Optimizing synthesis of Na2Ti2SiO7 · 2H2O (Na-CST) and ion exchange pathways for Cs0.4H1.6Ti2SiO7 · H2O (Cs-CST) determined from in situ synchrotron X-ray powder diffraction

Observation of wide angle diffraction data collected in situ during synthesis of Na-CST (Na2Ti2O3SiO4 · 2H2O) showed initial crystallization of a precursor phase (SNT) at 30 °C followed by conversion to CST after 1 h at 220 °C. In situ studies of Cs+ ion exchange into the H+ form of CST showed a site-by-site ion exchange pathway accompanied by a simultaneous structural transition from P42/mbc (cell parameters a = 11.0690(6) Å, c = 11.8842(6) Å) to P42/mcm (cell parameters a = 7.847(2) Å, c = 11.9100(6) Å). After approximately 18% Cs+ exchange into site designated Cs2 in space group P42/mcm, a site designated Cs1 in space group P42/mcm began to fill at the center of the 8MR windows until a maximum of approximately 22% exchange was achieved for Cs1. Bond valence sums of site Cs1 to framework O2− are 1.00 v.u., while bond valence sums of site Cs2 to framework O2− are 0.712 v.u. suggesting Cs1 to have a more stable bonding environment.