Khaled Boughzala

Spectroscopic studies and Rietveld refinement of strontium-britholites

Abstract Strontium-britholites whose chemical formula was Sr10−xLax(PO4)6−x(SiO4)xF2, where x=0, 1, 2, and 4 were prepared by solid state reaction. The structural refinement carried out using the Rietveld method indicated that La3+ ions were located into the two sites with a strong preference for metal (2) sites especially for low contents. A progressive shift of the F− position along the c-axis outside the centre of the triangle formed by metal (2)-atoms was observed with the increase of x. The infrared and Raman spectra exhibited the characteristic vibration modes of PO4 and SiO4 groups confirming the incorporation of this last group into the apatite structure. The 29Si MAS-NMR spectra exhibited one resonance peak confirming the data obtained by X-ray diffraction, indicating that P and Si were located in the same crystallographic site.

Structural analysis by Rietveld refinement of calcium and lanthanum phosphosilicate apatites

Abstract Britholites with the general formula Ca10–xLnx(PO4)6–x(SiO4)xF2, (0≤x≤6) are considered to be promising matrices for the confinement of the by-products in the nuclear industry. A thermodynamic study showed that the stability of these compounds decreased as the substitution rate increased. The present work was an attempt to gain more information about the structural changes induced by the substitution, in order to understand the observed stability decrease. The samples were successfully synthesized as a single-phase apatite by a solid-state reaction between 1200 and 1400 °C. The structural refinement indicated that the La3+ ions preferentially occupied the 6(h) sites. A progressive shift of F− along the c-axis outside its ideal position occurred as a result of the substitution increase. This might be the cause of the observed stability decrease, especially as the energies of the La–O, La–F and Si–O bonds are higher than those of Ca–O, Ca–F and P–O. The distribution of La3+ between the two non-equivalent sites was confirmed by the charge distribution method.