Study of sites occupation and chemical environment of Eu3+ in phosphate-silicates oxyapatites by luminescence

Silicate apatites called britholites have been studied due to their potential application as materials in the form of nuclear waste for the containment of actinides. The luminescence study of Eu3+ in the solid solution Ca10-xLax(SiO4)y(PO4)6-yOz2-z (with z=1+1/2 (x-y)) allows us to predict the location of an eventual actinide ion and provide structural information on the expected behaviour of the structure towards irradiation damage. The luminescence study confirms the preferential location of the Eu3+ ion in the 6h site (Cs point symmetry) of the space group P63/m, where a strong crystal field due to Eu3+–O(4) bond occurs. This bond competes with oxygen SiO4 and PO4 tetrahedra and the competing is found to be stronger when silicate groups are present. This is because silicate tetrahedra are less compact than phosphate tetrahedra and are able to approach Eu3+ much closer. It is also shown that the crystal field strength decreases with silicate content. The luminescence spectra tend towards those which are more common of Eu3+ luminescence in which phosphates are doped with rare earth ions. The spectra of luminescence versus temperature confirms that the crystal field between Eu3+(6h)–O(4) is stronger when the temperature is low and the cell parameters are small.