A Potential Wasteform for Cs Immobilization: Synthesis, Structure Determination, and Aqueous Durability of Cs₂TiNb₆O₁₈

Cs₂TiNb₆O₁₈ is a potential ceramic wasteform for the long-term immobilization of radioactive cesium. Cs₂TiNb₆O₁₈ was synthesized using the aqueous precursor method and a solid-state reaction, and its crystal structure was determined from the Rietveld refinement of synchrotron X-ray and neutron powder diffraction data. The structure is a pyrochlore analogue, space group P3Ìm1 with Cs in 9-fold coordination. The calculated bond valence sum from analysis of neutron diffraction data of 0.84 and high coordination number suggest that Cs has a strong bonding environment. The chemical aqueous durability was investigated using the MCC-1 and PCT-B standard test methods. The measured Cs leach rates of 3.8 × 10^-3 and 2.1 × 10^-3 g m^-2 day^-1 obtained via the MCC-1 and PCT-B methods, respectively, demonstrate good promise of a safe long-term immobilization material comparable to, if not better than, hollandite - the material in the multiphase titanate ceramics (Synroc) targeted for cesium sequestration.