laboratoire de Thermodynamique Appliquée, Tunis El Manar, Tunisie, 2092;(2) Laboratoire TECSEN, UMR 6122, CNRS-UPCAM III, Marseille Cedex 20, France, 13397, Case 251, Avenue Escadrille Normandie-Niemen;(3) Laboratoire des Matériaux à Finalités Spécifiques – Matériaux du Milieu Marin, Université de Toulon et du Var, La Garde, France, 83000 |
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Abstract: | Lanthanum-bearing silicate-oxyapatites or britholites, Ca10–xLax(PO4)6–x(SiO4)xO with 1≤x≤6, have been synthesized by solid state reaction
at high temperature. They were characterized by X-ray diffraction and IR spectroscopy.
Using two microcalorimeters, the heat of solution of these compounds have
been measured at 298 K in a solution of nitric and hydrofluoric acid. A strained
least squares method was applied to the experimental results to obtain the
solution enthalpies at infinite dilution, and the mixing enthalpy in two steps.
In the first step the mixing enthalpy obtained is referenced to the britholite
monosubstituted and to the oxysilicate. The mixing enthalpy referenced to
the oxyapatite and to the oxysilicate is then extrapolated. In order to determine
the enthalpies of formation of all the terms of the solution, thermochemical
cycles were proposed and complementary experiments were performed. The results
obtained show a decrease of the enthalpy of formation with the amount of Si
and La introduced in the lattice. This was explained by the difference in
the bond energies of (Ca–O, P–O) and (La–O, Si–O). |
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Keywords: | britholites enthalpy of formation solution calorimetry thermodynamics |
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