Preparation and characterization of lanthanum–gadolinium monazites as ceramics for radioactive waste storage

Several La1 − xGdxPO4 solid solutions were prepared in the monazite- or rhabdophane-type structures for various x values using three methods of preparation (direct evaporation, synthesis in closed PTFE containers on a sand bath or in autoclaves). Samples of rhabdophane-type La1 − xGdxPO4·nH2O (n ≈ 0.5) were prepared at 150 °C only for x ≥ 0.4. For x ≤ 0.3, the solids were precipitated as the monazite-type structure. These results were confirmed by the study of pure rare earth phosphates synthesized under the same conditions. By these means, well-crystallized and monophase samples of MPO4·nH2O (n ≈ 0.5–1) in the monazite (La, Ce), rhabdophane (Nd, Sm, Eu, Gd, Tb, Dy) or churchite (Ho, Er, Tm, Yb, Lu) forms were prepared. On the basis of the variation of the specific area versus the holding temperature and of the dilatometric studies, the optimal temperature of sintering for these solids was found to be between 1250 and 1400 °C. The effective relative densities of the pellets of GdPO4 prepared using a two-step procedure (pressing between 200 and 700 MPa, then heat treatment at 1300 °C) reached 96% of the value calculated from the XRD data. The chemical durability of sintered samples of GdPO4 was evaluated in several acidic media between room temperature and 90 °C. The very low normalized dissolution rates RL (between 10−6 and 10−3 g m−2 day−1) measured even in very acidic media confirmed the very good retention properties of this kind of phosphate-based matrix for the immobilization of radionuclides and especially of trivalent actinides.