First Rb Silicotitanate Phase and Its K-Structural Analogue: New Members of the SNL-A Family (Cc-A2TiSi6O15; A = K, Rb, Cs)

Abstract

Alkali silicotitanate ternary phases have been studied for a wide variety of applications including catalysis, selective ion exchange, and ceramic nuclear waste form materials. We report here the synthesis and structural characterization of Rb2TiSi6O15 and K2TiSi6O15, structural analogues to SNL-A (Cc-Cs2TiSi6O15). The Rb analogue is the first reported ternary oxide, the rubidium silicotitanate phase. Both the Rb and the K analogues were synthesized nearly pure by hydrothermal treatment of oxide precursor mixtures seeded with SNL-A and could not be synthesized without the addition of the seed. Rietveld refinement gave good structural models in the Cc space group with unit cell parameters: Rb2TiSi6O15, V = 1357.9 Å3, a = 12.736(2) Å, b = 7.3392(3) Å, c = 15.061(3) Å, β = 105.29(2)° and K2TiSi6O15, V = 1318.5 Å3, a = 12.570(2) Å, b = 7.2534(3) Å, c = 15.082(3) Å, β = 106.49(3)°. The two isostructural phases were also characterized by thermogravimetry and 29Si NMR. Unit cell parameters, melting temperatures, and 29Si chemical shifts were compared to those of the Cs analogue and related to variation in alkali radius (Cs > Rb > K). The volume, the a-axis, and the b-axis increase linearly with alkali radius, which is directly related to a “relaxation” of the folds of the corrugated silicate layers that lie in the xy plane.