Crystal chemistry and energetics of pharmacosiderite-related microporous phases in the K 2O–Cs 2O–SiO 2–TiO 2–H 2O system

Abstract

A complete series of solid solutions with compositions (K 1− x Cs x ) 3Ti 4Si 3O 15(OH) · nH 2O ( n=4–6, 0⩽ x⩽1) and having the pharmacosiderite structure (space group P 4 ̄ 3 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 · B 2O 3 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.