Integrated experimental and computational methods for structure determination and characterization of a new, highly stable cesium silicotitanate phase, Cs2TiSi6O15 (SNL-A)

Exploratory hydrothermal synthesis in the system Cs2O-SiO2-TiO2 has produce d a new polymorph of Cs2TiSi6O15 (SNL-A), whose structure was determined us ing a combination of experimental and theoretical techniques (Si-29 and Cs- 133 NMR, X-ray powder diffraction, and density functional theory). SNL-A cr ystallizes in the monoclinic space group Cc with unit cell parameters a = 1 2.998(2) Angstrom, b = 7.5014(3) Angstrom, c = 15.156(3) Angstrom, and beta = 105.80(3)degrees. The SNL-A framework is an unbranched drier single-laye r silicate with silicon tetrahedra and titanium octahedra that are linked i n 3-, 5-, 6-, 7-, and 8-membered rings in three dimensions. SNL-A is distin ctive from a previously reported C2/c polymorph of Cs2TiSi6O15 by orientati on of the Si2O52- layers and by different ring geometries. Similarities and differences between the two structures are discussed. Other characterizati ons of SNL-A include TGA-DTA, Cs/Si/Ti elemental analyses, and SEM/EDS. Fur thermore, the chemical and radiation durability of SNL-A was studied in int erest of ceramic waste form applications. These studies show that SNL-A is durable in both radioactive and rigorous chemical environments. Finally, ca lculated cohesive energies of the two CS2TiSi6O15 polymorphs suggest that t he Cc SNL-A phase (synthesized at 200 degreesC) is energetically more favor able than the C2/c polymorph (synthesized at 1050 degreesC).