Crystal chemistry and energetics of pharmacosiderite-related microporous phases in the K₂O-Cs₂O-SiO₂- TiO₂-H₂O system

A complete series of solid solutions with compositions (K_1-x Cs_x)₃Ti₄Si₃O₁₅( OH)· nH₂O (n=4-6, 0≤x≤1) and having the pharmacosiderite structure (space group P 4̄3 m) has been synthesized using hydrothermal and ion-exchange methods. Rietveld analysis of synchrotron XRD data shows that the unit-cell parameter a increases linearly with increasing Cs⁺ content. In the structure, K⁺ is situated in the center of the eight-membered titanosilicate ring, whereas Cs⁺ is displaced from the ring center, and the displacement increases with higher K⁺/ (Cs⁺ + K⁺) ratio.The enthalpies of formation from the oxides and from the elements were determined by drop solution calorimetry into molten 2PbO · B₂O₃ solvent at 974 K. The formation enthalpies from oxides become more exothermic with increasing Cs⁺/(Cs⁺ + K⁺), suggesting a stabilizing effect of K⁺ → Cs⁺ on the pharmacosiderite structure. Calculation of the enthalpy of the K⁺ → Cs⁺ exchange reaction based on the measured formation enthalpies indicates that the Cs⁺ uptake in these phases is probably thermodynamically (rather than kinetically) driven.