Preparation and structural study of Na(1-3x)EuxTi2(PO4)3 phosphates

The phosphates Na(1-3x)EuxTi2(PO4)(3) (0 less than or equal to x less than or equal to 0.25) have been obtained by solid state reaction at 950 degrees C. They crystallize in the Nasicon-type structure. The progressive substitution of europium for sodium results in an increase of the a(h) parameter and a decrease of the c(h) parameter. This variation is mainly due to the creation of vacancies in the M(1) sites. The structure of Na0.25Eu0.25Ti2(PO4)(3) has been solved by the Rietveld profilation method in the <R(3)over bar> space group. The europium and sodium occupy statistically half of the M(1) sites with a 1-1 vacancy-Na/Eu ordering along the c-axis. In this compound, the europium (III) emission spectrum evidences the actual location of the rare-earth in a unique non-centrosymmetric site. The strong absorption at high energy can be ascribed to the oxygen europium (III) and oxygen titanium (IV) charge transfers.