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Crystal chemistry and energetics of pharmacosiderite-related microporous phases in the K2O–Cs2O–SiO2–TiO2–H2O system by Hongwu Xu; Alexandra Navrotsky; May Nyman; Tina M. Nenoff is a Materials Science article available to read on EtoBox.

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A complete series of solid solutions with compositions (K 1Àx Cs x ) 3 Ti 4 Si 3 O 15 (OH) AE nH 2 O (n 1⁄4 4-6, 0 6 x 6 1) and having the pharmacosiderite structure (space group P 43m) 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 þ content. In the structure, K þ is situated in the center of the eight-membered titanosilicate ring, whereas Cs þ is displaced from the ring center, and the displacement increases with higher The enthalpies of formation from the oxides and from the elements were determined by drop solution calorimetry into molten 2PbO AE B 2 O 3 solvent at 974 K. The formation enthalpies from oxides become more exothermic with increasing Cs þ /(Cs þ + K þ ), suggesting a stabilizing effect of K þ fi Cs þ on the pharmacosiderite structure. Calculation of the enthalpy of the K þ fi Cs þ exchange reaction based on the measured formation enthalpies indicates that the Cs þ uptake in these phases is probably thermodynamically (rather than kinetically) driven.

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Author

Hongwu Xu; Alexandra Navrotsky; May Nyman; Tina M. Nenoff

Publisher

Elsevier Science; Elsevier ; Elsevier BV (ISSN 1387-1811)

Published

2004

Language

EN

Field

Materials Science (Physical Sciences)