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 \(P{\bar{4}}2_{1}m.\) 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: α1=7.2(4)×10−6×K−1+3.6(7)×10−9ΔT×K−2 and α3=15.0(1)×10−6×K−1. For TbCaAl[Al2O7] the respective values are: α1=7.0(2)×10−6×K−1+2.0(2)×10−9ΔT×K−2 and α3=8.5(2)×10−6×K−1+2.0(3)×10−9ΔT×K−2, and the thermal expansion coefficients for SmCaAl[Al2O7] are: α1=6.9(2)×10−6×K−1+1.7(2)×10−9ΔT×K−2 and α3=9.344(5)×10−6×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.
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Acknowledgements
We wish to thank the Deutsche Forschungsgemeinschaft (DFG) for financial support (De 412/27-1, 2), S. Bass and E. Kirchhof for their help with some of the syntheses and the workshop staff for the construction of the furnace. Additionally, we wish to thank the HASYLAB / DESY for providing beam time at the B2 diffractometer (grant II-04-033). Critical comments by Dr. Martin Ryan and an anonymous reviewer helped to improve the manuscript and are gratefully acknowledged.
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This article has been mistakenly published twice. The first and original version of it is available at http://dx.doi.org/10.1007/s00269-005-0015-4.
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Peters, L., Knorr, K., Knapp, M. et al. Thermal expansion of gehlenite, Ca2Al[AlSiO7], and the related aluminates LnCaAl[Al2O7] with Ln = Tb, Sm. Phys Chem Minerals 32, 546–551 (2005). https://doi.org/10.1007/s00269-005-0021-6
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DOI: https://doi.org/10.1007/s00269-005-0021-6