Two new zeolitic crystalline phases with stoichiometry,
CS2TiSi6O15, have been discovered.
CSTiSi2O6.5 has a crystal structure isomorphous to
the mineral pollucite, CsAlSi2O6, with Ti+4
replacing Al+3. This replacement requires a mechanism for charge
compensation. A combination of techniques including neutron diffraction,
single crystal x-ray diffraction and x-ray absorption spectroscopy have
revealed that eight extra oxygens are present per unit cell
Cs2TiSi2O6.5 as compared to pollucite.
As a result of the extra oxygen, the titanium coordination geometry is
five-fold. Pentacoordinate titanium and tetrahedral silicon form a network
structure with Cs residing in cages formed by the network. The crystal
structure of Cs2TiSi6O15 is unique, with
titanium octahedra and silicon tetrahedra forming an open framework
structure with the Cs residing in large cavities. The largest covalently
bonded ring opening to the Cs cavities in both compounds are smaller than a
Cs ion, revealing that the Cs ion has minimal mobility in the structure.
Cesium leach rates for both compounds are lower than or comparable to