Phase evolution and aqueous durability of high-entropy component Ln3+ doped zirconolite ceramics for nuclear waste forms

In this work, a series of high-entropy component Ln 3 + (HEcp- Ln 3 + ) doped zirconolite ceramics Ca 1- x ( Ln 1/5 ) x ZrTi 2- x Al x O 7 (Ln = La, Nd, Sm, Eu, Gd; x  = 0.0∼1.0) were successfully prepared by a high-temperature solid-state reaction method. The XRD and Raman results showed that the sample transforms from the zirconolite-2M structure to the -3O structure with increasing HEcp- Ln 3+ doping content to x  = 0.8. The SEM-EDS results revealed the as-prepared samples have dense microstructures and homogeneous elemental distribution. The MCC-1 results indicated that the representative samples of Ca 0.5 ( Ln 1/5 ) 0.5 ZrTi 1.5 Al 0.5 O 7 with 2M structure and ( Ln 1/5 )ZrTiAlO 7 with 3O structure show superior aqueous stability compared to a single-component Nd 3+ /Sm 3+ doped zirconolite system. The chemical durability of zirconolite-3O seemed to be stronger than that of zirconolite-2M due to the difference in the leaching mechanism at the initial leaching stage or due to the high-entropy effect. The present study could be helpful for the design of high-entropy zirconolite-based waste forms for immobilizing plutonium and various minor actinides.