Synthesis and Crystal Structure Characterization of Oxysilicate Apatites for Stabilization of Sr and Rare-Earth Elements

Strontium (Sr) containing rare-earth oxysilicate apatite A^I ₄A^II ₆(SiO₄)₆O₂ is considered a good matrix to accommodate radionuclide as its cation sites can incorporate lanthanide elements other than Sr. Here, we report a study on the synthesis of Nd₈Sr₂(SiO₄)₆O₂ and Yb₈Sr₂(SiO₄)₆O₂, which adopt P6₃/m apatite symmetry, as well as the characterization of crystallographic structures using X-ray diffraction and electron microscopy. It is found that A^I position is shared by Nd, Yb and Sr, and A^IO₆ polyhedra are face-connected, forming column structures, which are linked to SiO₄ tetrahedra, creating continuous channels. The Rietveld refinement shows that Nd and Yb prefer the A^II position in the channel. The twisted angle of adjacent triangle faces in an A^I-O polyhedron along [001] is a critical parameter to identify the channel volume and its value varies when different cations are incorporated. The twisted angles for Nd-apatite and Yb are 24.2°and 22.7°The findings provide a new insight into nuclear waste stabilization by apatite-type structure.