Aeschynite and euxenite structure-types as host phases for rare-earths and actinides from HLW

Aeschynite and euxenite structure-types are proposed as host phases for rare earth elements and actinides. Aeschynite and euxenite structure types are represented by the general formula: (REE, Y, Th, U, Ca)(Ti, Nb, Ta)2O6. In both structure-types, a variety of large cations such as rare-earths and actinides can be accommodated in the tunnels of connected holes in aeschynite or the interlayer space in euxenite. The waste loadings (rare-earths and actinides) in aeschynite and euxenite are at least 40 wt.% based on chemical compositions of naturally occurring aeschynite and euxenite. The sol-gel method has been successfully used to prepare aeschynite and euxenite as preferred crystalline ceramics. The aged gels and finally calcined products have been characterized by infrared and Raman spectroscopy, differential thermal analysis, and X-ray diffraction analysis. The fabrication technology of aeschynite and euxenite type ceramics is feasible by using hot-pressing as a final processing stage. The mechanistic processes of aeschynite and euxenite formation by the sol-gel method as described as REE(OH)3·Nb(OH)5·Ti(OH)4 coprecipitation; the polymerization of Nb and Ti hydroxides to form isolated (Nb,Ti)2O10 di-octahedra through oxygen and hydroxyl bridges accompanied by a dehydration reaction; welding together of isolated pieces of (Nb,Ti)2O10 to form longer (Nb,Ti)2O10 di-octahedral chains; REE being connected to the dioctahedral chains through oxygen and hydroxyl bridges; and to gradually form small three-dimensional nucleates of aeschynite and euxenite. Critical data from the literature on radiation effects and secondary alteration are reviewed as a part of the evaluation of the long-term performance of these phases