Reversible hydration in synthetic mixite, BiCu6(OH)6(AsO4)3·nH2O (n≤3): hydration kinetics and crystal chemistry |
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Authors: | R Miletich J Zemann M Nowak |
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Institution: | Bayerisches Geoinstitut, Universit?t Bayreuth, Universit?tsstra?e 30, D-95440 Bayreuth, Germany Tel. +49 (0)921 55-3730; Fax +49 (0)921 55-3769; e-mail: ronald.miletichuni-bayreuth.de, DE Institut für Mineralogie & Kristallographie der Universit?t Wien, Geozentrum, Althanstra?e 14, A-1090 Wien, Austria Tel. +43 (0)1 31336-1866 or 1825; FAX +43 (0)1 31336-783, AT
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Abstract: | The presence of zeolitic water, with a reversible hydration behaviour, was determined by structural and kinetic studies on
synthetic mixite BiCu6(OH)6(AsO4)3·nH2O (n≤3). X-ray diffraction and infrared-spectroscopic investigations were performed on single crystals. Isothermal thermogravimetric
experiments were carried out to determine the reaction kinetics of the de- and rehydration processes. The single-crystal structure
refinement of a fully hydrated crystal yielded five partially occupied Ow positions (Ow=oxygen atom of a H2O molecule) within the tube-like channels of the hexagonal BiCu6(OH)6(AsO4)3] framework. For the partially dehydrated form, with n≈1, at least two of these sites were found to be occupied significantly.
In addition, the structural investigations allowed two different intra-framework hydrogen bonds to be distinguished that are
independent of the extra-framework water distribution and are responsible for the stability of the self-supporting framework.
The kinetic analysis of the rate data in the 298–343K temperature range shows that the dehydration behaviour obeys a diffusion-controlled
reaction mechanism with an empirical activation energy of E
a
dehyd=54±4 kJ mol–1. A two-stage process controls rehydration of which the individual steps were attributed to an initial surface-controlled
(E
a
hyd-I=6±1 kJ mol–1) and subsequent diffusion-controlled reaction mechanism (E
a
hyd-II=12±1 kJ mol–1). The estimated hydration enthalpy of 42±5 kJ mol–1 supports the distribution model of molecular water within the channels based on a purely hydrogen-bonded network.
Received June 26, 1996 / Revised, accepted November 11, 1996 |
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