Role of the hydroxyl-water hydrogen-bond network in structural transitions and selectivity toward cesium in Cs_0.38(D_1.08H _0.54)SiTi₂O₇·(D_0.86H _0.14)₂O crystalline silicotitanate

The crystal structure of the selective Cs⁺ ion exchanger D _1.6H_0.4Ti₂SiO₇·D _2.66H_0.34O_1.5, known as crystalline silicotitanate or CST, has been determined in both native (D-CST) and in the Cs⁺-exchanged forms ((Cs, D)-CST) from angle-dispersive and time-of-flight neutron diffraction studies. The final fully exchange Cs ⁺ form transformed from D-CST with unit cell parameters a = 11.0704(3) Å c = 11.8917(5) A and space group P4₂/mbc, to one with a = 7.8902-(1) Å c = 11.9051(4) Å and space group P4 ₂/mcm. Rietveld structure refinements of both D-CST and (Cs, D)-CST suggest the transition, and ultimately the selectivity, is driven by changes in the positions of water molecules, in response to the initial introduction of Cs⁺. The changes in water position appear to disrupt the D-O-O-D dihedral associated with the CST framework in space group P4₂/mbc which ultimately leads to the structural transition. The new geometric arrangement of the water-deuteroxyl network in (Cs, D)-CST suggests that D _water-D_deuteroxyl repulsion forced by Cs⁺ exchange drives the structural transformation.