Optimizing synthesis of Na₂Ti₂SiO₇ • 2H₂O (Na-CST) and ion exchange pathways for Cs_0.4H _1.6Ti₂SiO₇ • 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 P4 ₂/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 O ^2- are 1.00 v.u., while bond valence sums of site Cs2 to framework O^2- are 0.712 v.u. suggesting Cs1 to have a more stable bonding environment.