Thermal expansion of gehlenite, Ca2Al AlSiO7 , and the related aluminates LnCaAl Al2O7 with Ln = Tb, Sm

Peters, L., Knorr, K., Knapp, M. and Depmeier, W. (2005) Thermal expansion of gehlenite, Ca2Al AlSiO7 , and the related aluminates LnCaAl Al2O7 with Ln = Tb, Sm Physics and Chemistry of Minerals, 32 (8-9). pp. 546-551. DOI 10.1007/s00269-005-0021-6.

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Abstract

The thermal expansion of gehlenite, Ca2Al[AlSiO7], ( up to T= 830 K), TbCaAl[Al2O7] (up to T= 1100 K) and SmCaAl[Al2O7] ( up to T= 1024 K) has been determined. All compounds are of the melilite structure type with space group P (4) over bar2(1)m. Thermal expansion data were obtained from in situ X-ray powder diffraction experiments in-house and at HASYLAB at the Deutsches Elektronen Synchrotron (DESY) in Hamburg ( Germany). The thermal expansion coefficients for gehlenite were found to be: alpha(1) = 7.2(4) x 10(-6) x K-1 + 3.6(7) x 10(-9) Delta T x K-2 and alpha(3) = 15.0(1) x 10(-6) x K-1. For TbCaAl[Al2O7] the respective values are: alpha(1) = 7.0(2) x 10(-6) x K-1 + 2.0(2) x 10(-9) Delta T x K-2 and alpha(3) = 8.5(2) x 10(-6) x K-1 + 2.0(3) x 10(-9) Delta T x K-2, and the thermal expansion coefficients for SmCaAl[Al2O7] are: alpha(1) = 6.9(2) x 10(-6) x K-1 + 1.7(2) x 10(-9) Delta T x K-2 and alpha(3) = 9.344(5) x 10(-6) x K-1. The expansion mechanisms of the three compounds are explained in terms of structural trends obtained from Rietveld refinements of the crystal structures of the compounds against the powder diffraction patterns. No structural phase transitions have been observed. While gehlenite behaves like a 'proper' layer structure, the aluminates show increased framework structure behavior. This is most probably explained by stronger coulombic interactions between the tetrahedral conformation and the layer-bridging cations due to the coupled substitution (Ca2+ + Si4+) ( Ln(3+)+ Al3+) in the melilite-type structure.

Document Type: Article
Keywords: gehlenite melilite coupled substitution thermal expansion x-ray diffraction nd
Research affiliation: Kiel University
Refereed: Yes
DOI etc.: 10.1007/s00269-005-0021-6
ISSN: 0342-1791
Date Deposited: 18 Jan 2012 05:21
Last Modified: 28 Mar 2018 10:16
URI: http://eprints.uni-kiel.de/id/eprint/16086

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