Role of the Hydroxyl−Water Hydrogen-Bond Network in Structural Transitions and Selectivity toward Cesium in Cs<sub>0.38</sub>(D<sub>1.08</sub>H<sub>0.54</sub>)SiTi<sub>2</sub>O<sub>7</sub>·(D<sub>0.86</sub>H<sub>0.14</sub>)<sub>2</sub>O Crystalline Silicotitanate
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The crystal structure of the selective Cs+ ion exchanger D1.6H0.4Ti2SiO7·D2.66H0.34O1.5, 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) Å and space group P42/mbc, to one with a = 7.8902(1) Å c = 11.9051(4) Å and space group P42/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 Dwater−Ddeuteroxyl repulsion forced by Cs+ exchange drives the structural transformation.
