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-H2O has produced a new polymorph of Cs2TiSi6O15 (SNL-A) whose structure was determined using a combination of experimental and theoretical techniques (29Si and 133Cs NMR, X-ray Rietveld refinement, and Density Functional Theory). SNL-A crystallizes in the monoclinic space-group Cc with unit cell parameters: a = 12.998(2) angstrom, b = 7.5014(3) angstrom, c = 15.156(3) angstrom, η = 105.80(3) degree. The SNL-A framework consists of silicon tetrahedra and titanium octahedra which are linked in 3-, 5-, 6-, 7- and 8-membered rings in three dimensions. SNL-A is distinctive from a previously reported C2/c polymorph of Cs2TiSi6O15 by different ring geometries. Similarities and differences between the two structures are discussed. Other characterizations of SNL-A include TGA-DTA, Cs/Si/Ti elemental analyses, and SEM/EDS. Furthermore, the chemical and radiation durability of SNL-A was studied in interest of ceramic waste form applications. These studies show that SNL-A is durable in both radioactive and rigorous chemical environments. Finally, calculated cohesive energies of the two Cs2TiSi6O15 polymorphs suggest that the SNL-A phase (synthesized at 200 C) is energetically more favorable than the C2/c polymorph (synthesized at 1,050 C)