The crystal structures of pressure-induced LiSrAlF6-II and LiCaAlF6-II

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Grzechnik, Andrzej ; Dmitriev, Vladimir ; Weber, Hans-Peter ; Gesland, Jean-Yves ; van Smaalen, Sander:
The crystal structures of pressure-induced LiSrAlF6-II and LiCaAlF6-II.
In: Journal of Physics: Condensed Matter. Bd. 16 (2004) Heft 7 . - S. 1033-1043.
ISSN 1361-648X
DOI: https://doi.org/10.1088/0953-8984/16/7/003

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Abstract

The crystal structures of LiCaAlF 6 -II and LiSrAlF 6 -II (both P 2 1 / c , Z = 4) occurring at high pressures and room temperature were studied with synchrotron angle-dispersive x-ray powder diffraction in diamond anvil cells. The structure of LiSrAlF 6 -II stable between 1.6 and 3.0 GPa was solved with a global optimisation algorithm and group theory considerations, and refined with the Rietveld method in the rigid-body approximation. It is a distorted variant of the ambient pressure polymorph (LiSrAlF 6 -I, ##IMG## http://ej.iop.org/images/0953-8984/16/7/003/cm173091ieqn1.gif {P\bar 3 1c, Z=2} ), in which each cation occupies a deformed octahedral site. LiCaAlF 6 transforms to this monoclinic polymorph II above about 7 GPa. The differences in the high-pressure behaviours of LiCaAlF 6 and LiSrAlF 6 are discussed by considering the ionic radii.