Cs⁺-exchange property of Na-bearing GTS-type titanosilicate and possible distribution of Cs⁺ ions in its Cs⁺-exchanged form

Abstract

Single phase of Na-bearing grace titanosilicate (Na-GTS) powder was prepared hydrothermally. Its Cs⁺-exchanged forms [Na_4(1−x)Cs_4xTi₄O₄(SiO₄)₃·nH₂O] with various compositions up to x = 1.0 were produced by shaking the Na-GTS in the CsCl aqueous solutions with the Cs-concentration (C_Cs) up to 1.0 mol/L at temperatures up to 80 °C for 24 hours. The Cs⁺-exchange rates (x) at each temperature increase steeply with increasing C_Cs up to 0.1 mol/L, after which they are almost constant. Meanwhile, up to 60 °C the x values do not depend significantly on treatment temperature, but at a higher temperature of 80 °C they increase dramatically up to unity. Thus, at least above 80 °C, the increase in treatment temperature is considerably effective for promotion of ion-exchange, as well as the increase in C_Cs. TG-DTA measurements show that the exchange of Na⁺ ions for Cs⁺ ions largely decreases H₂O contents, as a consequence of a remarkable reduction in available void-space within the cavities due to a remarkable increase in mean cation size. The comparison between the observed XRD pattern and the simulated ones suggests the cation-distribution model that Cs⁺ ions prefer the 6g site in the GTS cavity in the assumed pseudocubic structure to the 4e site. The unit cell volumes increase largely with increasing x although the H₂O contents reduce largely. This can be because moderate bonding forces acting between a Cs⁺ ion on the 6g site, located at the proximity of the centers of the 8-membered ring windows, and adjacent O atoms forming the windows directly influence the framework structure. Furthermore, these findings are compared with those of the previously reported Er³⁺-exchanged forms, in terms of the difference in valence and size of the extra-framework cations.