Novel complex ceramic oxides, Ln₂TiO₅ (Ln = La, Sm, Gd, Tb, Dy, Ho, Er, and Yb), for polyphase nuclear waste‐forms

Abstract

As part of a broader study of ceramic nuclear waste‐forms, four different lanthanide titanates were fabricated; La0.1Sm0.1Gd0.1Tb0.1Dy0.3Ho0.1Er0.2YbTiO5, Sm0.3Gd0.3Dy0.3Yb1.1TiO5, Sm0.1Gd0.4Dy0.4Yb1.1TiO5, and Sm0.2Gd0.2Dy0.2Yb1.4TiO5. The aim was to produce single‐phase novel materials with cubic symmetry, capable of incorporating a wide variety of cations and with acceptable radiation tolerance. The chemistry flexibility and radiation tolerance are some of the major desirable properties for nuclear waste‐form materials. By using multiple lanthanides the average lanthanide radius can be controlled and consequently the structure, along with properties such as radiation tolerance. The radiation tolerance was assessed using in situ 1 MeV krypton irradiation and transmission electron microscopy characterization. Those materials for which cubic symmetry was achieved displayed better radiation tolerance; a greater critical fluence of ions (Fc) was required for the crystalline to amorphous transition, and a lower temperature was required to maintain crystallinity (Tc) during irradiation.

Crystal Chemistry of Titanates and Zirconates of Rare Earths—Possible Matrices for Actinide Isolation

“…Zirconates of light REE of the Ce group crystallize in the pyrochlore structure, while the phases of smaller cations of the Y group have the fl uorite structure. For REE-2TiO 5 compounds, the situation is more complicated: REE phases from La to Sm have an orthorhombic structure; those from Er to Lu, and Sc titanate have a cubic structure, while REE phases from Eu to Ho and Y have a hexagonal crystal symmetry [46,48,56]. As the temperature increases, the boundaries between these phases shift with the expansion of the fi elds of the cubic and hexagonal phases.…”

“…REE compounds (Nd) are potential matrices for minor actinides. Promising actinide matrices are REE titanates and zirconates [38][39][40][41][42][43][44][45][46][47][48][49]. Among them, much attention was paid to phases with pyrochlore and fl uorite structures [38,39,44].…”

“…Among them, much attention was paid to phases with pyrochlore and fl uorite structures [38,39,44]. Data on REE titanates are scarce and concern mainly the infl uence of the composition (REE type) on the structure of REE 2 TiO 5 and REE 2 Ti 2 O 7 phases and their behavior upon ion irradiation [40,[45][46][47][48][49]. In view of the fact that Nd 3+ serves as an analog of Am 3+ and Cm 3+ , neodymium compounds are of great interest in the search for REE-MA fraction matrices.…”