Incorporation de terres rares dans les pérovskites: Cas de l'incorporation de Néodyme dans CaSnO3

International audienceSynroc is a synthetic assembly of titanate phases to immobilize radioactive waste and perovskite is one of its major constituent (≈25%). It is generally assumed that REE 3+ and transuranic elements substitute Ca in CaTiO 3 perovskite of SYNROC owing to the proximity of their ionic radii. In this case, the mechanism of charge compensation can be explained either by partial replacement of Ti by Al, A site vacancies or titanium reduction to Ti 3+ state. Several factors can affect the substitution mechanisms (such as the A/B cationic ratio, the percentage of REE 3+ incorporation, the ionic radii of the REE 3+ , the atmosphere of synthesis…). In this study, we investigated the incorporation of Nd 3+ in the CaSnO 3 perovskite, an analogue of CaTiO 3 with an orthorhombic structure Pbnm. CaSnO 3 powder was synthesized by a conventional solid state route. The sintered CaSnO 3 was then mixed with Nd(OH) 3 , pressed into pellet and sintered at 1550°C for 3 days. Samples with varying compositions (ranging from 5 to 60 wt% of Nd) were then analysed by XRD, SEM, EMPA and Raman spectroscopy. Incorporation of Nd in the perovskite phase was successful up to 50% wt. The evolution of the lattice parameters is following a Vegard's law. A phase transition from an orthorhombic Pbnm structure to a monoclinic P2 1 /n structure is observed around Nd=30 wt%. The grain size of the ceramics globally increases with the Nd incorporation. Thus, crystals of the Nd 40 wt% and Nd 50 wt% samples were adapted for a single crystal XRD investigation. The results show that Nd substitutes Ca in the A site of the structure. The excess of positive charge on the A site due to the substitution of a divalent cation by a trivalent one is compensated by a migration of the calcium substituted on A site, to the Sn B site. Depending on the different synthesis parameters, several cristallochemistry could be obtained due to the huge flexibility of the perovskite structure. Besides, the durability of those perovskites might be affected by the different substitutions mechanisms. Further studies on the CaTiO 3 doped perovskite are therefore necessary to apprehend its long-term behavior in SYNROC