Al/Si and Ca/Eu short range order in Ca/Eu-bicchulite, studied by FTIR spectroscopy

Rahmoun, Nouri-Said, Peters, L., Knorr, K. and Depmeier, W. (2008) Al/Si and Ca/Eu short range order in Ca/Eu-bicchulite, studied by FTIR spectroscopy Zeitschrift Fur Kristallographie, 223 (7). pp. 441-447. DOI 10.1524/zkri.2008.0046.

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Abstract

Powder FTIR spectra of the solid solution series vertical bar(Ca2-xEux)(4)(OH)(8)vertical bar[(Al2+xSi1-x)(4)O-24]-SOD, 0 <= x <= 1, were taken at room temperature. All members, including the mineral bicchulite, vertical bar Ca-8(OH)(8)vertical bar[Al8Si4O24]-SOD, have a sodalite-type structure. Spectra taken in the hydroxyl region (4000 - 3000 cm(-1)) yield information related to the local structural states of the pseudo-cubane-like (Ca2+xEu2-x)(OH)(4)-clusters, 0 <= x <= 2. Three different patterns of short range order could be identified. Depending on x, different concentrations of Ca-4(OH)(4), EuCa3(OH)(4), and Eu2Ca2(OH)(4) clusters are found, whereas EU3Ca(OH)(4) or EU4(OH)(4) clusters are not observed. Hence, short range order is maintained such that each SOD cage of an all-alumina framework contains two Ca and two Eu per cluster. The positions of the framework stretching and bending modes shift linearly towards lower wavenumbers with increasing Al content. The shifts are correlated with changes of the framework geometry. Furthermore, Al rich samples show splitting of the stretching and bending modes. This is interpreted as the change of the local environment of Al due to the substitution of Eu3+ for Ca2+. With the use of deuteration experiments a more detailed assignment could be achieved, especially for the M-OH absorption bands. Finally, changes in the line shape, resulting from cation ordering, are studied by auto-correlation analysis. In particular, samples of intermediate composition show large broadening parameters, Delta(corr), Suggesting high degrees of local structural heterogeneity.

Document Type: Article
Keywords: FTIR spectroscopy sodalites short range order bicchulite crystal-structure sodalite ca2<al2sio6>(oh)2 diffraction spectra ir
Research affiliation: Kiel University
DOI etc.: 10.1524/zkri.2008.0046
ISSN: 0044-2968
Date Deposited: 12 Jan 2012 05:07
Last Modified: 08 Oct 2012 10:15
URI: http://eprints.uni-kiel.de/id/eprint/15871

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