Publications Details
The structure of TAM5: A silicotitanate molecular sieve for radwaste clean-up
A sodium silicotitanate (TAM5) of ideal composition Na{sub 3}Si{sub 2}Ti{sub 4}O{sub 13}(OH) {center_dot} 4H{sub 2}O (jointly developed by Sandia and Texas A and M) is synthesized hydrothermally in highly alkaline media. The material selectively removes cesium cations from solutions containing up to 5.7 M Na{sup +} and for a pH range of less than 1 to greater than 14. The crystal structure of TAM5 has been refined from X-ray powder data by Rietveld refinement, using the mineral Sitinakite as a model. The compound is tetragonal. The titanium and niobium atoms occur in clusters of four and are octahedrally coordinated by oxygen atoms. These clusters are held together by tetrahedral linkages to the silicon atoms. The framework forms a three dimensional structure that has potential accessibility to the three cation sites (two in the cages, one in the framework wall) from three directions. However, the sodium cation that sits in the wall is tightly held due to octahedral bonding from a combination of framework and water oxygen atoms. Furthermore, that site is too small for cations greater in size than sodium. In a concurrent report, extensive monovalent and divalent cation exchange studies are presented, with analyses including structure refinement, elemental analyses and thermal stability of both powder and UOP engineered forms.