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Thermochemistry of microporous silicotitanate phases in the Na2O–Cs2O–SiO2–TiO2–H2O system

Published online by Cambridge University Press:  31 January 2011

Hongwu Xu ,
Alexandra Navrotsky ,
May D. Nyman  and
Tina M. Nenoff
Hongwu Xu
Affiliation:
Thermochemistry Facility, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California 95616
Alexandra Navrotsky
Affiliation:
Thermochemistry Facility, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California 95616
May D. Nyman
Affiliation:
Sandia National Laboratories, Catalysis and Chemical Technologies, P.O. Box 5800, M.S. 0710, Albuquerque, New Mexico 87185
Tina M. Nenoff
Affiliation:
Sandia National Laboratories, Catalysis and Chemical Technologies, P.O. Box 5800, M.S. 0710, Albuquerque, New Mexico 87185
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Abstract

Microporous silicotitanates can potentially be used as ion exchangers for removal of Cs+ from radioactive waste solutions. The enthalpies of formation from constituent oxides for two series of silicotitanates at 298 K have been determined by drop-solution calorimetry into molten 2PbO · B2O3 at 974 K: the (Na1−xCsx)3Ti4Si3O15(OH) · nH2O (n = 4 to 5) phases with a cubic structure (P43m), and the (Na1−xCsx)3Ti4Si2O13(OH) · nH2O (n = 4 to 5) phases with a tetragonal structure (P42/mcm). The enthalpies of formation from the oxides for the cubic series become more exothermic as Cs/(Na + Cs) increases, whereas those for the tetragonal series become less exothermic. This result indicates that the incorporation of Cs in the cubic phase is somewhat thermodynamically favorable, whereas that in the tetragonal phase is thermodynamically unfavorable and kinetically driven. In addition, the cubic phases are more stable than the corresponding tetragonal phases with the same Cs/Na ratios. These disparities in the energetic behavior between the two series are attributed to their differences in both local bonding configuration and degree of hydration.

Type
Articles
Information
Journal of Materials Research , Volume 15 , Issue 3 , March 2000 , pp. 815 - 823
Copyright
Copyright © Materials Research Society 2000

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