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

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

Research output: Contribution to journalArticlepeer-review

19 Scopus citations


A complete series of solid solutions with compositions (K1-x Csx)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.

Original languageEnglish (US)
Pages (from-to)209-218
Number of pages10
JournalMicroporous and Mesoporous Materials
Issue number1-3
StatePublished - Jul 8 2004
Externally publishedYes


  • Crystal chemistry
  • Enthalpy of formation
  • Pharmacosiderite
  • Thermodynamics
  • Titanosilicate

ASJC Scopus subject areas

  • Chemistry(all)
  • Materials Science(all)
  • Condensed Matter Physics
  • Mechanics of Materials


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