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Structural characterization of anhydrous and bishydrated calcium hexafluorosilicate by powder diffraction methods

Published online by Cambridge University Press:  05 March 2012

Simone Frisoni ,
Stefano Brenna  and
Norberto Masciocchi
Simone Frisoni
Affiliation:
Dipartimento di Scienze Chimiche e Ambientali, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy
Stefano Brenna
Affiliation:
Dipartimento di Scienze Chimiche e Ambientali, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy
Norberto Masciocchi*
Affiliation:
Dipartimento di Scienze Chimiche e Ambientali, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy
*
a)Author to whom correspondence should be addressed. Electronic mail: [email protected]
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Abstract

The synthesis and X-ray powder diffraction data for the long-known CaSiF6 and CaSiF6·2H2O species are reported. Their crystal structures have been determined from laboratory powder diffraction data by simulated annealing and full-profile Rietveld refinement methods. CaSiF6·2H2O was found to crystallize in the monoclinic P21/n space group with unit-cell parameters: a = 10.48107(9), b = 9.18272(7), c = 5.72973(5) Å, β = 98.9560(6)°, V = 544.733(8) Å3, and Z = 4. The crystal structure of CaSiF6·2H2O, eventually found to be isomorphous with SrSiF6·2H2O (but not with the Mg analogue—a hexahydrate phase), contains centrosymmetric [Ca(μ-H2O)2Ca]4+ dimers, interconnected by hexafluorosilicate anions, in a dense 3D framework. The crystal structure is completed by a further water molecule, terminally bound to the Ca2+ ion, which, consequently, attains a F5O3 octacoordination. Thermodiffractometric measurements allowed the determination of the linear and volumetric thermal expansion coefficients of CaSiF6·2H2O, which showed a minor contraction, along a, on heating. CaSiF6 is trigonal, space group R-3, a = 5.3497(3), c = 13.5831(11) Å, V = 336.66(5) Å3, and Z = 3, and isomorphous with several other species of MIIAIVF6 or MIAVF6 formulation, among which several silicates, germanates, and stannates.

Keywords

powder diffractioncalcium fluorosilicatestructure solutionRietveld refinementthermodiffractometry
Type
Technical Articles
Information
Powder Diffraction , Volume 26 , Issue 4 , December 2011 , pp. 308 - 312
Copyright
Copyright © Cambridge University Press 2011

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